ECMA-262, 15^th edition, June 2024
ECMAScript® 2024 Language Specification

About this Specification

The document at https://tc39.es/ecma262/ is the most accurate and up-to-date ECMAScript specification. It contains the content of the most recent yearly snapshot plus any finished proposals (those that have reached Stage 4 in the proposal process and thus are implemented in several implementations and will be in the next practical revision) since that snapshot was taken.

This document is available as a single page and as multiple pages.

Contributing to this Specification

This specification is developed on GitHub with the help of the ECMAScript community. There are a number of ways to contribute to the development of this specification:

GitHub Repository: https://github.com/tc39/ecma262
Issues: All Issues, File a New Issue
Pull Requests: All Pull Requests, Create a New Pull Request
Test Suite: Test262
Editors:
- Shu-yu Guo (@_shu)
- Michael Ficarra (@smooshMap)
- Kevin Gibbons (@bakkoting)
Community:
- Discourse: https://es.discourse.group
- Chat: Matrix
- Mailing List Archives: https://esdiscuss.org/

Refer to the colophon for more information on how this document is created.

Introduction

This Ecma Standard defines the ECMAScript 2024 Language. It is the fifteenth edition of the ECMAScript Language Specification. Since publication of the first edition in 1997, ECMAScript has grown to be one of the world's most widely used general-purpose programming languages. It is best known as the language embedded in web browsers but has also been widely adopted for server and embedded applications.

ECMAScript is based on several originating technologies, the most well-known being JavaScript (Netscape) and JScript (Microsoft). The language was invented by Brendan Eich at Netscape and first appeared in that company's Navigator 2.0 browser. It has appeared in all subsequent browsers from Netscape and in all browsers from Microsoft starting with Internet Explorer 3.0.

The development of the ECMAScript Language Specification started in November 1996. The first edition of this Ecma Standard was adopted by the Ecma General Assembly of June 1997.

That Ecma Standard was submitted to ISO/IEC JTC 1 for adoption under the fast-track procedure, and approved as international standard ISO/IEC 16262, in April 1998. The Ecma General Assembly of June 1998 approved the second edition of ECMA-262 to keep it fully aligned with ISO/IEC 16262. Changes between the first and the second edition are editorial in nature.

The third edition of the Standard introduced powerful regular expressions, better string handling, new control statements, try/catch exception handling, tighter definition of errors, formatting for numeric output and minor changes in anticipation of future language growth. The third edition of the ECMAScript standard was adopted by the Ecma General Assembly of December 1999 and published as ISO/IEC 16262:2002 in June 2002.

After publication of the third edition, ECMAScript achieved massive adoption in conjunction with the World Wide Web where it has become the programming language that is supported by essentially all web browsers. Significant work was done to develop a fourth edition of ECMAScript. However, that work was not completed and not published as the fourth edition of ECMAScript but some of it was incorporated into the development of the sixth edition.

The fifth edition of ECMAScript (published as ECMA-262 5^th edition) codified de facto interpretations of the language specification that have become common among browser implementations and added support for new features that had emerged since the publication of the third edition. Such features include accessor properties, reflective creation and inspection of objects, program control of property attributes, additional array manipulation functions, support for the JSON object encoding format, and a strict mode that provides enhanced error checking and program security. The fifth edition was adopted by the Ecma General Assembly of December 2009.

The fifth edition was submitted to ISO/IEC JTC 1 for adoption under the fast-track procedure, and approved as international standard ISO/IEC 16262:2011. Edition 5.1 of the ECMAScript Standard incorporated minor corrections and is the same text as ISO/IEC 16262:2011. The 5.1 Edition was adopted by the Ecma General Assembly of June 2011.

Focused development of the sixth edition started in 2009, as the fifth edition was being prepared for publication. However, this was preceded by significant experimentation and language enhancement design efforts dating to the publication of the third edition in 1999. In a very real sense, the completion of the sixth edition is the culmination of a fifteen year effort. The goals for this edition included providing better support for large applications, library creation, and for use of ECMAScript as a compilation target for other languages. Some of its major enhancements included modules, class declarations, lexical block scoping, iterators and generators, promises for asynchronous programming, destructuring patterns, and proper tail calls. The ECMAScript library of built-ins was expanded to support additional data abstractions including maps, sets, and arrays of binary numeric values as well as additional support for Unicode supplementary characters in strings and regular expressions. The built-ins were also made extensible via subclassing. The sixth edition provides the foundation for regular, incremental language and library enhancements. The sixth edition was adopted by the General Assembly of June 2015.

ECMAScript 2016 was the first ECMAScript edition released under Ecma TC39's new yearly release cadence and open development process. A plain-text source document was built from the ECMAScript 2015 source document to serve as the base for further development entirely on GitHub. Over the year of this standard's development, hundreds of pull requests and issues were filed representing thousands of bug fixes, editorial fixes and other improvements. Additionally, numerous software tools were developed to aid in this effort including Ecmarkup, Ecmarkdown, and Grammarkdown. ES2016 also included support for a new exponentiation operator and adds a new method to Array.prototype called includes.

ECMAScript 2017 introduced Async Functions, Shared Memory, and Atomics along with smaller language and library enhancements, bug fixes, and editorial updates. Async functions improve the asynchronous programming experience by providing syntax for promise-returning functions. Shared Memory and Atomics introduce a new memory model that allows multi-agent programs to communicate using atomic operations that ensure a well-defined execution order even on parallel CPUs. It also included new static methods on Object: Object.values, Object.entries, and Object.getOwnPropertyDescriptors.

ECMAScript 2018 introduced support for asynchronous iteration via the AsyncIterator protocol and async generators. It also included four new regular expression features: the dotAll flag, named capture groups, Unicode property escapes, and look-behind assertions. Lastly it included object rest and spread properties.

ECMAScript 2019 introduced a few new built-in functions: flat and flatMap on Array.prototype for flattening arrays, Object.fromEntries for directly turning the return value of Object.entries into a new Object, and trimStart and trimEnd on String.prototype as better-named alternatives to the widely implemented but non-standard String.prototype.trimLeft and trimRight built-ins. In addition, it included a few minor updates to syntax and semantics. Updated syntax included optional catch binding parameters and allowing U+2028 (LINE SEPARATOR) and U+2029 (PARAGRAPH SEPARATOR) in string literals to align with JSON. Other updates included requiring that Array.prototype.sort be a stable sort, requiring that JSON.stringify return well-formed UTF-8 regardless of input, and clarifying Function.prototype.toString by requiring that it either return the corresponding original source text or a standard placeholder.

ECMAScript 2020, the 11^th edition, introduced the matchAll method for Strings, to produce an iterator for all match objects generated by a global regular expression; import(), a syntax to asynchronously import Modules with a dynamic specifier; BigInt, a new number primitive for working with arbitrary precision integers; Promise.allSettled, a new Promise combinator that does not short-circuit; globalThis, a universal way to access the global this value; dedicated export * as ns from 'module' syntax for use within modules; increased standardization of for-in enumeration order; import.meta, a host- populated object available in Modules that may contain contextual information about the Module; as well as adding two new syntax features to improve working with “nullish” values (undefined or null): nullish coalescing, a value selection operator; and optional chaining, a property access and function invocation operator that short-circuits if the value to access/invoke is nullish.

ECMAScript 2021, the 12^th edition, introduced the replaceAll method for Strings; Promise.any, a Promise combinator that short-circuits when an input value is fulfilled; AggregateError, a new Error type to represent multiple errors at once; logical assignment operators (??=, &&=, ||=); WeakRef, for referring to a target object without preserving it from garbage collection, and FinalizationRegistry, to manage registration and unregistration of cleanup operations performed when target objects are garbage collected; separators for numeric literals (1_000); and Array.prototype.sort was made more precise, reducing the amount of cases that result in an implementation-defined sort order.

ECMAScript 2022, the 13^th edition, introduced top-level await, allowing the keyword to be used at the top level of modules; new class elements: public and private instance fields, public and private static fields, private instance methods and accessors, and private static methods and accessors; static blocks inside classes, to perform per-class evaluation initialization; the #x in obj syntax, to test for presence of private fields on objects; regular expression match indices via the /d flag, which provides start and end indices for matched substrings; the cause property on Error objects, which can be used to record a causation chain in errors; the at method for Strings, Arrays, and TypedArrays, which allows relative indexing; and Object.hasOwn, a convenient alternative to Object.prototype.hasOwnProperty.

ECMAScript 2023, the 14^th edition, introduced the toSorted, toReversed, with, findLast, and findLastIndex methods on Array.prototype and TypedArray.prototype, as well as the toSpliced method on Array.prototype; added support for #! comments at the beginning of files to better facilitate executable ECMAScript files; and allowed the use of most Symbols as keys in weak collections.

ECMAScript 2024, the 15^th edition, added facilities for resizing and transferring ArrayBuffers and SharedArrayBuffers; added a new RegExp /v flag for creating RegExps with more advanced features for working with sets of strings; and introduced the Promise.withResolvers convenience method for constructing Promises, the Object.groupBy and Map.groupBy methods for aggregating data, the Atomics.waitAsync method for asynchronously waiting for a change to shared memory, and the String.prototype.isWellFormed and String.prototype.toWellFormed methods for checking and ensuring that strings contain only well-formed Unicode.

Dozens of individuals representing many organizations have made very significant contributions within Ecma TC39 to the development of this edition and to the prior editions. In addition, a vibrant community has emerged supporting TC39's ECMAScript efforts. This community has reviewed numerous drafts, filed thousands of bug reports, performed implementation experiments, contributed test suites, and educated the world-wide developer community about ECMAScript. Unfortunately, it is impossible to identify and acknowledge every person and organization who has contributed to this effort.

Allen Wirfs-Brock
ECMA-262, Project Editor, 6^th Edition

Brian Terlson
ECMA-262, Project Editor, 7^th through 10^th Editions

Jordan Harband
ECMA-262, Project Editor, 10^th through 12^th Editions

Shu-yu Guo
ECMA-262, Project Editor, 12^th through 15^th Editions

Michael Ficarra
ECMA-262, Project Editor, 12^th through 15^th Editions

Kevin Gibbons
ECMA-262, Project Editor, 12^th through 15^th Editions

1 Scope

This Standard defines the ECMAScript 2024 general-purpose programming language.

2 Conformance

A conforming implementation of ECMAScript must provide and support all the types, values, objects, properties, functions, and program syntax and semantics described in this specification.

A conforming implementation of ECMAScript must interpret source text input in conformance with the latest version of the Unicode Standard and ISO/IEC 10646.

A conforming implementation of ECMAScript that provides an application programming interface (API) that supports programs that need to adapt to the linguistic and cultural conventions used by different human languages and countries must implement the interface defined by the most recent edition of ECMA-402 that is compatible with this specification.

A conforming implementation of ECMAScript may provide additional types, values, objects, properties, and functions beyond those described in this specification. In particular, a conforming implementation of ECMAScript may provide properties not described in this specification, and values for those properties, for objects that are described in this specification.

A conforming implementation of ECMAScript may support program and regular expression syntax not described in this specification. In particular, a conforming implementation of ECMAScript may support program syntax that makes use of any “future reserved words” noted in subclause 12.7.2 of this specification.

A conforming implementation of ECMAScript must not implement any extension that is listed as a Forbidden Extension in subclause 17.1.

A conforming implementation of ECMAScript must not redefine any facilities that are not implementation-defined, implementation-approximated, or host-defined.

A conforming implementation of ECMAScript may choose to implement or not implement Normative Optional subclauses. If any Normative Optional behaviour is implemented, all of the behaviour in the containing Normative Optional clause must be implemented. A Normative Optional clause is denoted in this specification with the words "Normative Optional" in a coloured box, as shown below.

Normative Optional

2.1 Example Normative Optional Clause Heading

Example clause contents.

A conforming implementation of ECMAScript must implement Legacy subclauses, unless they are also marked as Normative Optional. All of the language features and behaviours specified within Legacy subclauses have one or more undesirable characteristics. However, their continued usage in existing applications prevents their removal from this specification. These features are not considered part of the core ECMAScript language. Programmers should not use or assume the existence of these features and behaviours when writing new ECMAScript code.

Legacy

2.2 Example Legacy Clause Heading

Example clause contents.

Normative Optional, Legacy

2.3 Example Legacy Normative Optional Clause Heading

Example clause contents.

3 Normative References

The following referenced documents are indispensable for the application of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.

IEEE 754-2019, IEEE Standard for Floating-Point Arithmetic.

The Unicode Standard.
https://unicode.org/versions/latest

ISO/IEC 10646, Information Technology — Universal Multiple-Octet Coded Character Set (UCS) plus Amendment 1:2005, Amendment 2:2006, Amendment 3:2008, Amendment 4:2008, and additional amendments and corrigenda, or successor.

ECMA-402, ECMAScript Internationalization API Specification, specifically the annual edition corresponding to this edition of this specification.
https://www.ecma-international.org/publications-and-standards/standards/ecma-402/

ECMA-404, The JSON Data Interchange Format.
https://www.ecma-international.org/publications-and-standards/standards/ecma-404/

4 Overview

This section contains a non-normative overview of the ECMAScript language.

ECMAScript is an object-oriented programming language for performing computations and manipulating computational objects within a host environment. ECMAScript as defined here is not intended to be computationally self-sufficient; indeed, there are no provisions in this specification for input of external data or output of computed results. Instead, it is expected that the computational environment of an ECMAScript program will provide not only the objects and other facilities described in this specification but also certain environment-specific objects, whose description and behaviour are beyond the scope of this specification except to indicate that they may provide certain properties that can be accessed and certain functions that can be called from an ECMAScript program.

ECMAScript was originally designed to be used as a scripting language, but has become widely used as a general-purpose programming language. A scripting language is a programming language that is used to manipulate, customize, and automate the facilities of an existing system. In such systems, useful functionality is already available through a user interface, and the scripting language is a mechanism for exposing that functionality to program control. In this way, the existing system is said to provide a host environment of objects and facilities, which completes the capabilities of the scripting language. A scripting language is intended for use by both professional and non-professional programmers.

ECMAScript was originally designed to be a Web scripting language, providing a mechanism to enliven Web pages in browsers and to perform server computation as part of a Web-based client-server architecture. ECMAScript is now used to provide core scripting capabilities for a variety of host environments. Therefore the core language is specified in this document apart from any particular host environment.

ECMAScript usage has moved beyond simple scripting and it is now used for the full spectrum of programming tasks in many different environments and scales. As the usage of ECMAScript has expanded, so have the features and facilities it provides. ECMAScript is now a fully featured general-purpose programming language.

4.1 Web Scripting

A web browser provides an ECMAScript host environment for client-side computation including, for instance, objects that represent windows, menus, pop-ups, dialog boxes, text areas, anchors, frames, history, cookies, and input/output. Further, the host environment provides a means to attach scripting code to events such as change of focus, page and image loading, unloading, error and abort, selection, form submission, and mouse actions. Scripting code appears within the HTML and the displayed page is a combination of user interface elements and fixed and computed text and images. The scripting code is reactive to user interaction, and there is no need for a main program.

A web server provides a different host environment for server-side computation including objects representing requests, clients, and files; and mechanisms to lock and share data. By using browser-side and server-side scripting together, it is possible to distribute computation between the client and server while providing a customized user interface for a Web-based application.

Each Web browser and server that supports ECMAScript supplies its own host environment, completing the ECMAScript execution environment.

4.2 Hosts and Implementations

To aid integrating ECMAScript into host environments, this specification defers the definition of certain facilities (e.g., abstract operations), either in whole or in part, to a source outside of this specification. Editorially, this specification distinguishes the following kinds of deferrals.

An implementation is an external source that further defines facilities enumerated in Annex D or those that are marked as implementation-defined or implementation-approximated. In informal use, an implementation refers to a concrete artefact, such as a particular web browser.

An implementation-defined facility is one that defers its definition to an external source without further qualification. This specification does not make any recommendations for particular behaviours, and conforming implementations are free to choose any behaviour within the constraints put forth by this specification.

An implementation-approximated facility is one that defers its definition to an external source while recommending an ideal behaviour. While conforming implementations are free to choose any behaviour within the constraints put forth by this specification, they are encouraged to strive to approximate the ideal. Some mathematical operations, such as Math.exp, are implementation-approximated.

A host is an external source that further defines facilities listed in Annex D but does not further define other implementation-defined or implementation-approximated facilities. In informal use, a host refers to the set of all implementations, such as the set of all web browsers, that interface with this specification in the same way via Annex D. A host is often an external specification, such as WHATWG HTML (https://html.spec.whatwg.org/). In other words, facilities that are host-defined are often further defined in external specifications.

A host hook is an abstract operation that is defined in whole or in part by an external source. All host hooks must be listed in Annex D. A host hook must conform to at least the following requirements:

It must return either a normal completion or a throw completion.

A host-defined facility is one that defers its definition to an external source without further qualification and is listed in Annex D. Implementations that are not hosts may also provide definitions for host-defined facilities.

A host environment is a particular choice of definition for all host-defined facilities. A host environment typically includes objects or functions which allow obtaining input and providing output as host-defined properties of the global object.

This specification follows the editorial convention of always using the most specific term. For example, if a facility is host-defined, it should not be referred to as implementation-defined.

Both hosts and implementations may interface with this specification via the language types, specification types, abstract operations, grammar productions, intrinsic objects, and intrinsic symbols defined herein.

4.3 ECMAScript Overview

The following is an informal overview of ECMAScript—not all parts of the language are described. This overview is not part of the standard proper.

ECMAScript is object-based: basic language and host facilities are provided by objects, and an ECMAScript program is a cluster of communicating objects. In ECMAScript, an object is a collection of zero or more properties each with attributes that determine how each property can be used—for example, when the Writable attribute for a property is set to false, any attempt by executed ECMAScript code to assign a different value to the property fails. Properties are containers that hold other objects, primitive values, or functions. A primitive value is a member of one of the following built-in types: Undefined, Null, Boolean, Number, BigInt, String, and Symbol; an object is a member of the built-in type Object; and a function is a callable object. A function that is associated with an object via a property is called a method.

ECMAScript defines a collection of built-in objects that round out the definition of ECMAScript entities. These built-in objects include the global object; objects that are fundamental to the runtime semantics of the language including Object, Function, Boolean, Symbol, and various Error objects; objects that represent and manipulate numeric values including Math, Number, and Date; the text processing objects String and RegExp; objects that are indexed collections of values including Array and nine different kinds of Typed Arrays whose elements all have a specific numeric data representation; keyed collections including Map and Set objects; objects supporting structured data including the JSON object, ArrayBuffer, SharedArrayBuffer, and DataView; objects supporting control abstractions including generator functions and Promise objects; and reflection objects including Proxy and Reflect.

ECMAScript also defines a set of built-in operators. ECMAScript operators include various unary operations, multiplicative operators, additive operators, bitwise shift operators, relational operators, equality operators, binary bitwise operators, binary logical operators, assignment operators, and the comma operator.

Large ECMAScript programs are supported by modules which allow a program to be divided into multiple sequences of statements and declarations. Each module explicitly identifies declarations it uses that need to be provided by other modules and which of its declarations are available for use by other modules.

ECMAScript syntax intentionally resembles Java syntax. ECMAScript syntax is relaxed to enable it to serve as an easy-to-use scripting language. For example, a variable is not required to have its type declared nor are types associated with properties, and defined functions are not required to have their declarations appear textually before calls to them.

4.3.1 Objects

Even though ECMAScript includes syntax for class definitions, ECMAScript objects are not fundamentally class-based such as those in C++, Smalltalk, or Java. Instead objects may be created in various ways including via a literal notation or via constructors which create objects and then execute code that initializes all or part of them by assigning initial values to their properties. Each constructor is a function that has a property named "prototype" that is used to implement prototype-based inheritance and shared properties. Objects are created by using constructors in new expressions; for example, new Date(2009, 11) creates a new Date object. Invoking a constructor without using new has consequences that depend on the constructor. For example, Date() produces a string representation of the current date and time rather than an object.

Every object created by a constructor has an implicit reference (called the object's prototype) to the value of its constructor's "prototype" property. Furthermore, a prototype may have a non-null implicit reference to its prototype, and so on; this is called the prototype chain. When a reference is made to a property in an object, that reference is to the property of that name in the first object in the prototype chain that contains a property of that name. In other words, first the object mentioned directly is examined for such a property; if that object contains the named property, that is the property to which the reference refers; if that object does not contain the named property, the prototype for that object is examined next; and so on.

An image of lots of boxes and arrows. — Figure 1: Object/Prototype Relationships

In a class-based object-oriented language, in general, state is carried by instances, methods are carried by classes, and inheritance is only of structure and behaviour. In ECMAScript, the state and methods are carried by objects, while structure, behaviour, and state are all inherited.

All objects that do not directly contain a particular property that their prototype contains share that property and its value. Figure 1 illustrates this:

CF is a constructor (and also an object). Five objects have been created by using new expressions: cf₁, cf₂, cf₃, cf₄, and cf₅. Each of these objects contains properties named "q1" and "q2". The dashed lines represent the implicit prototype relationship; so, for example, cf₃'s prototype is CF_p. The constructor, CF, has two properties itself, named "P1" and "P2", which are not visible to CF_p, cf₁, cf₂, cf₃, cf₄, or cf₅. The property named "CFP1" in CF_p is shared by cf₁, cf₂, cf₃, cf₄, and cf₅ (but not by CF), as are any properties found in CF_p's implicit prototype chain that are not named "q1", "q2", or "CFP1". Notice that there is no implicit prototype link between CF and CF_p.

Unlike most class-based object languages, properties can be added to objects dynamically by assigning values to them. That is, constructors are not required to name or assign values to all or any of the constructed object's properties. In the above diagram, one could add a new shared property for cf₁, cf₂, cf₃, cf₄, and cf₅ by assigning a new value to the property in CF_p.

Although ECMAScript objects are not inherently class-based, it is often convenient to define class-like abstractions based upon a common pattern of constructor functions, prototype objects, and methods. The ECMAScript built-in objects themselves follow such a class-like pattern. Beginning with ECMAScript 2015, the ECMAScript language includes syntactic class definitions that permit programmers to concisely define objects that conform to the same class-like abstraction pattern used by the built-in objects.

4.3.2 The Strict Variant of ECMAScript

The ECMAScript Language recognizes the possibility that some users of the language may wish to restrict their usage of some features available in the language. They might do so in the interests of security, to avoid what they consider to be error-prone features, to get enhanced error checking, or for other reasons of their choosing. In support of this possibility, ECMAScript defines a strict variant of the language. The strict variant of the language excludes some specific syntactic and semantic features of the regular ECMAScript language and modifies the detailed semantics of some features. The strict variant also specifies additional error conditions that must be reported by throwing error exceptions in situations that are not specified as errors by the non-strict form of the language.

The strict variant of ECMAScript is commonly referred to as the strict mode of the language. Strict mode selection and use of the strict mode syntax and semantics of ECMAScript is explicitly made at the level of individual ECMAScript source text units as described in 11.2.2. Because strict mode is selected at the level of a syntactic source text unit, strict mode only imposes restrictions that have local effect within such a source text unit. Strict mode does not restrict or modify any aspect of the ECMAScript semantics that must operate consistently across multiple source text units. A complete ECMAScript program may be composed of both strict mode and non-strict mode ECMAScript source text units. In this case, strict mode only applies when actually executing code that is defined within a strict mode source text unit.

In order to conform to this specification, an ECMAScript implementation must implement both the full unrestricted ECMAScript language and the strict variant of the ECMAScript language as defined by this specification. In addition, an implementation must support the combination of unrestricted and strict mode source text units into a single composite program.

4.4 Terms and Definitions

For the purposes of this document, the following terms and definitions apply.

4.4.1 implementation-approximated

an implementation-approximated facility is defined in whole or in part by an external source but has a recommended, ideal behaviour in this specification

4.4.2 implementation-defined

an implementation-defined facility is defined in whole or in part by an external source to this specification

4.4.3 host-defined

same as implementation-defined

Note

Editorially, see clause 4.2.

4.4.4 type

set of data values as defined in clause 6

4.4.5 primitive value

member of one of the types Undefined, Null, Boolean, Number, BigInt, Symbol, or String as defined in clause 6

Note

A primitive value is a datum that is represented directly at the lowest level of the language implementation.

4.4.6 object

member of the type Object

Note

An object is a collection of properties and has a single prototype object. The prototype may be null.

4.4.7 constructor

function object that creates and initializes objects

Note

The value of a constructor's "prototype" property is a prototype object that is used to implement inheritance and shared properties.

4.4.8 prototype

object that provides shared properties for other objects

Note

When a constructor creates an object, that object implicitly references the constructor's "prototype" property for the purpose of resolving property references. The constructor's "prototype" property can be referenced by the program expression constructor.prototype, and properties added to an object's prototype are shared, through inheritance, by all objects sharing the prototype. Alternatively, a new object may be created with an explicitly specified prototype by using the Object.create built-in function.

4.4.9 ordinary object

object that has the default behaviour for the essential internal methods that must be supported by all objects

4.4.10 exotic object

object that does not have the default behaviour for one or more of the essential internal methods

Note

Any object that is not an ordinary object is an exotic object.

4.4.11 standard object

object whose semantics are defined by this specification

4.4.12 built-in object

object specified and supplied by an ECMAScript implementation

Note

Standard built-in objects are defined in this specification. An ECMAScript implementation may specify and supply additional kinds of built-in objects.

4.4.13 undefined value

primitive value used when a variable has not been assigned a value

4.4.14 Undefined type

type whose sole value is the undefined value

4.4.15 null value

primitive value that represents the intentional absence of any object value

4.4.16 Null type

type whose sole value is the null value

4.4.17 Boolean value

member of the Boolean type

Note

There are only two Boolean values, true and false.

4.4.18 Boolean type

type consisting of the primitive values true and false

4.4.19 Boolean object

member of the Object type that is an instance of the standard built-in Boolean constructor

Note

A Boolean object is created by using the Boolean constructor in a new expression, supplying a Boolean value as an argument. The resulting object has an internal slot whose value is the Boolean value. A Boolean object can be coerced to a Boolean value.

4.4.20 String value

primitive value that is a finite ordered sequence of zero or more 16-bit unsigned integer values

Note

A String value is a member of the String type. Each integer value in the sequence usually represents a single 16-bit unit of UTF-16 text. However, ECMAScript does not place any restrictions or requirements on the values except that they must be 16-bit unsigned integers.

4.4.21 String type

set of all possible String values

4.4.22 String object

member of the Object type that is an instance of the standard built-in String constructor

Note

A String object is created by using the String constructor in a new expression, supplying a String value as an argument. The resulting object has an internal slot whose value is the String value. A String object can be coerced to a String value by calling the String constructor as a function (22.1.1.1).

4.4.23 Number value

primitive value corresponding to a double-precision 64-bit binary format IEEE 754-2019 value

Note

A Number value is a member of the Number type and is a direct representation of a number.

4.4.24 Number type

set of all possible Number values including the special “Not-a-Number” (NaN) value, positive infinity, and negative infinity

4.4.25 Number object

member of the Object type that is an instance of the standard built-in Number constructor

Note

A Number object is created by using the Number constructor in a new expression, supplying a Number value as an argument. The resulting object has an internal slot whose value is the Number value. A Number object can be coerced to a Number value by calling the Number constructor as a function (21.1.1.1).

4.4.26 Infinity

Number value that is the positive infinite Number value

4.4.27 NaN

Number value that is an IEEE 754-2019 “Not-a-Number” value

4.4.28 BigInt value

primitive value corresponding to an arbitrary-precision integer value

4.4.29 BigInt type

set of all possible BigInt values

4.4.30 BigInt object

member of the Object type that is an instance of the standard built-in BigInt constructor

4.4.31 Symbol value

primitive value that represents a unique, non-String Object property key

4.4.32 Symbol type

set of all possible Symbol values

4.4.33 Symbol object

member of the Object type that is an instance of the standard built-in Symbol constructor

4.4.34 function

member of the Object type that may be invoked as a subroutine

Note

In addition to its properties, a function contains executable code and state that determine how it behaves when invoked. A function's code may or may not be written in ECMAScript.

4.4.35 built-in function

built-in object that is a function

Note

Examples of built-in functions include parseInt and Math.exp. A host or implementation may provide additional built-in functions that are not described in this specification.

4.4.36 built-in constructor

built-in function that is a constructor

Note

Examples of built-in constructors include Object and Function. A host or implementation may provide additional built-in constructors that are not described in this specification.

4.4.37 property

part of an object that associates a key (either a String value or a Symbol value) and a value

Note

Depending upon the form of the property the value may be represented either directly as a data value (a primitive value, an object, or a function object) or indirectly by a pair of accessor functions.

4.4.38 method

function that is the value of a property

Note

When a function is called as a method of an object, the object is passed to the function as its this value.

4.4.39 built-in method

method that is a built-in function

Note

Standard built-in methods are defined in this specification. A host or implementation may provide additional built-in methods that are not described in this specification.

4.4.40 attribute

internal value that defines some characteristic of a property

4.4.41 own property

property that is directly contained by its object

4.4.42 inherited property

property of an object that is not an own property but is a property (either own or inherited) of the object's prototype

4.5 Organization of This Specification

The remainder of this specification is organized as follows:

Clause 5 defines the notational conventions used throughout the specification.

Clauses 6 through 10 define the execution environment within which ECMAScript programs operate.

Clauses 11 through 17 define the actual ECMAScript programming language including its syntactic encoding and the execution semantics of all language features.

Clauses 18 through 28 define the ECMAScript standard library. They include the definitions of all of the standard objects that are available for use by ECMAScript programs as they execute.

Clause 29 describes the memory consistency model of accesses on SharedArrayBuffer-backed memory and methods of the Atomics object.

5 Notational Conventions

5.1 Syntactic and Lexical Grammars

5.1.1 Context-Free Grammars

A context-free grammar consists of a number of productions. Each production has an abstract symbol called a nonterminal as its left-hand side, and a sequence of zero or more nonterminal and terminal symbols as its right-hand side. For each grammar, the terminal symbols are drawn from a specified alphabet.

A chain production is a production that has exactly one nonterminal symbol on its right-hand side along with zero or more terminal symbols.

Starting from a sentence consisting of a single distinguished nonterminal, called the goal symbol, a given context-free grammar specifies a language, namely, the (perhaps infinite) set of possible sequences of terminal symbols that can result from repeatedly replacing any nonterminal in the sequence with a right-hand side of a production for which the nonterminal is the left-hand side.

5.1.2 The Lexical and RegExp Grammars

A lexical grammar for ECMAScript is given in clause 12. This grammar has as its terminal symbols Unicode code points that conform to the rules for SourceCharacter defined in 11.1. It defines a set of productions, starting from the goal symbol InputElementDiv, InputElementTemplateTail, InputElementRegExp, InputElementRegExpOrTemplateTail, or InputElementHashbangOrRegExp, that describe how sequences of such code points are translated into a sequence of input elements.

Input elements other than white space and comments form the terminal symbols for the syntactic grammar for ECMAScript and are called ECMAScript tokens. These tokens are the reserved words, identifiers, literals, and punctuators of the ECMAScript language. Moreover, line terminators, although not considered to be tokens, also become part of the stream of input elements and guide the process of automatic semicolon insertion (12.10). Simple white space and single-line comments are discarded and do not appear in the stream of input elements for the syntactic grammar. A MultiLineComment (that is, a comment of the form /*…*/ regardless of whether it spans more than one line) is likewise simply discarded if it contains no line terminator; but if a MultiLineComment contains one or more line terminators, then it is replaced by a single line terminator, which becomes part of the stream of input elements for the syntactic grammar.

A RegExp grammar for ECMAScript is given in 22.2.1. This grammar also has as its terminal symbols the code points as defined by SourceCharacter. It defines a set of productions, starting from the goal symbol Pattern, that describe how sequences of code points are translated into regular expression patterns.

Productions of the lexical and RegExp grammars are distinguished by having two colons “::” as separating punctuation. The lexical and RegExp grammars share some productions.

5.1.3 The Numeric String Grammar

A numeric string grammar appears in 7.1.4.1. It has as its terminal symbols SourceCharacter, and is used for translating Strings into numeric values starting from the goal symbol StringNumericLiteral (which is similar to but distinct from the lexical grammar for numeric literals).

Productions of the numeric string grammar are distinguished by having three colons “:::” as punctuation, and are never used for parsing source text.

5.1.4 The Syntactic Grammar

The syntactic grammar for ECMAScript is given in clauses 13 through 16. This grammar has ECMAScript tokens defined by the lexical grammar as its terminal symbols (5.1.2). It defines a set of productions, starting from two alternative goal symbols Script and Module, that describe how sequences of tokens form syntactically correct independent components of ECMAScript programs.

When a stream of code points is to be parsed as an ECMAScript Script or Module, it is first converted to a stream of input elements by repeated application of the lexical grammar; this stream of input elements is then parsed by a single application of the syntactic grammar. The input stream is syntactically in error if the tokens in the stream of input elements cannot be parsed as a single instance of the goal nonterminal (Script or Module), with no tokens left over.

When a parse is successful, it constructs a parse tree, a rooted tree structure in which each node is a Parse Node. Each Parse Node is an instance of a symbol in the grammar; it represents a span of the source text that can be derived from that symbol. The root node of the parse tree, representing the whole of the source text, is an instance of the parse's goal symbol. When a Parse Node is an instance of a nonterminal, it is also an instance of some production that has that nonterminal as its left-hand side. Moreover, it has zero or more children, one for each symbol on the production's right-hand side: each child is a Parse Node that is an instance of the corresponding symbol.

New Parse Nodes are instantiated for each invocation of the parser and never reused between parses even of identical source text. Parse Nodes are considered the same Parse Node if and only if they represent the same span of source text, are instances of the same grammar symbol, and resulted from the same parser invocation.

Note 1

Parsing the same String multiple times will lead to different Parse Nodes. For example, consider:

let str = "1 + 1;";
eval(str);
eval(str);

Each call to eval converts the value of str into ECMAScript source text and performs an independent parse that creates its own separate tree of Parse Nodes. The trees are distinct even though each parse operates upon a source text that was derived from the same String value.

Note 2

Parse Nodes are specification artefacts, and implementations are not required to use an analogous data structure.

Productions of the syntactic grammar are distinguished by having just one colon “:” as punctuation.

The syntactic grammar as presented in clauses 13 through 16 is not a complete account of which token sequences are accepted as a correct ECMAScript Script or Module. Certain additional token sequences are also accepted, namely, those that would be described by the grammar if only semicolons were added to the sequence in certain places (such as before line terminator characters). Furthermore, certain token sequences that are described by the grammar are not considered acceptable if a line terminator character appears in certain “awkward” places.

In certain cases, in order to avoid ambiguities, the syntactic grammar uses generalized productions that permit token sequences that do not form a valid ECMAScript Script or Module. For example, this technique is used for object literals and object destructuring patterns. In such cases a more restrictive supplemental grammar is provided that further restricts the acceptable token sequences. Typically, an early error rule will then state that, in certain contexts, "P must cover an N", where P is a Parse Node (an instance of the generalized production) and N is a nonterminal from the supplemental grammar. This means:

The sequence of tokens originally matched by P is parsed again using N as the goal symbol. If N takes grammatical parameters, then they are set to the same values used when P was originally parsed.
If the sequence of tokens can be parsed as a single instance of N, with no tokens left over, then:
1. We refer to that instance of N (a Parse Node, unique for a given P) as "the N that is covered by P".
2. All Early Error rules for N and its derived productions also apply to the N that is covered by P.
Otherwise (if the parse fails), it is an early Syntax Error.

5.1.5 Grammar Notation

5.1.5.1 Terminal Symbols

In the ECMAScript grammars, some terminal symbols are shown in fixed-width font. These are to appear in a source text exactly as written. All terminal symbol code points specified in this way are to be understood as the appropriate Unicode code points from the Basic Latin block, as opposed to any similar-looking code points from other Unicode ranges. A code point in a terminal symbol cannot be expressed by a \ UnicodeEscapeSequence.

In grammars whose terminal symbols are individual Unicode code points (i.e., the lexical, RegExp, and numeric string grammars), a contiguous run of multiple fixed-width code points appearing in a production is a simple shorthand for the same sequence of code points, written as standalone terminal symbols.

For example, the production:

is a shorthand for:

In contrast, in the syntactic grammar, a contiguous run of fixed-width code points is a single terminal symbol.

Terminal symbols come in two other forms:

In the lexical and RegExp grammars, Unicode code points without a conventional printed representation are instead shown in the form "<ABBREV>" where "ABBREV" is a mnemonic for the code point or set of code points. These forms are defined in Unicode Format-Control Characters, White Space, and Line Terminators.
In the syntactic grammar, certain terminal symbols (e.g. IdentifierName and RegularExpressionLiteral) are shown in italics, as they refer to the nonterminals of the same name in the lexical grammar.

5.1.5.2 Nonterminal Symbols and Productions

Nonterminal symbols are shown in italic type. The definition of a nonterminal (also called a “production”) is introduced by the name of the nonterminal being defined followed by one or more colons. (The number of colons indicates to which grammar the production belongs.) One or more alternative right-hand sides for the nonterminal then follow on succeeding lines. For example, the syntactic definition:

WhileStatement

while

(

Expression

)

Statement

states that the nonterminal WhileStatement represents the token while, followed by a left parenthesis token, followed by an Expression, followed by a right parenthesis token, followed by a Statement. The occurrences of Expression and Statement are themselves nonterminals. As another example, the syntactic definition:

states that an ArgumentList may represent either a single AssignmentExpression or an ArgumentList, followed by a comma, followed by an AssignmentExpression. This definition of ArgumentList is recursive, that is, it is defined in terms of itself. The result is that an ArgumentList may contain any positive number of arguments, separated by commas, where each argument expression is an AssignmentExpression. Such recursive definitions of nonterminals are common.

5.1.5.3 Optional Symbols

The subscripted suffix “_opt”, which may appear after a terminal or nonterminal, indicates an optional symbol. The alternative containing the optional symbol actually specifies two right-hand sides, one that omits the optional element and one that includes it. This means that:

VariableDeclaration

BindingIdentifier

Initializer

opt

is a convenient abbreviation for:

and that:

for

(

LexicalDeclaration

Expression

opt

;

Expression

opt

)

Statement

is a convenient abbreviation for:

ForStatement

for

(

LexicalDeclaration

;

Expression

opt

)

Statement

for

(

LexicalDeclaration

Expression

;

Expression

opt

)

Statement

which in turn is an abbreviation for:

ForStatement

for

(

LexicalDeclaration

;

)

Statement

for

(

LexicalDeclaration

;

Expression

)

Statement

for

(

LexicalDeclaration

Expression

;

)

Statement

for

(

;

)

so, in this example, the nonterminal ForStatement actually has four alternative right-hand sides.

5.1.5.4 Grammatical Parameters

A production may be parameterized by a subscripted annotation of the form “_[parameters]”, which may appear as a suffix to the nonterminal symbol defined by the production. “_parameters” may be either a single name or a comma separated list of names. A parameterized production is shorthand for a set of productions defining all combinations of the parameter names, preceded by an underscore, appended to the parameterized nonterminal symbol. This means that:

StatementList

[Return]

ReturnStatement

ExpressionStatement

is a convenient abbreviation for:

and that:

[Return, In]

is an abbreviation for:

StatementList_Return_In

ReturnStatement

ExpressionStatement

Multiple parameters produce a combinatory number of productions, not all of which are necessarily referenced in a complete grammar.

References to nonterminals on the right-hand side of a production can also be parameterized. For example:

StatementList

ReturnStatement

ExpressionStatement

[+In]

is equivalent to saying:

StatementList

ReturnStatement

ExpressionStatement_In

and:

StatementList

ReturnStatement

ExpressionStatement

[~In]

is equivalent to:

StatementList

ReturnStatement

ExpressionStatement

A nonterminal reference may have both a parameter list and an “_opt” suffix. For example:

VariableDeclaration

BindingIdentifier

Initializer

[+In]

opt

is an abbreviation for:

VariableDeclaration

BindingIdentifier

Initializer_In

Prefixing a parameter name with “_?” on a right-hand side nonterminal reference makes that parameter value dependent upon the occurrence of the parameter name on the reference to the current production's left-hand side symbol. For example:

VariableDeclaration

[In]

BindingIdentifier

Initializer

[?In]

is an abbreviation for:

VariableDeclaration

BindingIdentifier

Initializer

VariableDeclaration_In

BindingIdentifier

Initializer_In

If a right-hand side alternative is prefixed with “[+parameter]” that alternative is only available if the named parameter was used in referencing the production's nonterminal symbol. If a right-hand side alternative is prefixed with “[~parameter]” that alternative is only available if the named parameter was not used in referencing the production's nonterminal symbol. This means that:

StatementList

[Return]

[+Return]

ReturnStatement

ExpressionStatement

is an abbreviation for:

and that:

[Return]

[~Return]

ReturnStatement

ExpressionStatement

is an abbreviation for:

5.1.5.5 one of

When the words “one of” follow the colon(s) in a grammar definition, they signify that each of the terminal symbols on the following line or lines is an alternative definition. For example, the lexical grammar for ECMAScript contains the production:

NonZeroDigit

one of

which is merely a convenient abbreviation for:

NonZeroDigit

5.1.5.6 [empty]

If the phrase “[empty]” appears as the right-hand side of a production, it indicates that the production's right-hand side contains no terminals or nonterminals.

5.1.5.7 Lookahead Restrictions

If the phrase “[lookahead = seq]” appears in the right-hand side of a production, it indicates that the production may only be used if the token sequence seq is a prefix of the immediately following input token sequence. Similarly, “[lookahead ∈ set]”, where set is a finite non-empty set of token sequences, indicates that the production may only be used if some element of set is a prefix of the immediately following token sequence. For convenience, the set can also be written as a nonterminal, in which case it represents the set of all token sequences to which that nonterminal could expand. It is considered an editorial error if the nonterminal could expand to infinitely many distinct token sequences.

These conditions may be negated. “[lookahead ≠ seq]” indicates that the containing production may only be used if seq is not a prefix of the immediately following input token sequence, and “[lookahead ∉ set]” indicates that the production may only be used if no element of set is a prefix of the immediately following token sequence.

As an example, given the definitions:

one of

the definition:

[lookahead ∉ { 1, 3, 5, 7, 9 }]

DecimalDigits

DecimalDigit

[lookahead ∉ DecimalDigit]

matches either the letter n followed by one or more decimal digits the first of which is even, or a decimal digit not followed by another decimal digit.

Note that when these phrases are used in the syntactic grammar, it may not be possible to unambiguously identify the immediately following token sequence because determining later tokens requires knowing which lexical goal symbol to use at later positions. As such, when these are used in the syntactic grammar, it is considered an editorial error for a token sequence seq to appear in a lookahead restriction (including as part of a set of sequences) if the choices of lexical goal symbols to use could change whether or not seq would be a prefix of the resulting token sequence.

5.1.5.8 [no LineTerminator here]

If the phrase “[no LineTerminator here]” appears in the right-hand side of a production of the syntactic grammar, it indicates that the production is a restricted production: it may not be used if a LineTerminator occurs in the input stream at the indicated position. For example, the production:

ThrowStatement

throw

[no LineTerminator here]

Expression

;

indicates that the production may not be used if a LineTerminator occurs in the script between the throw token and the Expression.

Unless the presence of a LineTerminator is forbidden by a restricted production, any number of occurrences of LineTerminator may appear between any two consecutive tokens in the stream of input elements without affecting the syntactic acceptability of the script.

5.1.5.9 but not

The right-hand side of a production may specify that certain expansions are not permitted by using the phrase “but not” and then indicating the expansions to be excluded. For example, the production:

Identifier

IdentifierName

but not ReservedWord

means that the nonterminal Identifier may be replaced by any sequence of code points that could replace IdentifierName provided that the same sequence of code points could not replace ReservedWord.

5.1.5.10 Descriptive Phrases

Finally, a few nonterminal symbols are described by a descriptive phrase in sans-serif type in cases where it would be impractical to list all the alternatives:

SourceCharacter

any Unicode code point

5.2 Algorithm Conventions

The specification often uses a numbered list to specify steps in an algorithm. These algorithms are used to precisely specify the required semantics of ECMAScript language constructs. The algorithms are not intended to imply the use of any specific implementation technique. In practice, there may be more efficient algorithms available to implement a given feature.

Algorithms may be explicitly parameterized with an ordered, comma-separated sequence of alias names which may be used within the algorithm steps to reference the argument passed in that position. Optional parameters are denoted with surrounding brackets ([ , name ]) and are no different from required parameters within algorithm steps. A rest parameter may appear at the end of a parameter list, denoted with leading ellipsis (, ...name). The rest parameter captures all of the arguments provided following the required and optional parameters into a List. If there are no such additional arguments, that List is empty.

Algorithm steps may be subdivided into sequential substeps. Substeps are indented and may themselves be further divided into indented substeps. Outline numbering conventions are used to identify substeps with the first level of substeps labelled with lowercase alphabetic characters and the second level of substeps labelled with lowercase roman numerals. If more than three levels are required these rules repeat with the fourth level using numeric labels. For example:

1. Top-level step
1. Substep.
2. b. Substep.
  1. i. Subsubstep.
    1. 1. Subsubsubstep
      1. Subsubsubsubstep
        Subsubsubsubsubstep

A step or substep may be written as an “if” predicate that conditions its substeps. In this case, the substeps are only applied if the predicate is true. If a step or substep begins with the word “else”, it is a predicate that is the negation of the preceding “if” predicate step at the same level.

A step may specify the iterative application of its substeps.

A step that begins with “Assert:” asserts an invariant condition of its algorithm. Such assertions are used to make explicit algorithmic invariants that would otherwise be implicit. Such assertions add no additional semantic requirements and hence need not be checked by an implementation. They are used simply to clarify algorithms.

Algorithm steps may declare named aliases for any value using the form “Let x be someValue”. These aliases are reference-like in that both x and someValue refer to the same underlying data and modifications to either are visible to both. Algorithm steps that want to avoid this reference-like behaviour should explicitly make a copy of the right-hand side: “Let x be a copy of someValue” creates a shallow copy of someValue.

Once declared, an alias may be referenced in any subsequent steps and must not be referenced from steps prior to the alias's declaration. Aliases may be modified using the form “Set x to someOtherValue”.

5.2.1 Abstract Operations

In order to facilitate their use in multiple parts of this specification, some algorithms, called abstract operations, are named and written in parameterized functional form so that they may be referenced by name from within other algorithms. Abstract operations are typically referenced using a functional application style such as OperationName(arg1, arg2). Some abstract operations are treated as polymorphically dispatched methods of class-like specification abstractions. Such method-like abstract operations are typically referenced using a method application style such as someValue.OperationName(arg1, arg2).

5.2.2 Syntax-Directed Operations

A syntax-directed operation is a named operation whose definition consists of algorithms, each of which is associated with one or more productions from one of the ECMAScript grammars. A production that has multiple alternative definitions will typically have a distinct algorithm for each alternative. When an algorithm is associated with a grammar production, it may reference the terminal and nonterminal symbols of the production alternative as if they were parameters of the algorithm. When used in this manner, nonterminal symbols refer to the actual alternative definition that is matched when parsing the source text. The source text matched by a grammar production or Parse Node derived from it is the portion of the source text that starts at the beginning of the first terminal that participated in the match and ends at the end of the last terminal that participated in the match.

When an algorithm is associated with a production alternative, the alternative is typically shown without any “[ ]” grammar annotations. Such annotations should only affect the syntactic recognition of the alternative and have no effect on the associated semantics for the alternative.

Syntax-directed operations are invoked with a parse node and, optionally, other parameters by using the conventions on steps 1, 3, and 4 in the following algorithm:

Let status be SyntaxDirectedOperation of SomeNonTerminal.
Let someParseNode be the parse of some source text.
Perform SyntaxDirectedOperation of someParseNode.
Perform SyntaxDirectedOperation of someParseNode with argument "value".

Unless explicitly specified otherwise, all chain productions have an implicit definition for every operation that might be applied to that production's left-hand side nonterminal. The implicit definition simply reapplies the same operation with the same parameters, if any, to the chain production's sole right-hand side nonterminal and then returns the result. For example, assume that some algorithm has a step of the form: “Return Evaluation of Block” and that there is a production:

Block

{

StatementList

}

but the Evaluation operation does not associate an algorithm with that production. In that case, the Evaluation operation implicitly includes an association of the form:

Runtime Semantics: Evaluation

Block

{

StatementList

}

Return Evaluation of StatementList.

5.2.3 Runtime Semantics

Algorithms which specify semantics that must be called at runtime are called runtime semantics. Runtime semantics are defined by abstract operations or syntax-directed operations.

5.2.3.1 Completion ( `completionRecord` )

The abstract operation Completion takes argument completionRecord (a Completion Record) and returns a Completion Record. It is used to emphasize that a Completion Record is being returned. It performs the following steps when called:

Assert: completionRecord is a Completion Record.
Return completionRecord.

5.2.3.2 Throw an Exception

Algorithms steps that say to throw an exception, such as

Throw a TypeError exception.

mean the same things as:

Return ThrowCompletion(a newly created TypeError object).

5.2.3.3 ReturnIfAbrupt

Algorithms steps that say or are otherwise equivalent to:

ReturnIfAbrupt(argument).

mean the same thing as:

Assert: argument is a Completion Record.
If argument is an abrupt completion, return Completion(argument).
Else, set argument to argument.[[Value]].

Algorithms steps that say or are otherwise equivalent to:

ReturnIfAbrupt(AbstractOperation()).

mean the same thing as:

Let hygienicTemp be AbstractOperation().
Assert: hygienicTemp is a Completion Record.
If hygienicTemp is an abrupt completion, return Completion(hygienicTemp).
Else, set hygienicTemp to hygienicTemp.[[Value]].

Where hygienicTemp is ephemeral and visible only in the steps pertaining to ReturnIfAbrupt.

Algorithms steps that say or are otherwise equivalent to:

Let result be AbstractOperation(ReturnIfAbrupt(argument)).

mean the same thing as:

Assert: argument is a Completion Record.
If argument is an abrupt completion, return Completion(argument).
Else, set argument to argument.[[Value]].
Let result be AbstractOperation(argument).

5.2.3.4 ReturnIfAbrupt Shorthands

Invocations of abstract operations and syntax-directed operations that are prefixed by ? indicate that ReturnIfAbrupt should be applied to the resulting Completion Record. For example, the step:

? OperationName().

is equivalent to the following step:

ReturnIfAbrupt(OperationName()).

Similarly, for method application style, the step:

? someValue.OperationName().

is equivalent to:

ReturnIfAbrupt(someValue.OperationName()).

Similarly, prefix ! is used to indicate that the following invocation of an abstract or syntax-directed operation will never return an abrupt completion and that the resulting Completion Record's [[Value]] field should be used in place of the return value of the operation. For example, the step:

Let val be ! OperationName().

is equivalent to the following steps:

Let val be OperationName().
Assert: val is a normal completion.
Set val to val.[[Value]].

Syntax-directed operations for runtime semantics make use of this shorthand by placing ! or ? before the invocation of the operation:

Perform ! SyntaxDirectedOperation of NonTerminal.

5.2.3.5 Implicit Normal Completion

In algorithms within abstract operations which are declared to return a Completion Record, and within all built-in functions, the returned value is first passed to NormalCompletion, and the result is used instead. This rule does not apply within the Completion algorithm or when the value being returned is clearly marked as a Completion Record in that step; these cases are:

when the result of applying Completion, NormalCompletion, or ThrowCompletion is directly returned
when the result of constructing a Completion Record is directly returned

It is an editorial error if a Completion Record is returned from such an abstract operation through any other means. For example, within these abstract operations,

Return true.

means the same things as any of

Return NormalCompletion(true).

Let completion be NormalCompletion(true).
Return Completion(completion).

Return Completion Record { [[Type]]: normal, [[Value]]: true, [[Target]]: empty }.

Note that, through the ReturnIfAbrupt expansion, the following example is allowed, as within the expanded steps, the result of applying Completion is returned directly in the abrupt case and the implicit NormalCompletion application occurs after unwrapping in the normal case.

Return ? completion.

The following example would be an editorial error because a Completion Record is being returned without being annotated in that step.

Let completion be NormalCompletion(true).
Return completion.

5.2.4 Static Semantics

Context-free grammars are not sufficiently powerful to express all the rules that define whether a stream of input elements form a valid ECMAScript Script or Module that may be evaluated. In some situations additional rules are needed that may be expressed using either ECMAScript algorithm conventions or prose requirements. Such rules are always associated with a production of a grammar and are called the static semantics of the production.

Static Semantic Rules have names and typically are defined using an algorithm. Named Static Semantic Rules are associated with grammar productions and a production that has multiple alternative definitions will typically have for each alternative a distinct algorithm for each applicable named static semantic rule.

A special kind of static semantic rule is an Early Error Rule. Early error rules define early error conditions (see clause 17) that are associated with specific grammar productions. Evaluation of most early error rules are not explicitly invoked within the algorithms of this specification. A conforming implementation must, prior to the first evaluation of a Script or Module, validate all of the early error rules of the productions used to parse that Script or Module. If any of the early error rules are violated the Script or Module is invalid and cannot be evaluated.

5.2.5 Mathematical Operations

This specification makes reference to these kinds of numeric values:

Mathematical values: Arbitrary real numbers, used as the default numeric type.
Extended mathematical values: Mathematical values together with +∞ and -∞.
Numbers: IEEE 754-2019 double-precision floating point values.
BigInts: ECMAScript language values representing arbitrary integers in a one-to-one correspondence.

In the language of this specification, numerical values are distinguished among different numeric kinds using subscript suffixes. The subscript _𝔽 refers to Numbers, and the subscript _ℤ refers to BigInts. Numeric values without a subscript suffix refer to mathematical values.

Numeric operators such as +, ×, =, and ≥ refer to those operations as determined by the type of the operands. When applied to mathematical values, the operators refer to the usual mathematical operations. When applied to extended mathematical values, the operators refer to the usual mathematical operations over the extended real numbers; indeterminate forms are not defined and their use in this specification should be considered an editorial error. When applied to Numbers, the operators refer to the relevant operations within IEEE 754-2019. When applied to BigInts, the operators refer to the usual mathematical operations applied to the mathematical value of the BigInt.

In general, when this specification refers to a numerical value, such as in the phrase, "the length of y" or "the integer represented by the four hexadecimal digits ...", without explicitly specifying a numeric kind, the phrase refers to a mathematical value. Phrases which refer to a Number or a BigInt value are explicitly annotated as such; for example, "the Number value for the number of code points in …" or "the BigInt value for …".

Numeric operators applied to mixed-type operands (such as a Number and a mathematical value) are not defined and should be considered an editorial error in this specification.

This specification denotes most numeric values in base 10; it also uses numeric values of the form 0x followed by digits 0-9 or A-F as base-16 values.

When the term integer is used in this specification, it refers to a mathematical value which is in the set of integers, unless otherwise stated. When the term integral Number is used in this specification, it refers to a Number value whose mathematical value is in the set of integers.

Conversions between mathematical values and Numbers or BigInts are always explicit in this document. A conversion from a mathematical value or extended mathematical value x to a Number is denoted as "the Number value for x" or 𝔽(x), and is defined in 6.1.6.1. A conversion from an integer x to a BigInt is denoted as "the BigInt value for x" or ℤ(x). A conversion from a Number or BigInt x to a mathematical value is denoted as "the mathematical value of x", or ℝ(x). The mathematical value of +0_𝔽 and -0_𝔽 is the mathematical value 0. The mathematical value of non-finite values is not defined. The extended mathematical value of x is the mathematical value of x for finite values, and is +∞ and -∞ for +∞_𝔽 and -∞_𝔽 respectively; it is not defined for NaN.

The mathematical function abs(x) produces the absolute value of x, which is -x if x < 0 and otherwise is x itself.

The mathematical function min(x1, x2, … , xN) produces the mathematically smallest of x1 through xN. The mathematical function max(x1, x2, ..., xN) produces the mathematically largest of x1 through xN. The domain and range of these mathematical functions are the extended mathematical values.

The notation “x modulo y” (y must be finite and non-zero) computes a value k of the same sign as y (or zero) such that abs(k) < abs(y) and x - k = q × y for some integer q.

The phrase "the result of clamping x between lower and upper" (where x is an extended mathematical value and lower and upper are mathematical values such that lower ≤ upper) produces lower if x < lower, produces upper if x > upper, and otherwise produces x.

The mathematical function floor(x) produces the largest integer (closest to +∞) that is not larger than x.

Note

floor(x) = x - (x modulo 1).

The mathematical function truncate(x) removes the fractional part of x by rounding towards zero, producing -floor(-x) if x < 0 and otherwise producing floor(x).

Mathematical functions min, max, abs, floor, and truncate are not defined for Numbers and BigInts, and any usage of those methods that have non-mathematical value arguments would be an editorial error in this specification.

An interval from lower bound a to upper bound b is a possibly-infinite, possibly-empty set of numeric values of the same numeric type. Each bound will be described as either inclusive or exclusive, but not both. There are four kinds of intervals, as follows:

An interval from a (inclusive) to b (inclusive), also called an inclusive interval from a to b, includes all values x of the same numeric type such that a ≤ x ≤ b, and no others.
An interval from a (inclusive) to b (exclusive) includes all values x of the same numeric type such that a ≤ x < b, and no others.
An interval from a (exclusive) to b (inclusive) includes all values x of the same numeric type such that a < x ≤ b, and no others.
An interval from a (exclusive) to b (exclusive) includes all values x of the same numeric type such that a < x < b, and no others.

For example, the interval from 1 (inclusive) to 2 (exclusive) consists of all mathematical values between 1 and 2, including 1 and not including 2. For the purpose of defining intervals, -0_𝔽 < +0_𝔽, so, for example, an inclusive interval with a lower bound of +0_𝔽 includes +0_𝔽 but not -0_𝔽. NaN is never included in an interval.

5.2.6 Value Notation

In this specification, ECMAScript language values are displayed in bold. Examples include null, true, or "hello". These are distinguished from ECMAScript source text such as Function.prototype.apply or let n = 42;.

5.2.7 Identity

In this specification, both specification values and ECMAScript language values are compared for equality. When comparing for equality, values fall into one of two categories. Values without identity are equal to other values without identity if all of their innate characteristics are the same — characteristics such as the magnitude of an integer or the length of a sequence. Values without identity may be manifest without prior reference by fully describing their characteristics. In contrast, each value with identity is unique and therefore only equal to itself. Values with identity are like values without identity but with an additional unguessable, unchangeable, universally-unique characteristic called identity. References to existing values with identity cannot be manifest simply by describing them, as the identity itself is indescribable; instead, references to these values must be explicitly passed from one place to another. Some values with identity are mutable and therefore can have their characteristics (except their identity) changed in-place, causing all holders of the value to observe the new characteristics. A value without identity is never equal to a value with identity.

From the perspective of this specification, the word “is” is used to compare two values for equality, as in “If bool is true, then ...”, and the word “contains” is used to search for a value inside lists using equality comparisons, as in "If list contains a Record r such that r.[[Foo]] is true, then ...". The specification identity of values determines the result of these comparisons and is axiomatic in this specification.

From the perspective of the ECMAScript language, language values are compared for equality using the SameValue abstract operation and the abstract operations it transitively calls. The algorithms of these comparison abstract operations determine language identity of ECMAScript language values.

For specification values, examples of values without specification identity include, but are not limited to: mathematical values and extended mathematical values; ECMAScript source text, surrogate pairs, Directive Prologues, etc; UTF-16 code units; Unicode code points; enums; abstract operations, including syntax-directed operations, host hooks, etc; and ordered pairs. Examples of specification values with specification identity include, but are not limited to: any kind of Records, including Property Descriptors, PrivateElements, etc; Parse Nodes; Lists; Sets and Relations; Abstract Closures; Data Blocks; Private Names; execution contexts and execution context stacks; agent signifiers; and WaiterList Records.

Specification identity agrees with language identity for all ECMAScript language values except Symbol values produced by Symbol.for. The ECMAScript language values without specification identity and without language identity are undefined, null, Booleans, Strings, Numbers, and BigInts. The ECMAScript language values with specification identity and language identity are Symbols not produced by Symbol.for and Objects. Symbol values produced by Symbol.for have specification identity, but not language identity.

6 ECMAScript Data Types and Values

Algorithms within this specification manipulate values each of which has an associated type. The possible value types are exactly those defined in this clause. Types are further subclassified into ECMAScript language types and specification types.

Within this specification, the notation “Type(x)” is used as shorthand for “the type of x” where “type” refers to the ECMAScript language and specification types defined in this clause.

6.1 ECMAScript Language Types

An ECMAScript language type corresponds to values that are directly manipulated by an ECMAScript programmer using the ECMAScript language. The ECMAScript language types are Undefined, Null, Boolean, String, Symbol, Number, BigInt, and Object. An ECMAScript language value is a value that is characterized by an ECMAScript language type.

6.1.1 The Undefined Type

The Undefined type has exactly one value, called undefined. Any variable that has not been assigned a value has the value undefined.

6.1.2 The Null Type

The Null type has exactly one value, called null.

6.1.3 The Boolean Type

The Boolean type represents a logical entity having two values, called true and false.

6.1.4 The String Type

The String type is the set of all ordered sequences of zero or more 16-bit unsigned integer values (“elements”) up to a maximum length of 2⁵³ - 1 elements. The String type is generally used to represent textual data in a running ECMAScript program, in which case each element in the String is treated as a UTF-16 code unit value. Each element is regarded as occupying a position within the sequence. These positions are indexed with non-negative integers. The first element (if any) is at index 0, the next element (if any) at index 1, and so on. The length of a String is the number of elements (i.e., 16-bit values) within it. The empty String has length zero and therefore contains no elements.

ECMAScript operations that do not interpret String contents apply no further semantics. Operations that do interpret String values treat each element as a single UTF-16 code unit. However, ECMAScript does not restrict the value of or relationships between these code units, so operations that further interpret String contents as sequences of Unicode code points encoded in UTF-16 must account for ill-formed subsequences. Such operations apply special treatment to every code unit with a numeric value in the inclusive interval from 0xD800 to 0xDBFF (defined by the Unicode Standard as a leading surrogate, or more formally as a high-surrogate code unit) and every code unit with a numeric value in the inclusive interval from 0xDC00 to 0xDFFF (defined as a trailing surrogate, or more formally as a low-surrogate code unit) using the following rules:

A code unit that is not a leading surrogate and not a trailing surrogate is interpreted as a code point with the same value.
A sequence of two code units, where the first code unit c1 is a leading surrogate and the second code unit c2 a trailing surrogate, is a surrogate pair and is interpreted as a code point with the value (c1 - 0xD800) × 0x400 + (c2 - 0xDC00) + 0x10000. (See 11.1.3)
A code unit that is a leading surrogate or trailing surrogate, but is not part of a surrogate pair, is interpreted as a code point with the same value.

The function String.prototype.normalize (see 22.1.3.15) can be used to explicitly normalize a String value. String.prototype.localeCompare (see 22.1.3.12) internally normalizes String values, but no other operations implicitly normalize the strings upon which they operate. Operation results are not language- and/or locale-sensitive unless stated otherwise.

Note

The rationale behind this design was to keep the implementation of Strings as simple and high-performing as possible. If ECMAScript source text is in Normalized Form C, string literals are guaranteed to also be normalized, as long as they do not contain any Unicode escape sequences.

In this specification, the phrase "the string-concatenation of A, B, ..." (where each argument is a String value, a code unit, or a sequence of code units) denotes the String value whose sequence of code units is the concatenation of the code units (in order) of each of the arguments (in order).

The phrase "the substring of S from inclusiveStart to exclusiveEnd" (where S is a String value or a sequence of code units and inclusiveStart and exclusiveEnd are integers) denotes the String value consisting of the consecutive code units of S beginning at index inclusiveStart and ending immediately before index exclusiveEnd (which is the empty String when inclusiveStart = exclusiveEnd). If the "to" suffix is omitted, the length of S is used as the value of exclusiveEnd.

The phrase "the ASCII word characters" denotes the following String value, which consists solely of every letter and number in the Unicode Basic Latin block along with U+005F (LOW LINE):
"ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789_".
For historical reasons, it has significance to various algorithms.

6.1.4.1 StringIndexOf ( `string`, `searchValue`, `fromIndex` )

The abstract operation StringIndexOf takes arguments string (a String), searchValue (a String), and fromIndex (a non-negative integer) and returns an integer. It performs the following steps when called:

Let len be the length of string.
If searchValue is the empty String and fromIndex ≤ len, return fromIndex.
Let searchLen be the length of searchValue.
4. For each integer i such that fromIndex ≤ i ≤ len - searchLen, in ascending order, do
1. Let candidate be the substring of string from i to i + searchLen.
2. If candidate is searchValue, return i.
Return -1.

Note 1

If searchValue is the empty String and fromIndex ≤ the length of string, this algorithm returns fromIndex. The empty String is effectively found at every position within a string, including after the last code unit.

Note 2

This algorithm always returns -1 if fromIndex + the length of searchValue > the length of string.

6.1.5 The Symbol Type

The Symbol type is the set of all non-String values that may be used as the key of an Object property (6.1.7).

Each possible Symbol value is unique and immutable.

Each Symbol value immutably holds an associated value called [[Description]] that is either undefined or a String value.

6.1.5.1 Well-Known Symbols

Well-known symbols are built-in Symbol values that are explicitly referenced by algorithms of this specification. They are typically used as the keys of properties whose values serve as extension points of a specification algorithm. Unless otherwise specified, well-known symbols values are shared by all realms (9.3).

Within this specification a well-known symbol is referred to by using a notation of the form @@name, where “name” is one of the values listed in Table 1.

Table 1: Well-known Symbols

Specification Name	`[[Description]]`	Value and Purpose
@@asyncIterator	"Symbol.asyncIterator"	A method that returns the default AsyncIterator for an object. Called by the semantics of the `for`-`await`-`of` statement.
@@hasInstance	"Symbol.hasInstance"	A method that determines if a constructor object recognizes an object as one of the constructor's instances. Called by the semantics of the `instanceof` operator.
@@isConcatSpreadable	"Symbol.isConcatSpreadable"	A Boolean valued property that if true indicates that an object should be flattened to its array elements by `Array.prototype.concat`.
@@iterator	"Symbol.iterator"	A method that returns the default Iterator for an object. Called by the semantics of the for-of statement.
@@match	"Symbol.match"	A regular expression method that matches the regular expression against a string. Called by the `String.prototype.match` method.
@@matchAll	"Symbol.matchAll"	A regular expression method that returns an iterator, that yields matches of the regular expression against a string. Called by the `String.prototype.matchAll` method.
@@replace	"Symbol.replace"	A regular expression method that replaces matched substrings of a string. Called by the `String.prototype.replace` method.
@@search	"Symbol.search"	A regular expression method that returns the index within a string that matches the regular expression. Called by the `String.prototype.search` method.
@@species	"Symbol.species"	A function valued property that is the constructor function that is used to create derived objects.
@@split	"Symbol.split"	A regular expression method that splits a string at the indices that match the regular expression. Called by the `String.prototype.split` method.
@@toPrimitive	"Symbol.toPrimitive"	A method that converts an object to a corresponding primitive value. Called by the ToPrimitive abstract operation.
@@toStringTag	"Symbol.toStringTag"	A String valued property that is used in the creation of the default string description of an object. Accessed by the built-in method `Object.prototype.toString`.
@@unscopables	"Symbol.unscopables"	An object valued property whose own and inherited property names are property names that are excluded from the `with` environment bindings of the associated object.

6.1.6 Numeric Types

ECMAScript has two built-in numeric types: Number and BigInt. The following abstract operations are defined over these numeric types. The "Result" column shows the return type, along with an indication if it is possible for some invocations of the operation to return an abrupt completion.

Table 2: Numeric Type Operations

Operation	Example source	Invoked by the Evaluation semantics of ...	Result
Number::unaryMinus	`-x`	Unary `-` Operator	Number
BigInt::unaryMinus	`-x`	Unary `-` Operator	BigInt
Number::bitwiseNOT	`~x`	Bitwise NOT Operator ( `~` )	Number
BigInt::bitwiseNOT	`~x`	Bitwise NOT Operator ( `~` )	BigInt
Number::exponentiate	`x ** y`	Exponentiation Operator and Math.pow ( `base`, `exponent` )	Number
BigInt::exponentiate	`x ** y`	Exponentiation Operator and Math.pow ( `base`, `exponent` )	either a normal completion containing a BigInt or a throw completion
Number::multiply	`x * y`	Multiplicative Operators	Number
BigInt::multiply	`x * y`	Multiplicative Operators	BigInt
Number::divide	`x / y`	Multiplicative Operators	Number
BigInt::divide	`x / y`	Multiplicative Operators	either a normal completion containing a BigInt or a throw completion
Number::remainder	`x % y`	Multiplicative Operators	Number
BigInt::remainder	`x % y`	Multiplicative Operators	either a normal completion containing a BigInt or a throw completion
Number::add	`x ++` `++ x` `x + y`	Postfix Increment Operator, Prefix Increment Operator, and The Addition Operator ( `+` )	Number
BigInt::add	`x ++` `++ x` `x + y`		BigInt
Number::subtract	`x --` `-- x` `x - y`	Postfix Decrement Operator, Prefix Decrement Operator, and The Subtraction Operator ( `-` )	Number
BigInt::subtract	`x --` `-- x` `x - y`		BigInt
Number::leftShift	`x << y`	The Left Shift Operator ( `<<` )	Number
BigInt::leftShift	`x << y`	The Left Shift Operator ( `<<` )	BigInt
Number::signedRightShift	`x >> y`	The Signed Right Shift Operator ( `>>` )	Number
BigInt::signedRightShift	`x >> y`	The Signed Right Shift Operator ( `>>` )	BigInt
Number::unsignedRightShift	`x >>> y`	The Unsigned Right Shift Operator ( `>>>` )	Number
BigInt::unsignedRightShift	`x >>> y`	The Unsigned Right Shift Operator ( `>>>` )	a throw completion
Number::lessThan	`x < y` `x > y` `x <= y` `x >= y`	Relational Operators, via IsLessThan ( `x`, `y`, `LeftFirst` )	Boolean or undefined (for unordered inputs)
BigInt::lessThan	`x < y` `x > y` `x <= y` `x >= y`		Boolean
Number::equal	`x == y` `x != y` `x === y` `x !== y`	Equality Operators, via IsStrictlyEqual ( `x`, `y` )	Boolean
BigInt::equal	`x == y` `x != y` `x === y` `x !== y`	Equality Operators, via IsStrictlyEqual ( `x`, `y` )	Boolean
Number::sameValue	`Object.is(x, y)`	Object internal methods, via SameValue ( `x`, `y` ), to test exact value equality	Boolean
Number::sameValueZero	`[x].includes(y)`	Array, Map, and Set methods, via SameValueZero ( `x`, `y` ), to test value equality, ignoring the difference between +0_𝔽 and -0_𝔽	Boolean
Number::bitwiseAND	`x & y`	Binary Bitwise Operators	Number
BigInt::bitwiseAND	`x & y`		BigInt
Number::bitwiseXOR	`x ^ y`		Number
BigInt::bitwiseXOR	`x ^ y`		BigInt
Number::bitwiseOR	`x \| y`		Number
BigInt::bitwiseOR	`x \| y`		BigInt
Number::toString	`String(x)`	Many expressions and built-in functions, via ToString ( `argument` )	String
BigInt::toString	`String(x)`		String

Because the numeric types are in general not convertible without loss of precision or truncation, the ECMAScript language provides no implicit conversion among these types. Programmers must explicitly call Number and BigInt functions to convert among types when calling a function which requires another type.

Note

The first and subsequent editions of ECMAScript have provided, for certain operators, implicit numeric conversions that could lose precision or truncate. These legacy implicit conversions are maintained for backward compatibility, but not provided for BigInt in order to minimize opportunity for programmer error, and to leave open the option of generalized value types in a future edition.

6.1.6.1 The Number Type

The Number type has exactly 18,437,736,874,454,810,627 (that is, 2⁶⁴ - 2⁵³ + 3) values, representing the double-precision 64-bit format IEEE 754-2019 values as specified in the IEEE Standard for Binary Floating-Point Arithmetic, except that the 9,007,199,254,740,990 (that is, 2⁵³ - 2) distinct “Not-a-Number” values of the IEEE Standard are represented in ECMAScript as a single special NaN value. (Note that the NaN value is produced by the program expression NaN.) In some implementations, external code might be able to detect a difference between various Not-a-Number values, but such behaviour is implementation-defined; to ECMAScript code, all NaN values are indistinguishable from each other.

Note

The bit pattern that might be observed in an ArrayBuffer (see 25.1) or a SharedArrayBuffer (see 25.2) after a Number value has been stored into it is not necessarily the same as the internal representation of that Number value used by the ECMAScript implementation.

There are two other special values, called positive Infinity and negative Infinity. For brevity, these values are also referred to for expository purposes by the symbols +∞_𝔽 and -∞_𝔽, respectively. (Note that these two infinite Number values are produced by the program expressions +Infinity (or simply Infinity) and -Infinity.)

The other 18,437,736,874,454,810,624 (that is, 2⁶⁴ - 2⁵³) values are called the finite numbers. Half of these are positive numbers and half are negative numbers; for every finite positive Number value there is a corresponding negative value having the same magnitude.

Note that there is both a positive zero and a negative zero. For brevity, these values are also referred to for expository purposes by the symbols +0_𝔽 and -0_𝔽, respectively. (Note that these two different zero Number values are produced by the program expressions +0 (or simply 0) and -0.)

The 18,437,736,874,454,810,622 (that is, 2⁶⁴ - 2⁵³ - 2) finite non-zero values are of two kinds:

18,428,729,675,200,069,632 (that is, 2⁶⁴ - 2⁵⁴) of them are normalized, having the form

s \times m \times 2 ** e

where s is 1 or -1, m is an integer in the interval from 2⁵² (inclusive) to 2⁵³ (exclusive), and e is an integer in the inclusive interval from -1074 to 971.

The remaining 9,007,199,254,740,990 (that is, 2⁵³ - 2) values are denormalized, having the form

s \times m \times 2 ** e

where s is 1 or -1, m is an integer in the interval from 0 (exclusive) to 2⁵² (exclusive), and e is -1074.

Note that all the positive and negative integers whose magnitude is no greater than 2⁵³ are representable in the Number type. The integer 0 has two representations in the Number type: +0_𝔽 and -0_𝔽.

A finite number has an odd significand if it is non-zero and the integer m used to express it (in one of the two forms shown above) is odd. Otherwise, it has an even significand.

In this specification, the phrase “the Number value for x” where x represents an exact real mathematical quantity (which might even be an irrational number such as π) means a Number value chosen in the following manner. Consider the set of all finite values of the Number type, with -0_𝔽 removed and with two additional values added to it that are not representable in the Number type, namely 2¹⁰²⁴ (which is +1 × 2⁵³ × 2⁹⁷¹) and -2¹⁰²⁴ (which is -1 × 2⁵³ × 2⁹⁷¹). Choose the member of this set that is closest in value to x. If two values of the set are equally close, then the one with an even significand is chosen; for this purpose, the two extra values 2¹⁰²⁴ and -2¹⁰²⁴ are considered to have even significands. Finally, if 2¹⁰²⁴ was chosen, replace it with +∞_𝔽; if -2¹⁰²⁴ was chosen, replace it with -∞_𝔽; if +0_𝔽 was chosen, replace it with -0_𝔽 if and only if x < 0; any other chosen value is used unchanged. The result is the Number value for x. (This procedure corresponds exactly to the behaviour of the IEEE 754-2019 roundTiesToEven mode.)

The Number value for +∞ is +∞_𝔽, and the Number value for -∞ is -∞_𝔽.

Some ECMAScript operators deal only with integers in specific ranges such as the inclusive interval from -2³¹ to 2³¹ - 1 or the inclusive interval from 0 to 2¹⁶ - 1. These operators accept any value of the Number type but first convert each such value to an integer value in the expected range. See the descriptions of the numeric conversion operations in 7.1.

6.1.6.1.1 Number::unaryMinus ( `x` )

The abstract operation Number::unaryMinus takes argument x (a Number) and returns a Number. It performs the following steps when called:

If x is NaN, return NaN.
Return the result of negating x; that is, compute a Number with the same magnitude but opposite sign.

6.1.6.1.2 Number::bitwiseNOT ( `x` )

The abstract operation Number::bitwiseNOT takes argument x (a Number) and returns an integral Number. It performs the following steps when called:

Let oldValue be ! ToInt32(x).
Return the result of applying bitwise complement to oldValue. The mathematical value of the result is exactly representable as a 32-bit two's complement bit string.

6.1.6.1.3 Number::exponentiate ( `base`, `exponent` )

The abstract operation Number::exponentiate takes arguments base (a Number) and exponent (a Number) and returns a Number. It returns an implementation-approximated value representing the result of raising base to the exponent power. It performs the following steps when called:

If exponent is NaN, return NaN.
If exponent is either +0_𝔽 or -0_𝔽, return 1_𝔽.
If base is NaN, return NaN.
4. If base is +∞_𝔽, then
1. If exponent > +0_𝔽, return +∞_𝔽. Otherwise, return +0_𝔽.
5. If base is -∞_𝔽, then
1. a. If exponent > +0_𝔽, then
  1. If exponent is an odd integral Number, return -∞_𝔽. Otherwise, return +∞_𝔽.
2. b. Else,
  1. If exponent is an odd integral Number, return -0_𝔽. Otherwise, return +0_𝔽.
6. If base is +0_𝔽, then
1. If exponent > +0_𝔽, return +0_𝔽. Otherwise, return +∞_𝔽.
7. If base is -0_𝔽, then
1. a. If exponent > +0_𝔽, then
  1. If exponent is an odd integral Number, return -0_𝔽. Otherwise, return +0_𝔽.
2. b. Else,
  1. If exponent is an odd integral Number, return -∞_𝔽. Otherwise, return +∞_𝔽.
Assert: base is finite and is neither +0_𝔽 nor -0_𝔽.
9. If exponent is +∞_𝔽, then
1. If abs(ℝ(base)) > 1, return +∞_𝔽.
2. If abs(ℝ(base)) = 1, return NaN.
3. If abs(ℝ(base)) < 1, return +0_𝔽.
10. If exponent is -∞_𝔽, then
1. If abs(ℝ(base)) > 1, return +0_𝔽.
2. If abs(ℝ(base)) = 1, return NaN.
3. If abs(ℝ(base)) < 1, return +∞_𝔽.
Assert: exponent is finite and is neither +0_𝔽 nor -0_𝔽.
If base < -0_𝔽 and exponent is not an integral Number, return NaN.
Return an implementation-approximated Number value representing the result of raising ℝ(base) to the ℝ(exponent) power.

Note

The result of base ** exponent when base is 1_𝔽 or -1_𝔽 and exponent is +∞_𝔽 or -∞_𝔽, or when base is 1_𝔽 and exponent is NaN, differs from IEEE 754-2019. The first edition of ECMAScript specified a result of NaN for this operation, whereas later revisions of IEEE 754 specified 1_𝔽. The historical ECMAScript behaviour is preserved for compatibility reasons.

6.1.6.1.4 Number::multiply ( `x`, `y` )

The abstract operation Number::multiply takes arguments x (a Number) and y (a Number) and returns a Number. It performs multiplication according to the rules of IEEE 754-2019 binary double-precision arithmetic, producing the product of x and y. It performs the following steps when called:

If x is NaN or y is NaN, return NaN.
2. If x is either +∞_𝔽 or -∞_𝔽, then
1. If y is either +0_𝔽 or -0_𝔽, return NaN.
2. If y > +0_𝔽, return x.
3. Return -x.
3. If y is either +∞_𝔽 or -∞_𝔽, then
1. If x is either +0_𝔽 or -0_𝔽, return NaN.
2. If x > +0_𝔽, return y.
3. Return -y.
4. If x is -0_𝔽, then
1. If y is -0_𝔽 or y < -0_𝔽, return +0_𝔽.
2. Else, return -0_𝔽.
5. If y is -0_𝔽, then
1. If x < -0_𝔽, return +0_𝔽.
2. Else, return -0_𝔽.
Return 𝔽(ℝ(x) × ℝ(y)).

Note

Finite-precision multiplication is commutative, but not always associative.

6.1.6.1.5 Number::divide ( `x`, `y` )

The abstract operation Number::divide takes arguments x (a Number) and y (a Number) and returns a Number. It performs division according to the rules of IEEE 754-2019 binary double-precision arithmetic, producing the quotient of x and y where x is the dividend and y is the divisor. It performs the following steps when called:

If x is NaN or y is NaN, return NaN.
2. If x is either +∞_𝔽 or -∞_𝔽, then
1. If y is either +∞_𝔽 or -∞_𝔽, return NaN.
2. If y is +0_𝔽 or y > +0_𝔽, return x.
3. Return -x.
3. If y is +∞_𝔽, then
1. If x is +0_𝔽 or x > +0_𝔽, return +0_𝔽. Otherwise, return -0_𝔽.
4. If y is -∞_𝔽, then
1. If x is +0_𝔽 or x > +0_𝔽, return -0_𝔽. Otherwise, return +0_𝔽.
5. If x is either +0_𝔽 or -0_𝔽, then
1. If y is either +0_𝔽 or -0_𝔽, return NaN.
2. If y > +0_𝔽, return x.
3. Return -x.
6. If y is +0_𝔽, then
1. If x > +0_𝔽, return +∞_𝔽. Otherwise, return -∞_𝔽.
7. If y is -0_𝔽, then
1. If x > +0_𝔽, return -∞_𝔽. Otherwise, return +∞_𝔽.
Return 𝔽(ℝ(x) / ℝ(y)).

6.1.6.1.6 Number::remainder ( `n`, `d` )

The abstract operation Number::remainder takes arguments n (a Number) and d (a Number) and returns a Number. It yields the remainder from an implied division of its operands where n is the dividend and d is the divisor. It performs the following steps when called:

If n is NaN or d is NaN, return NaN.
If n is either +∞_𝔽 or -∞_𝔽, return NaN.
If d is either +∞_𝔽 or -∞_𝔽, return n.
If d is either +0_𝔽 or -0_𝔽, return NaN.
If n is either +0_𝔽 or -0_𝔽, return n.
Assert: n and d are finite and non-zero.
Let quotient be ℝ(n) / ℝ(d).
Let q be truncate(quotient).
Let r be ℝ(n) - (ℝ(d) × q).
If r = 0 and n < -0_𝔽, return -0_𝔽.
Return 𝔽(r).

Note 1

In C and C++, the remainder operator accepts only integral operands; in ECMAScript, it also accepts floating-point operands.

Note 2

The result of a floating-point remainder operation as computed by the % operator is not the same as the “remainder” operation defined by IEEE 754-2019. The IEEE 754-2019 “remainder” operation computes the remainder from a rounding division, not a truncating division, and so its behaviour is not analogous to that of the usual integer remainder operator. Instead the ECMAScript language defines % on floating-point operations to behave in a manner analogous to that of the Java integer remainder operator; this may be compared with the C library function fmod.

6.1.6.1.7 Number::add ( `x`, `y` )

The abstract operation Number::add takes arguments x (a Number) and y (a Number) and returns a Number. It performs addition according to the rules of IEEE 754-2019 binary double-precision arithmetic, producing the sum of its arguments. It performs the following steps when called:

If x is NaN or y is NaN, return NaN.
If x is +∞_𝔽 and y is -∞_𝔽, return NaN.
If x is -∞_𝔽 and y is +∞_𝔽, return NaN.
If x is either +∞_𝔽 or -∞_𝔽, return x.
If y is either +∞_𝔽 or -∞_𝔽, return y.
Assert: x and y are both finite.
If x is -0_𝔽 and y is -0_𝔽, return -0_𝔽.
Return 𝔽(ℝ(x) + ℝ(y)).

Note

Finite-precision addition is commutative, but not always associative.

6.1.6.1.8 Number::subtract ( `x`, `y` )

The abstract operation Number::subtract takes arguments x (a Number) and y (a Number) and returns a Number. It performs subtraction, producing the difference of its operands; x is the minuend and y is the subtrahend. It performs the following steps when called:

Return Number::add(x, Number::unaryMinus(y)).

Note

It is always the case that x - y produces the same result as x + (-y).

6.1.6.1.9 Number::leftShift ( `x`, `y` )

The abstract operation Number::leftShift takes arguments x (a Number) and y (a Number) and returns an integral Number. It performs the following steps when called:

Let lnum be ! ToInt32(x).
Let rnum be ! ToUint32(y).
Let shiftCount be ℝ(rnum) modulo 32.
Return the result of left shifting lnum by shiftCount bits. The mathematical value of the result is exactly representable as a 32-bit two's complement bit string.

6.1.6.1.10 Number::signedRightShift ( `x`, `y` )

The abstract operation Number::signedRightShift takes arguments x (a Number) and y (a Number) and returns an integral Number. It performs the following steps when called:

Let lnum be ! ToInt32(x).
Let rnum be ! ToUint32(y).
Let shiftCount be ℝ(rnum) modulo 32.
Return the result of performing a sign-extending right shift of lnum by shiftCount bits. The most significant bit is propagated. The mathematical value of the result is exactly representable as a 32-bit two's complement bit string.

6.1.6.1.11 Number::unsignedRightShift ( `x`, `y` )

The abstract operation Number::unsignedRightShift takes arguments x (a Number) and y (a Number) and returns an integral Number. It performs the following steps when called:

Let lnum be ! ToUint32(x).
Let rnum be ! ToUint32(y).
Let shiftCount be ℝ(rnum) modulo 32.
Return the result of performing a zero-filling right shift of lnum by shiftCount bits. Vacated bits are filled with zero. The mathematical value of the result is exactly representable as a 32-bit unsigned bit string.

6.1.6.1.12 Number::lessThan ( `x`, `y` )

The abstract operation Number::lessThan takes arguments x (a Number) and y (a Number) and returns a Boolean or undefined. It performs the following steps when called:

If x is NaN, return undefined.
If y is NaN, return undefined.
If x is y, return false.
If x is +0_𝔽 and y is -0_𝔽, return false.
If x is -0_𝔽 and y is +0_𝔽, return false.
If x is +∞_𝔽, return false.
If y is +∞_𝔽, return true.
If y is -∞_𝔽, return false.
If x is -∞_𝔽, return true.
Assert: x and y are finite.
If ℝ(x) < ℝ(y), return true. Otherwise, return false.

6.1.6.1.13 Number::equal ( `x`, `y` )

The abstract operation Number::equal takes arguments x (a Number) and y (a Number) and returns a Boolean. It performs the following steps when called:

If x is NaN, return false.
If y is NaN, return false.
If x is y, return true.
If x is +0_𝔽 and y is -0_𝔽, return true.
If x is -0_𝔽 and y is +0_𝔽, return true.
Return false.

6.1.6.1.14 Number::sameValue ( `x`, `y` )

The abstract operation Number::sameValue takes arguments x (a Number) and y (a Number) and returns a Boolean. It performs the following steps when called:

If x is NaN and y is NaN, return true.
If x is +0_𝔽 and y is -0_𝔽, return false.
If x is -0_𝔽 and y is +0_𝔽, return false.
If x is y, return true.
Return false.

6.1.6.1.15 Number::sameValueZero ( `x`, `y` )

The abstract operation Number::sameValueZero takes arguments x (a Number) and y (a Number) and returns a Boolean. It performs the following steps when called:

If x is NaN and y is NaN, return true.
If x is +0_𝔽 and y is -0_𝔽, return true.
If x is -0_𝔽 and y is +0_𝔽, return true.
If x is y, return true.
Return false.

6.1.6.1.16 NumberBitwiseOp ( `op`, `x`, `y` )

The abstract operation NumberBitwiseOp takes arguments op (&, ^, or |), x (a Number), and y (a Number) and returns an integral Number. It performs the following steps when called:

Let lnum be ! ToInt32(x).
Let rnum be ! ToInt32(y).
Let lbits be the 32-bit two's complement bit string representing ℝ(lnum).
Let rbits be the 32-bit two's complement bit string representing ℝ(rnum).
5. If op is &, then
1. Let result be the result of applying the bitwise AND operation to lbits and rbits.
6. Else if op is ^, then
1. Let result be the result of applying the bitwise exclusive OR (XOR) operation to lbits and rbits.
7. Else,
1. Assert: op is |.
2. Let result be the result of applying the bitwise inclusive OR operation to lbits and rbits.
Return the Number value for the integer represented by the 32-bit two's complement bit string result.

6.1.6.1.17 Number::bitwiseAND ( `x`, `y` )

The abstract operation Number::bitwiseAND takes arguments x (a Number) and y (a Number) and returns an integral Number. It performs the following steps when called:

Return NumberBitwiseOp(&, x, y).

6.1.6.1.18 Number::bitwiseXOR ( `x`, `y` )

The abstract operation Number::bitwiseXOR takes arguments x (a Number) and y (a Number) and returns an integral Number. It performs the following steps when called:

Return NumberBitwiseOp(^, x, y).

6.1.6.1.19 Number::bitwiseOR ( `x`, `y` )

The abstract operation Number::bitwiseOR takes arguments x (a Number) and y (a Number) and returns an integral Number. It performs the following steps when called:

Return NumberBitwiseOp(|, x, y).

6.1.6.1.20 Number::toString ( `x`, `radix` )

The abstract operation Number::toString takes arguments x (a Number) and radix (an integer in the inclusive interval from 2 to 36) and returns a String. It represents x as a String using a positional numeral system with radix radix. The digits used in the representation of a number using radix r are taken from the first r code units of "0123456789abcdefghijklmnopqrstuvwxyz" in order. The representation of numbers with magnitude greater than or equal to 1_𝔽 never includes leading zeroes. It performs the following steps when called:

If x is NaN, return "NaN".
If x is either +0_𝔽 or -0_𝔽, return "0".
If x < -0_𝔽, return the string-concatenation of "-" and Number::toString(-x, radix).
If x is +∞_𝔽, return "Infinity".
Let n, k, and s be integers such that k ≥ 1, radix^{k - 1} ≤ s < radix^k, 𝔽(s × radix^{n - k}) is x, and k is as small as possible. Note that k is the number of digits in the representation of s using radix radix, that s is not divisible by radix, and that the least significant digit of s is not necessarily uniquely determined by these criteria.
6. If radix ≠ 10 or n is in the inclusive interval from -5 to 21, then
1. a. If n ≥ k, then
  1. i. Return the string-concatenation of:
    - the code units of the k digits of the representation of s using radix radix
    - n - k occurrences of the code unit 0x0030 (DIGIT ZERO)
2. b. Else if n > 0, then
  1. i. Return the string-concatenation of:
    - the code units of the most significant n digits of the representation of s using radix radix
    - the code unit 0x002E (FULL STOP)
    - the code units of the remaining k - n digits of the representation of s using radix radix
3. c. Else,
  1. Assert: n ≤ 0.
  2. ii. Return the string-concatenation of:
    - the code unit 0x0030 (DIGIT ZERO)
    - the code unit 0x002E (FULL STOP)
    - -n occurrences of the code unit 0x0030 (DIGIT ZERO)
    - the code units of the k digits of the representation of s using radix radix
NOTE: In this case, the input will be represented using scientific E notation, such as 1.2e+3.
Assert: radix is 10.
9. If n < 0, then
1. Let exponentSign be the code unit 0x002D (HYPHEN-MINUS).
10. Else,
1. Let exponentSign be the code unit 0x002B (PLUS SIGN).
11. If k = 1, then
1. a. Return the string-concatenation of:
  - the code unit of the single digit of s
  - the code unit 0x0065 (LATIN SMALL LETTER E)
  - exponentSign
  - the code units of the decimal representation of abs(n - 1)
12. Return the string-concatenation of:
- the code unit of the most significant digit of the decimal representation of s
- the code unit 0x002E (FULL STOP)
- the code units of the remaining k - 1 digits of the decimal representation of s
- the code unit 0x0065 (LATIN SMALL LETTER E)
- exponentSign
- the code units of the decimal representation of abs(n - 1)

Note 1

The following observations may be useful as guidelines for implementations, but are not part of the normative requirements of this Standard:

If x is any Number value other than -0_𝔽, then ToNumber(ToString(x)) is x.
The least significant digit of s is not always uniquely determined by the requirements listed in step 5.

Note 2

For implementations that provide more accurate conversions than required by the rules above, it is recommended that the following alternative version of step 5 be used as a guideline:

Let n, k, and s be integers such that k ≥ 1, radix^{k - 1} ≤ s < radix^k, 𝔽(s × radix^{n - k}) is x, and k is as small as possible. If there are multiple possibilities for s, choose the value of s for which s × radix^{n - k} is closest in value to ℝ(x). If there are two such possible values of s, choose the one that is even. Note that k is the number of digits in the representation of s using radix radix and that s is not divisible by radix.

Note 3

Implementers of ECMAScript may find useful the paper and code written by David M. Gay for binary-to-decimal conversion of floating-point numbers:

Gay, David M. Correctly Rounded Binary-Decimal and Decimal-Binary Conversions. Numerical Analysis, Manuscript 90-10. AT&T Bell Laboratories (Murray Hill, New Jersey). 30 November 1990. Available as
http://ampl.com/REFS/abstracts.html#rounding. Associated code available as
http://netlib.sandia.gov/fp/dtoa.c and as
http://netlib.sandia.gov/fp/g_fmt.c and may also be found at the various netlib mirror sites.

6.1.6.2 The BigInt Type

The BigInt type represents an integer value. The value may be any size and is not limited to a particular bit-width. Generally, where not otherwise noted, operations are designed to return exact mathematically-based answers. For binary operations, BigInts act as two's complement binary strings, with negative numbers treated as having bits set infinitely to the left.

6.1.6.2.1 BigInt::unaryMinus ( `x` )

The abstract operation BigInt::unaryMinus takes argument x (a BigInt) and returns a BigInt. It performs the following steps when called:

If x is 0_ℤ, return 0_ℤ.
Return -x.

6.1.6.2.2 BigInt::bitwiseNOT ( `x` )

The abstract operation BigInt::bitwiseNOT takes argument x (a BigInt) and returns a BigInt. It returns the one's complement of x. It performs the following steps when called:

Return -x - 1_ℤ.

6.1.6.2.3 BigInt::exponentiate ( `base`, `exponent` )

The abstract operation BigInt::exponentiate takes arguments base (a BigInt) and exponent (a BigInt) and returns either a normal completion containing a BigInt or a throw completion. It performs the following steps when called:

If exponent < 0_ℤ, throw a RangeError exception.
If base is 0_ℤ and exponent is 0_ℤ, return 1_ℤ.
Return base raised to the power exponent.

6.1.6.2.4 BigInt::multiply ( `x`, `y` )

The abstract operation BigInt::multiply takes arguments x (a BigInt) and y (a BigInt) and returns a BigInt. It performs the following steps when called:

Return x × y.

Note

Even if the result has a much larger bit width than the input, the exact mathematical answer is given.

6.1.6.2.5 BigInt::divide ( `x`, `y` )

The abstract operation BigInt::divide takes arguments x (a BigInt) and y (a BigInt) and returns either a normal completion containing a BigInt or a throw completion. It performs the following steps when called:

If y is 0_ℤ, throw a RangeError exception.
Let quotient be ℝ(x) / ℝ(y).
Return ℤ(truncate(quotient)).

6.1.6.2.6 BigInt::remainder ( `n`, `d` )

The abstract operation BigInt::remainder takes arguments n (a BigInt) and d (a BigInt) and returns either a normal completion containing a BigInt or a throw completion. It performs the following steps when called:

If d is 0_ℤ, throw a RangeError exception.
If n is 0_ℤ, return 0_ℤ.
Let quotient be ℝ(n) / ℝ(d).
Let q be ℤ(truncate(quotient)).
Return n - (d × q).

Note

The sign of the result is the sign of the dividend.

6.1.6.2.7 BigInt::add ( `x`, `y` )

The abstract operation BigInt::add takes arguments x (a BigInt) and y (a BigInt) and returns a BigInt. It performs the following steps when called:

Return x + y.

6.1.6.2.8 BigInt::subtract ( `x`, `y` )

The abstract operation BigInt::subtract takes arguments x (a BigInt) and y (a BigInt) and returns a BigInt. It performs the following steps when called:

Return x - y.

6.1.6.2.9 BigInt::leftShift ( `x`, `y` )

The abstract operation BigInt::leftShift takes arguments x (a BigInt) and y (a BigInt) and returns a BigInt. It performs the following steps when called:

1. If y < 0_ℤ, then
1. Return ℤ(floor(ℝ(x) / 2^-ℝ(y))).
Return x × 2_ℤ^y.

Note

Semantics here should be equivalent to a bitwise shift, treating the BigInt as an infinite length string of binary two's complement digits.

6.1.6.2.10 BigInt::signedRightShift ( `x`, `y` )

The abstract operation BigInt::signedRightShift takes arguments x (a BigInt) and y (a BigInt) and returns a BigInt. It performs the following steps when called:

Return BigInt::leftShift(x, -y).

6.1.6.2.11 BigInt::unsignedRightShift ( `x`, `y` )

The abstract operation BigInt::unsignedRightShift takes arguments x (a BigInt) and y (a BigInt) and returns a throw completion. It performs the following steps when called:

Throw a TypeError exception.

6.1.6.2.12 BigInt::lessThan ( `x`, `y` )

The abstract operation BigInt::lessThan takes arguments x (a BigInt) and y (a BigInt) and returns a Boolean. It performs the following steps when called:

If ℝ(x) < ℝ(y), return true; otherwise return false.

6.1.6.2.13 BigInt::equal ( `x`, `y` )

The abstract operation BigInt::equal takes arguments x (a BigInt) and y (a BigInt) and returns a Boolean. It performs the following steps when called:

If ℝ(x) = ℝ(y), return true; otherwise return false.

6.1.6.2.14 BinaryAnd ( `x`, `y` )

The abstract operation BinaryAnd takes arguments x (0 or 1) and y (0 or 1) and returns 0 or 1. It performs the following steps when called:

If x = 1 and y = 1, return 1.
Else, return 0.

6.1.6.2.15 BinaryOr ( `x`, `y` )

The abstract operation BinaryOr takes arguments x (0 or 1) and y (0 or 1) and returns 0 or 1. It performs the following steps when called:

If x = 1 or y = 1, return 1.
Else, return 0.

6.1.6.2.16 BinaryXor ( `x`, `y` )

The abstract operation BinaryXor takes arguments x (0 or 1) and y (0 or 1) and returns 0 or 1. It performs the following steps when called:

If x = 1 and y = 0, return 1.
Else if x = 0 and y = 1, return 1.
Else, return 0.

6.1.6.2.17 BigIntBitwiseOp ( `op`, `x`, `y` )

The abstract operation BigIntBitwiseOp takes arguments op (&, ^, or |), x (a BigInt), and y (a BigInt) and returns a BigInt. It performs the following steps when called:

Set x to ℝ(x).
Set y to ℝ(y).
Let result be 0.
Let shift be 0.
5. Repeat, until (x = 0 or x = -1) and (y = 0 or y = -1),
1. Let xDigit be x modulo 2.
2. Let yDigit be y modulo 2.
3. c. If op is &, then
  1. Set result to result + 2^shift × BinaryAnd(xDigit, yDigit).
4. d. Else if op is |, then
  1. Set result to result + 2^shift × BinaryOr(xDigit, yDigit).
5. e. Else,
  1. Assert: op is ^.
  2. Set result to result + 2^shift × BinaryXor(xDigit, yDigit).
6. Set shift to shift + 1.
7. Set x to (x - xDigit) / 2.
8. Set y to (y - yDigit) / 2.
6. If op is &, then
1. Let tmp be BinaryAnd(x modulo 2, y modulo 2).
7. Else if op is |, then
1. Let tmp be BinaryOr(x modulo 2, y modulo 2).
8. Else,
1. Assert: op is ^.
2. Let tmp be BinaryXor(x modulo 2, y modulo 2).
9. If tmp ≠ 0, then
1. Set result to result - 2^shift.
2. NOTE: This extends the sign.
Return the BigInt value for result.

6.1.6.2.18 BigInt::bitwiseAND ( `x`, `y` )

The abstract operation BigInt::bitwiseAND takes arguments x (a BigInt) and y (a BigInt) and returns a BigInt. It performs the following steps when called:

Return BigIntBitwiseOp(&, x, y).

6.1.6.2.19 BigInt::bitwiseXOR ( `x`, `y` )

The abstract operation BigInt::bitwiseXOR takes arguments x (a BigInt) and y (a BigInt) and returns a BigInt. It performs the following steps when called:

Return BigIntBitwiseOp(^, x, y).

6.1.6.2.20 BigInt::bitwiseOR ( `x`, `y` )

The abstract operation BigInt::bitwiseOR takes arguments x (a BigInt) and y (a BigInt) and returns a BigInt. It performs the following steps when called:

Return BigIntBitwiseOp(|, x, y).

6.1.6.2.21 BigInt::toString ( `x`, `radix` )

The abstract operation BigInt::toString takes arguments x (a BigInt) and radix (an integer in the inclusive interval from 2 to 36) and returns a String. It represents x as a String using a positional numeral system with radix radix. The digits used in the representation of a BigInt using radix r are taken from the first r code units of "0123456789abcdefghijklmnopqrstuvwxyz" in order. The representation of BigInts other than 0_ℤ never includes leading zeroes. It performs the following steps when called:

If x < 0_ℤ, return the string-concatenation of "-" and BigInt::toString(-x, radix).
Return the String value consisting of the representation of x using radix radix.

6.1.7 The Object Type

Each instance of the Object type, also referred to simply as “an Object”, represents a collection of properties. Each property is either a data property, or an accessor property:

A data property associates a key value with an ECMAScript language value and a set of Boolean attributes.
An accessor property associates a key value with one or two accessor functions, and a set of Boolean attributes. The accessor functions are used to store or retrieve an ECMAScript language value that is associated with the property.

The properties of an object are uniquely identified using property keys. A property key is either a String or a Symbol. All Strings and Symbols, including the empty String, are valid as property keys. A property name is a property key that is a String.

An integer index is a property name n such that CanonicalNumericIndexString(n) returns an integral Number in the inclusive interval from +0_𝔽 to 𝔽(2⁵³ - 1). An array index is an integer index n such that CanonicalNumericIndexString(n) returns an integral Number in the inclusive interval from +0_𝔽 to 𝔽(2³² - 2).

Note

Every non-negative safe integer has a corresponding integer index. Every 32-bit unsigned integer except 2³² - 1 has a corresponding array index. "-0" is neither an integer index nor an array index.

Property keys are used to access properties and their values. There are two kinds of access for properties: get and set, corresponding to value retrieval and assignment, respectively. The properties accessible via get and set access includes both own properties that are a direct part of an object and inherited properties which are provided by another associated object via a property inheritance relationship. Inherited properties may be either own or inherited properties of the associated object. Each own property of an object must each have a key value that is distinct from the key values of the other own properties of that object.

All objects are logically collections of properties, but there are multiple forms of objects that differ in their semantics for accessing and manipulating their properties. Please see 6.1.7.2 for definitions of the multiple forms of objects.

In addition, some objects are callable; these are referred to as functions or function objects and are described further below. All functions in ECMAScript are members of the Object type.

6.1.7.1 Property Attributes

Attributes are used in this specification to define and explain the state of Object properties as described in Table 3. Unless specified explicitly, the initial value of each attribute is its Default Value.

Table 3: Attributes of an Object property

Attribute Name	Types of property for which it is present	Value Domain	Default Value	Description
`[[Value]]`	data property	an ECMAScript language value	undefined	The value retrieved by a get access of the property.
`[[Writable]]`	data property	a Boolean	false	If false, attempts by ECMAScript code to change the property's `[[Value]]` attribute using `[[Set]]` will not succeed.
`[[Get]]`	accessor property	an Object or undefined	undefined	If the value is an Object it must be a function object. The function's `[[Call]]` internal method (Table 5) is called with an empty arguments list to retrieve the property value each time a get access of the property is performed.
`[[Set]]`	accessor property	an Object or undefined	undefined	If the value is an Object it must be a function object. The function's `[[Call]]` internal method (Table 5) is called with an arguments list containing the assigned value as its sole argument each time a set access of the property is performed. The effect of a property's `[[Set]]` internal method may, but is not required to, have an effect on the value returned by subsequent calls to the property's `[[Get]]` internal method.
`[[Enumerable]]`	data property or accessor property	a Boolean	false	If true, the property will be enumerated by a for-in enumeration (see 14.7.5). Otherwise, the property is said to be non-enumerable.
`[[Configurable]]`	data property or accessor property	a Boolean	false	If false, attempts to delete the property, change it from a data property to an accessor property or from an accessor property to a data property, or make any changes to its attributes (other than replacing an existing `[[Value]]` or setting `[[Writable]]` to false) will fail.

6.1.7.2 Object Internal Methods and Internal Slots

The actual semantics of objects, in ECMAScript, are specified via algorithms called internal methods. Each object in an ECMAScript engine is associated with a set of internal methods that defines its runtime behaviour. These internal methods are not part of the ECMAScript language. They are defined by this specification purely for expository purposes. However, each object within an implementation of ECMAScript must behave as specified by the internal methods associated with it. The exact manner in which this is accomplished is determined by the implementation.

Internal method names are polymorphic. This means that different object values may perform different algorithms when a common internal method name is invoked upon them. That actual object upon which an internal method is invoked is the “target” of the invocation. If, at runtime, the implementation of an algorithm attempts to use an internal method of an object that the object does not support, a TypeError exception is thrown.

Internal slots correspond to internal state that is associated with objects and used by various ECMAScript specification algorithms. Internal slots are not object properties and they are not inherited. Depending upon the specific internal slot specification, such state may consist of values of any ECMAScript language type or of specific ECMAScript specification type values. Unless explicitly specified otherwise, internal slots are allocated as part of the process of creating an object and may not be dynamically added to an object. Unless specified otherwise, the initial value of an internal slot is the value undefined. Various algorithms within this specification create objects that have internal slots. However, the ECMAScript language provides no direct way to associate internal slots with an object.

All objects have an internal slot named [[PrivateElements]], which is a List of PrivateElements. This List represents the values of the private fields, methods, and accessors for the object. Initially, it is an empty List.

Internal methods and internal slots are identified within this specification using names enclosed in double square brackets [[ ]].

Table 4 summarizes the essential internal methods used by this specification that are applicable to all objects created or manipulated by ECMAScript code. Every object must have algorithms for all of the essential internal methods. However, all objects do not necessarily use the same algorithms for those methods.

An ordinary object is an object that satisfies all of the following criteria:

For the internal methods listed in Table 4, the object uses those defined in 10.1.
If the object has a [[Call]] internal method, it uses either the one defined in 10.2.1 or the one defined in 10.3.1.
If the object has a [[Construct]] internal method, it uses either the one defined in 10.2.2 or the one defined in 10.3.2.

An exotic object is an object that is not an ordinary object.

This specification recognizes different kinds of exotic objects by those objects' internal methods. An object that is behaviourally equivalent to a particular kind of exotic object (such as an Array exotic object or a bound function exotic object), but does not have the same collection of internal methods specified for that kind, is not recognized as that kind of exotic object.

The “Signature” column of Table 4 and other similar tables describes the invocation pattern for each internal method. The invocation pattern always includes a parenthesized list of descriptive parameter names. If a parameter name is the same as an ECMAScript type name then the name describes the required type of the parameter value. If an internal method explicitly returns a value, its parameter list is followed by the symbol “→” and the type name of the returned value. The type names used in signatures refer to the types defined in clause 6 augmented by the following additional names. “any” means the value may be any ECMAScript language type.

In addition to its parameters, an internal method always has access to the object that is the target of the method invocation.

An internal method implicitly returns a Completion Record, either a normal completion that wraps a value of the return type shown in its invocation pattern, or a throw completion.

Table 4: Essential Internal Methods

Internal Method	Signature	Description
`[[GetPrototypeOf]]`	( ) → Object \| Null	Determine the object that provides inherited properties for this object. A null value indicates that there are no inherited properties.
`[[SetPrototypeOf]]`	(Object \| Null) → Boolean	Associate this object with another object that provides inherited properties. Passing null indicates that there are no inherited properties. Returns true indicating that the operation was completed successfully or false indicating that the operation was not successful.
`[[IsExtensible]]`	( ) → Boolean	Determine whether it is permitted to add additional properties to this object.
`[[PreventExtensions]]`	( ) → Boolean	Control whether new properties may be added to this object. Returns true if the operation was successful or false if the operation was unsuccessful.
`[[GetOwnProperty]]`	(`propertyKey`) → Undefined \| Property Descriptor	Return a Property Descriptor for the own property of this object whose key is `propertyKey`, or undefined if no such property exists.
`[[DefineOwnProperty]]`	(`propertyKey`, `PropertyDescriptor`) → Boolean	Create or alter the own property, whose key is `propertyKey`, to have the state described by `PropertyDescriptor`. Return true if that property was successfully created/updated or false if the property could not be created or updated.
`[[HasProperty]]`	(`propertyKey`) → Boolean	Return a Boolean value indicating whether this object already has either an own or inherited property whose key is `propertyKey`.
`[[Get]]`	(`propertyKey`, `Receiver`) → any	Return the value of the property whose key is `propertyKey` from this object. If any ECMAScript code must be executed to retrieve the property value, `Receiver` is used as the this value when evaluating the code.
`[[Set]]`	(`propertyKey`, `value`, `Receiver`) → Boolean	Set the value of the property whose key is `propertyKey` to `value`. If any ECMAScript code must be executed to set the property value, `Receiver` is used as the this value when evaluating the code. Returns true if the property value was set or false if it could not be set.
`[[Delete]]`	(`propertyKey`) → Boolean	Remove the own property whose key is `propertyKey` from this object. Return false if the property was not deleted and is still present. Return true if the property was deleted or is not present.
`[[OwnPropertyKeys]]`	( ) → List of property keys	Return a List whose elements are all of the own property keys for the object.

Table 5 summarizes additional essential internal methods that are supported by objects that may be called as functions. A function object is an object that supports the [[Call]] internal method. A constructor is an object that supports the [[Construct]] internal method. Every object that supports [[Construct]] must support [[Call]]; that is, every constructor must be a function object. Therefore, a constructor may also be referred to as a constructor function or constructor function object.

Table 5: Additional Essential Internal Methods of Function Objects

Internal Method	Signature	Description
`[[Call]]`	(any, a List of any) → any	Executes code associated with this object. Invoked via a function call expression. The arguments to the internal method are a this value and a List whose elements are the arguments passed to the function by a call expression. Objects that implement this internal method are callable.
`[[Construct]]`	(a List of any, Object) → Object	Creates an object. Invoked via the `new` operator or a `super` call. The first argument to the internal method is a List whose elements are the arguments of the constructor invocation or the `super` call. The second argument is the object to which the `new` operator was initially applied. Objects that implement this internal method are called constructors. A function object is not necessarily a constructor and such non-constructor function objects do not have a `[[Construct]]` internal method.

The semantics of the essential internal methods for ordinary objects and standard exotic objects are specified in clause 10. If any specified use of an internal method of an exotic object is not supported by an implementation, that usage must throw a TypeError exception when attempted.

6.1.7.3 Invariants of the Essential Internal Methods

The Internal Methods of Objects of an ECMAScript engine must conform to the list of invariants specified below. Ordinary ECMAScript Objects as well as all standard exotic objects in this specification maintain these invariants. ECMAScript Proxy objects maintain these invariants by means of runtime checks on the result of traps invoked on the [[ProxyHandler]] object.

Any implementation provided exotic objects must also maintain these invariants for those objects. Violation of these invariants may cause ECMAScript code to have unpredictable behaviour and create security issues. However, violation of these invariants must never compromise the memory safety of an implementation.

An implementation must not allow these invariants to be circumvented in any manner such as by providing alternative interfaces that implement the functionality of the essential internal methods without enforcing their invariants.

Definitions:

The target of an internal method is the object upon which the internal method is called.
A target is non-extensible if it has been observed to return false from its [[IsExtensible]] internal method, or true from its [[PreventExtensions]] internal method.
A non-existent property is a property that does not exist as an own property on a non-extensible target.
All references to SameValue are according to the definition of the SameValue algorithm.

Return value:

The value returned by any internal method must be a Completion Record with either:

[[Type]] = normal, [[Target]] = empty, and [[Value]] = a value of the "normal return type" shown below for that internal method, or
[[Type]] = throw, [[Target]] = empty, and [[Value]] = any ECMAScript language value.

Note 1

An internal method must not return a continue completion, a break completion, or a return completion.

`[[GetPrototypeOf]]` ( )

The normal return type is either Object or Null.
If target is non-extensible, and [[GetPrototypeOf]] returns a value V, then any future calls to [[GetPrototypeOf]] should return the SameValue as V.

Note 2

An object's prototype chain should have finite length (that is, starting from any object, recursively applying the [[GetPrototypeOf]] internal method to its result should eventually lead to the value null). However, this requirement is not enforceable as an object level invariant if the prototype chain includes any exotic objects that do not use the ordinary object definition of [[GetPrototypeOf]]. Such a circular prototype chain may result in infinite loops when accessing object properties.

`[[SetPrototypeOf]]` ( `V` )

The normal return type is Boolean.
If target is non-extensible, [[SetPrototypeOf]] must return false, unless V is the SameValue as the target's observed [[GetPrototypeOf]] value.

`[[IsExtensible]]` ( )

The normal return type is Boolean.
If [[IsExtensible]] returns false, all future calls to [[IsExtensible]] on the target must return false.

`[[PreventExtensions]]` ( )

The normal return type is Boolean.
If [[PreventExtensions]] returns true, all future calls to [[IsExtensible]] on the target must return false and the target is now considered non-extensible.

`[[GetOwnProperty]]` ( `P` )

The normal return type is either Property Descriptor or Undefined.
If the Type of the return value is Property Descriptor, the return value must be a fully populated Property Descriptor.
If P is described as a non-configurable, non-writable own data property, all future calls to [[GetOwnProperty]] ( P ) must return Property Descriptor whose [[Value]] is SameValue as P's [[Value]] attribute.
If P's attributes other than [[Writable]] and [[Value]] may change over time, or if the property might be deleted, then P's [[Configurable]] attribute must be true.
If the [[Writable]] attribute may change from false to true, then the [[Configurable]] attribute must be true.
If the target is non-extensible and P is non-existent, then all future calls to [[GetOwnProperty]] (P) on the target must describe P as non-existent (i.e. [[GetOwnProperty]] (P) must return undefined).

Note 3

As a consequence of the third invariant, if a property is described as a data property and it may return different values over time, then either or both of the [[Writable]] and [[Configurable]] attributes must be true even if no mechanism to change the value is exposed via the other essential internal methods.

`[[DefineOwnProperty]]` ( `P`, `Desc` )

The normal return type is Boolean.
[[DefineOwnProperty]] must return false if P has previously been observed as a non-configurable own property of the target, unless either:
1. P is a writable data property. A non-configurable writable data property can be changed into a non-configurable non-writable data property.
2. All attributes of Desc are the SameValue as P's attributes.
[[DefineOwnProperty]] (P, Desc) must return false if target is non-extensible and P is a non-existent own property. That is, a non-extensible target object cannot be extended with new properties.

`[[HasProperty]]` ( `P` )

The normal return type is Boolean.
If P was previously observed as a non-configurable own data or accessor property of the target, [[HasProperty]] must return true.

`[[Get]]` ( `P`, `Receiver` )

The normal return type is any ECMAScript language type.
If P was previously observed as a non-configurable, non-writable own data property of the target with value V, then [[Get]] must return the SameValue as V.
If P was previously observed as a non-configurable own accessor property of the target whose [[Get]] attribute is undefined, the [[Get]] operation must return undefined.

`[[Set]]` ( `P`, `V`, `Receiver` )

The normal return type is Boolean.
If P was previously observed as a non-configurable, non-writable own data property of the target, then [[Set]] must return false unless V is the SameValue as P's [[Value]] attribute.
If P was previously observed as a non-configurable own accessor property of the target whose [[Set]] attribute is undefined, the [[Set]] operation must return false.

`[[Delete]]` ( `P` )

The normal return type is Boolean.
If P was previously observed as a non-configurable own data or accessor property of the target, [[Delete]] must return false.

`[[OwnPropertyKeys]]` ( )

The normal return type is List.
The returned List must not contain any duplicate entries.
The Type of each element of the returned List is either String or Symbol.
The returned List must contain at least the keys of all non-configurable own properties that have previously been observed.
If the target is non-extensible, the returned List must contain only the keys of all own properties of the target that are observable using [[GetOwnProperty]].

`[[Call]]` ( )

The normal return type is any ECMAScript language type.

`[[Construct]]` ( )

The normal return type is Object.
The target must also have a [[Call]] internal method.

6.1.7.4 Well-Known Intrinsic Objects

Well-known intrinsics are built-in objects that are explicitly referenced by the algorithms of this specification and which usually have realm- specific identities. Unless otherwise specified each intrinsic object actually corresponds to a set of similar objects, one per realm.

Within this specification a reference such as %name% means the intrinsic object, associated with the current realm, corresponding to the name. A reference such as %name.a.b% means, as if the "b" property of the value of the "a" property of the intrinsic object %name% was accessed prior to any ECMAScript code being evaluated. Determination of the current realm and its intrinsics is described in 9.4. The well-known intrinsics are listed in Table 6.

Table 6: Well-Known Intrinsic Objects

Intrinsic Name	Global Name	ECMAScript Language Association
%AggregateError%	`AggregateError`	The `AggregateError` constructor (20.5.7.1)
%Array%	`Array`	The Array constructor (23.1.1)
%ArrayBuffer%	`ArrayBuffer`	The ArrayBuffer constructor (25.1.4)
%ArrayIteratorPrototype%		The prototype of Array iterator objects (23.1.5)
%AsyncFromSyncIteratorPrototype%		The prototype of async-from-sync iterator objects (27.1.4)
%AsyncFunction%		The constructor of async function objects (27.7.1)
%AsyncGeneratorFunction%		The constructor of async iterator objects (27.4.1)
%AsyncIteratorPrototype%		An object that all standard built-in async iterator objects indirectly inherit from
%Atomics%	`Atomics`	The `Atomics` object (25.4)
%BigInt%	`BigInt`	The BigInt constructor (21.2.1)
%BigInt64Array%	`BigInt64Array`	The BigInt64Array constructor (23.2)
%BigUint64Array%	`BigUint64Array`	The BigUint64Array constructor (23.2)
%Boolean%	`Boolean`	The Boolean constructor (20.3.1)
%DataView%	`DataView`	The DataView constructor (25.3.2)
%Date%	`Date`	The Date constructor (21.4.2)
%decodeURI%	`decodeURI`	The `decodeURI` function (19.2.6.1)
%decodeURIComponent%	`decodeURIComponent`	The `decodeURIComponent` function (19.2.6.2)
%encodeURI%	`encodeURI`	The `encodeURI` function (19.2.6.3)
%encodeURIComponent%	`encodeURIComponent`	The `encodeURIComponent` function (19.2.6.4)
%Error%	`Error`	The Error constructor (20.5.1)
%eval%	`eval`	The `eval` function (19.2.1)
%EvalError%	`EvalError`	The EvalError constructor (20.5.5.1)
%FinalizationRegistry%	`FinalizationRegistry`	The FinalizationRegistry constructor (26.2.1)
%Float32Array%	`Float32Array`	The Float32Array constructor (23.2)
%Float64Array%	`Float64Array`	The Float64Array constructor (23.2)
%ForInIteratorPrototype%		The prototype of For-In iterator objects (14.7.5.10)
%Function%	`Function`	The Function constructor (20.2.1)
%GeneratorFunction%		The constructor of Generators (27.3.1)
%Int8Array%	`Int8Array`	The Int8Array constructor (23.2)
%Int16Array%	`Int16Array`	The Int16Array constructor (23.2)
%Int32Array%	`Int32Array`	The Int32Array constructor (23.2)
%isFinite%	`isFinite`	The `isFinite` function (19.2.2)
%isNaN%	`isNaN`	The `isNaN` function (19.2.3)
%IteratorPrototype%		An object that all standard built-in iterator objects indirectly inherit from
%JSON%	`JSON`	The `JSON` object (25.5)
%Map%	`Map`	The Map constructor (24.1.1)
%MapIteratorPrototype%		The prototype of Map iterator objects (24.1.5)
%Math%	`Math`	The `Math` object (21.3)
%Number%	`Number`	The Number constructor (21.1.1)
%Object%	`Object`	The Object constructor (20.1.1)
%parseFloat%	`parseFloat`	The `parseFloat` function (19.2.4)
%parseInt%	`parseInt`	The `parseInt` function (19.2.5)
%Promise%	`Promise`	The Promise constructor (27.2.3)
%Proxy%	`Proxy`	The Proxy constructor (28.2.1)
%RangeError%	`RangeError`	The RangeError constructor (20.5.5.2)
%ReferenceError%	`ReferenceError`	The ReferenceError constructor (20.5.5.3)
%Reflect%	`Reflect`	The `Reflect` object (28.1)
%RegExp%	`RegExp`	The RegExp constructor (22.2.4)
%RegExpStringIteratorPrototype%		The prototype of RegExp String Iterator objects (22.2.9)
%Set%	`Set`	The Set constructor (24.2.1)
%SetIteratorPrototype%		The prototype of Set iterator objects (24.2.5)
%SharedArrayBuffer%	`SharedArrayBuffer`	The SharedArrayBuffer constructor (25.2.3)
%String%	`String`	The String constructor (22.1.1)
%StringIteratorPrototype%		The prototype of String iterator objects (22.1.5)
%Symbol%	`Symbol`	The Symbol constructor (20.4.1)
%SyntaxError%	`SyntaxError`	The SyntaxError constructor (20.5.5.4)
%ThrowTypeError%		A function object that unconditionally throws a new instance of %TypeError%
%TypedArray%		The super class of all typed Array constructors (23.2.1)
%TypeError%	`TypeError`	The TypeError constructor (20.5.5.5)
%Uint8Array%	`Uint8Array`	The Uint8Array constructor (23.2)
%Uint8ClampedArray%	`Uint8ClampedArray`	The Uint8ClampedArray constructor (23.2)
%Uint16Array%	`Uint16Array`	The Uint16Array constructor (23.2)
%Uint32Array%	`Uint32Array`	The Uint32Array constructor (23.2)
%URIError%	`URIError`	The URIError constructor (20.5.5.6)
%WeakMap%	`WeakMap`	The WeakMap constructor (24.3.1)
%WeakRef%	`WeakRef`	The WeakRef constructor (26.1.1)
%WeakSet%	`WeakSet`	The WeakSet constructor (24.4.1)

Note

Additional entries in Table 97.

6.2 ECMAScript Specification Types

A specification type corresponds to meta-values that are used within algorithms to describe the semantics of ECMAScript language constructs and ECMAScript language types. The specification types include Reference, List, Completion Record, Property Descriptor, Environment Record, Abstract Closure, and Data Block. Specification type values are specification artefacts that do not necessarily correspond to any specific entity within an ECMAScript implementation. Specification type values may be used to describe intermediate results of ECMAScript expression evaluation but such values cannot be stored as properties of objects or values of ECMAScript language variables.

6.2.1 The Enum Specification Type

Enums are values which are internal to the specification and not directly observable from ECMAScript code. Enums are denoted using a sans-serif typeface. For instance, a Completion Record's [[Type]] field takes on values like normal, return, or throw. Enums have no characteristics other than their name. The name of an enum serves no purpose other than to distinguish it from other enums, and implies nothing about its usage or meaning in context.

6.2.2 The List and Record Specification Types

The List type is used to explain the evaluation of argument lists (see 13.3.8) in new expressions, in function calls, and in other algorithms where a simple ordered list of values is needed. Values of the List type are simply ordered sequences of list elements containing the individual values. These sequences may be of any length. The elements of a list may be randomly accessed using 0-origin indices. For notational convenience an array-like syntax can be used to access List elements. For example, arguments[2] is shorthand for saying the 3^rd element of the List arguments.

When an algorithm iterates over the elements of a List without specifying an order, the order used is the order of the elements in the List.

For notational convenience within this specification, a literal syntax can be used to express a new List value. For example, « 1, 2 » defines a List value that has two elements each of which is initialized to a specific value. A new empty List can be expressed as « ».

In this specification, the phrase "the list-concatenation of A, B, ..." (where each argument is a possibly empty List) denotes a new List value whose elements are the concatenation of the elements (in order) of each of the arguments (in order).

The Record type is used to describe data aggregations within the algorithms of this specification. A Record type value consists of one or more named fields. The value of each field is an ECMAScript language value or specification value. Field names are always enclosed in double brackets, for example [[Value]].

For notational convenience within this specification, an object literal-like syntax can be used to express a Record value. For example, { [[Field1]]: 42, [[Field2]]: false, [[Field3]]: empty } defines a Record value that has three fields, each of which is initialized to a specific value. Field name order is not significant. Any fields that are not explicitly listed are considered to be absent.

In specification text and algorithms, dot notation may be used to refer to a specific field of a Record value. For example, if R is the record shown in the previous paragraph then R.[[Field2]] is shorthand for “the field of R named [[Field2]]”.

Schema for commonly used Record field combinations may be named, and that name may be used as a prefix to a literal Record value to identify the specific kind of aggregations that is being described. For example: PropertyDescriptor { [[Value]]: 42, [[Writable]]: false, [[Configurable]]: true }.

6.2.3 The Set and Relation Specification Types

The Set type is used to explain a collection of unordered elements for use in the memory model. It is distinct from the ECMAScript collection type of the same name. To disambiguate, instances of the ECMAScript collection are consistently referred to as "Set objects" within this specification. Values of the Set type are simple collections of elements, where no element appears more than once. Elements may be added to and removed from Sets. Sets may be unioned, intersected, or subtracted from each other.

The Relation type is used to explain constraints on Sets. Values of the Relation type are Sets of ordered pairs of values from its value domain. For example, a Relation on events is a set of ordered pairs of events. For a Relation R and two values a and b in the value domain of R, a R b is shorthand for saying the ordered pair (a, b) is a member of R. A Relation is least with respect to some conditions when it is the smallest Relation that satisfies those conditions.

A strict partial order is a Relation value R that satisfies the following.

For all a, b, and c in R's domain:
- It is not the case that a R a, and
- If a R b and b R c, then a R c.

Note 1

The two properties above are called irreflexivity and transitivity, respectively.

A strict total order is a Relation value R that satisfies the following.

For all a, b, and c in R's domain:
- a is b or a R b or b R a, and
- It is not the case that a R a, and
- If a R b and b R c, then a R c.

Note 2

The three properties above are called totality, irreflexivity, and transitivity, respectively.

6.2.4 The Completion Record Specification Type

The Completion Record specification type is used to explain the runtime propagation of values and control flow such as the behaviour of statements (break, continue, return and throw) that perform nonlocal transfers of control.

Completion Records have the fields defined in Table 7.

Table 7: Completion Record Fields

Field Name	Value	Meaning
`[[Type]]`	normal, break, continue, return, or throw	The type of completion that occurred.
`[[Value]]`	any value except a Completion Record	The value that was produced.
`[[Target]]`	a String or empty	The target label for directed control transfers.

The following shorthand terms are sometimes used to refer to Completion Records.

normal completion refers to any Completion Record with a [[Type]] value of normal.
break completion refers to any Completion Record with a [[Type]] value of break.
continue completion refers to any Completion Record with a [[Type]] value of continue.
return completion refers to any Completion Record with a [[Type]] value of return.
throw completion refers to any Completion Record with a [[Type]] value of throw.
abrupt completion refers to any Completion Record with a [[Type]] value other than normal.
a normal completion containing some type of value refers to a normal completion that has a value of that type in its [[Value]] field.

Callable objects that are defined in this specification only return a normal completion or a throw completion. Returning any other kind of Completion Record is considered an editorial error.

Implementation-defined callable objects must return either a normal completion or a throw completion.

6.2.4.1 NormalCompletion ( `value` )

The abstract operation NormalCompletion takes argument value (any value except a Completion Record) and returns a normal completion. It performs the following steps when called:

Return Completion Record { [[Type]]: normal, [[Value]]: value, [[Target]]: empty }.

6.2.4.2 ThrowCompletion ( `value` )

The abstract operation ThrowCompletion takes argument value (an ECMAScript language value) and returns a throw completion. It performs the following steps when called:

Return Completion Record { [[Type]]: throw, [[Value]]: value, [[Target]]: empty }.

6.2.4.3 UpdateEmpty ( `completionRecord`, `value` )

The abstract operation UpdateEmpty takes arguments completionRecord (a Completion Record) and value (any value except a Completion Record) and returns a Completion Record. It performs the following steps when called:

Assert: If completionRecord is either a return completion or a throw completion, then completionRecord.[[Value]] is not empty.
If completionRecord.[[Value]] is not empty, return ? completionRecord.
Return Completion Record { [[Type]]: completionRecord.[[Type]], [[Value]]: value, [[Target]]: completionRecord.[[Target]] }.

6.2.5 The Reference Record Specification Type

The Reference Record type is used to explain the behaviour of such operators as delete, typeof, the assignment operators, the super keyword and other language features. For example, the left-hand operand of an assignment is expected to produce a Reference Record.

A Reference Record is a resolved name or property binding; its fields are defined by Table 8.

Table 8: Reference Record Fields

Field Name	Value	Meaning
`[[Base]]`	an ECMAScript language value, an Environment Record, or unresolvable	The value or Environment Record which holds the binding. A `[[Base]]` of unresolvable indicates that the binding could not be resolved.
`[[ReferencedName]]`	a String, a Symbol, or a Private Name	The name of the binding. Always a String if `[[Base]]` value is an Environment Record.
`[[Strict]]`	a Boolean	true if the Reference Record originated in strict mode code, false otherwise.
`[[ThisValue]]`	an ECMAScript language value or empty	If not empty, the Reference Record represents a property binding that was expressed using the `super` keyword; it is called a Super Reference Record and its `[[Base]]` value will never be an Environment Record. In that case, the `[[ThisValue]]` field holds the this value at the time the Reference Record was created.

The following abstract operations are used in this specification to operate upon Reference Records:

6.2.5.1 IsPropertyReference ( `V` )

The abstract operation IsPropertyReference takes argument V (a Reference Record) and returns a Boolean. It performs the following steps when called:

If V.[[Base]] is unresolvable, return false.
If V.[[Base]] is an Environment Record, return false; otherwise return true.

6.2.5.2 IsUnresolvableReference ( `V` )

The abstract operation IsUnresolvableReference takes argument V (a Reference Record) and returns a Boolean. It performs the following steps when called:

If V.[[Base]] is unresolvable, return true; otherwise return false.

6.2.5.3 IsSuperReference ( `V` )

The abstract operation IsSuperReference takes argument V (a Reference Record) and returns a Boolean. It performs the following steps when called:

If V.[[ThisValue]] is not empty, return true; otherwise return false.

6.2.5.4 IsPrivateReference ( `V` )

The abstract operation IsPrivateReference takes argument V (a Reference Record) and returns a Boolean. It performs the following steps when called:

If V.[[ReferencedName]] is a Private Name, return true; otherwise return false.

6.2.5.5 GetValue ( `V` )

The abstract operation GetValue takes argument V (a Reference Record or an ECMAScript language value) and returns either a normal completion containing an ECMAScript language value or an abrupt completion. It performs the following steps when called:

If V is not a Reference Record, return V.
If IsUnresolvableReference(V) is true, throw a ReferenceError exception.
3. If IsPropertyReference(V) is true, then
1. Let baseObj be ? ToObject(V.[[Base]]).
2. b. If IsPrivateReference(V) is true, then
  1. Return ? PrivateGet(baseObj, V.[[ReferencedName]]).
3. Return ? baseObj.[[Get]](V.[[ReferencedName]], GetThisValue(V)).
4. Else,
1. Let base be V.[[Base]].
2. Assert: base is an Environment Record.
3. Return ? base.GetBindingValue(V.[[ReferencedName]], V.[[Strict]]) (see 9.1).

Note

The object that may be created in step 3.a is not accessible outside of the above abstract operation and the ordinary object [[Get]] internal method. An implementation might choose to avoid the actual creation of the object.

6.2.5.6 PutValue ( `V`, `W` )

The abstract operation PutValue takes arguments V (a Reference Record or an ECMAScript language value) and W (an ECMAScript language value) and returns either a normal completion containing unused or an abrupt completion. It performs the following steps when called:

If V is not a Reference Record, throw a ReferenceError exception.
2. If IsUnresolvableReference(V) is true, then
1. If V.[[Strict]] is true, throw a ReferenceError exception.
2. Let globalObj be GetGlobalObject().
3. Perform ? Set(globalObj, V.[[ReferencedName]], W, false).
4. Return unused.
3. If IsPropertyReference(V) is true, then
1. Let baseObj be ? ToObject(V.[[Base]]).
2. b. If IsPrivateReference(V) is true, then
  1. Return ? PrivateSet(baseObj, V.[[ReferencedName]], W).
3. Let succeeded be ? baseObj.[[Set]](V.[[ReferencedName]], W, GetThisValue(V)).
4. If succeeded is false and V.[[Strict]] is true, throw a TypeError exception.
5. Return unused.
4. Else,
1. Let base be V.[[Base]].
2. Assert: base is an Environment Record.
3. Return ? base.SetMutableBinding(V.[[ReferencedName]], W, V.[[Strict]]) (see 9.1).

Note

The object that may be created in step 3.a is not accessible outside of the above abstract operation and the ordinary object [[Set]] internal method. An implementation might choose to avoid the actual creation of that object.

6.2.5.7 GetThisValue ( `V` )

The abstract operation GetThisValue takes argument V (a Reference Record) and returns an ECMAScript language value. It performs the following steps when called:

Assert: IsPropertyReference(V) is true.
If IsSuperReference(V) is true, return V.[[ThisValue]]; otherwise return V.[[Base]].

6.2.5.8 InitializeReferencedBinding ( `V`, `W` )

The abstract operation InitializeReferencedBinding takes arguments V (a Reference Record) and W (an ECMAScript language value) and returns either a normal completion containing unused or an abrupt completion. It performs the following steps when called:

Assert: IsUnresolvableReference(V) is false.
Let base be V.[[Base]].
Assert: base is an Environment Record.
Return ? base.InitializeBinding(V.[[ReferencedName]], W).

6.2.5.9 MakePrivateReference ( `baseValue`, `privateIdentifier` )

The abstract operation MakePrivateReference takes arguments baseValue (an ECMAScript language value) and privateIdentifier (a String) and returns a Reference Record. It performs the following steps when called:

Let privEnv be the running execution context's PrivateEnvironment.
Assert: privEnv is not null.
Let privateName be ResolvePrivateIdentifier(privEnv, privateIdentifier).
Return the Reference Record { [[Base]]: baseValue, [[ReferencedName]]: privateName, [[Strict]]: true, [[ThisValue]]: empty }.

6.2.6 The Property Descriptor Specification Type

The Property Descriptor type is used to explain the manipulation and reification of Object property attributes. A Property Descriptor is a Record with zero or more fields, where each field's name is an attribute name and its value is a corresponding attribute value as specified in 6.1.7.1. The schema name used within this specification to tag literal descriptions of Property Descriptor records is “PropertyDescriptor”.

Property Descriptor values may be further classified as data Property Descriptors and accessor Property Descriptors based upon the existence or use of certain fields. A data Property Descriptor is one that includes any fields named either [[Value]] or [[Writable]]. An accessor Property Descriptor is one that includes any fields named either [[Get]] or [[Set]]. Any Property Descriptor may have fields named [[Enumerable]] and [[Configurable]]. A Property Descriptor value may not be both a data Property Descriptor and an accessor Property Descriptor; however, it may be neither (in which case it is a generic Property Descriptor). A fully populated Property Descriptor is one that is either an accessor Property Descriptor or a data Property Descriptor and that has all of the corresponding fields defined in Table 3.

The following abstract operations are used in this specification to operate upon Property Descriptor values:

6.2.6.1 IsAccessorDescriptor ( `Desc` )

The abstract operation IsAccessorDescriptor takes argument Desc (a Property Descriptor or undefined) and returns a Boolean. It performs the following steps when called:

If Desc is undefined, return false.
If Desc has a [[Get]] field, return true.
If Desc has a [[Set]] field, return true.
Return false.

6.2.6.2 IsDataDescriptor ( `Desc` )

The abstract operation IsDataDescriptor takes argument Desc (a Property Descriptor or undefined) and returns a Boolean. It performs the following steps when called:

If Desc is undefined, return false.
If Desc has a [[Value]] field, return true.
If Desc has a [[Writable]] field, return true.
Return false.

6.2.6.3 IsGenericDescriptor ( `Desc` )

The abstract operation IsGenericDescriptor takes argument Desc (a Property Descriptor or undefined) and returns a Boolean. It performs the following steps when called:

If Desc is undefined, return false.
If IsAccessorDescriptor(Desc) is true, return false.
If IsDataDescriptor(Desc) is true, return false.
Return true.

6.2.6.4 FromPropertyDescriptor ( `Desc` )

The abstract operation FromPropertyDescriptor takes argument Desc (a Property Descriptor or undefined) and returns an Object or undefined. It performs the following steps when called:

If Desc is undefined, return undefined.
Let obj be OrdinaryObjectCreate(%Object.prototype%).
Assert: obj is an extensible ordinary object with no own properties.
4. If Desc has a [[Value]] field, then
1. Perform ! CreateDataPropertyOrThrow(obj, "value", Desc.[[Value]]).
5. If Desc has a [[Writable]] field, then
1. Perform ! CreateDataPropertyOrThrow(obj, "writable", Desc.[[Writable]]).
6. If Desc has a [[Get]] field, then
1. Perform ! CreateDataPropertyOrThrow(obj, "get", Desc.[[Get]]).
7. If Desc has a [[Set]] field, then
1. Perform ! CreateDataPropertyOrThrow(obj, "set", Desc.[[Set]]).
8. If Desc has an [[Enumerable]] field, then
1. Perform ! CreateDataPropertyOrThrow(obj, "enumerable", Desc.[[Enumerable]]).
9. If Desc has a [[Configurable]] field, then
1. Perform ! CreateDataPropertyOrThrow(obj, "configurable", Desc.[[Configurable]]).
Return obj.

6.2.6.5 ToPropertyDescriptor ( `Obj` )

The abstract operation ToPropertyDescriptor takes argument Obj (an ECMAScript language value) and returns either a normal completion containing a Property Descriptor or a throw completion. It performs the following steps when called:

If Obj is not an Object, throw a TypeError exception.
Let desc be a new Property Descriptor that initially has no fields.
Let hasEnumerable be ? HasProperty(Obj, "enumerable").
4. If hasEnumerable is true, then
1. Let enumerable be ToBoolean(? Get(Obj, "enumerable")).
2. Set desc.[[Enumerable]] to enumerable.
Let hasConfigurable be ? HasProperty(Obj, "configurable").
6. If hasConfigurable is true, then
1. Let configurable be ToBoolean(? Get(Obj, "configurable")).
2. Set desc.[[Configurable]] to configurable.
Let hasValue be ? HasProperty(Obj, "value").
8. If hasValue is true, then
1. Let value be ? Get(Obj, "value").
2. Set desc.[[Value]] to value.
Let hasWritable be ? HasProperty(Obj, "writable").
10. If hasWritable is true, then
1. Let writable be ToBoolean(? Get(Obj, "writable")).
2. Set desc.[[Writable]] to writable.
Let hasGet be ? HasProperty(Obj, "get").
12. If hasGet is true, then
1. Let getter be ? Get(Obj, "get").
2. If IsCallable(getter) is false and getter is not undefined, throw a TypeError exception.
3. Set desc.[[Get]] to getter.
Let hasSet be ? HasProperty(Obj, "set").
14. If hasSet is true, then
1. Let setter be ? Get(Obj, "set").
2. If IsCallable(setter) is false and setter is not undefined, throw a TypeError exception.
3. Set desc.[[Set]] to setter.
15. If desc has a [[Get]] field or desc has a [[Set]] field, then
1. If desc has a [[Value]] field or desc has a [[Writable]] field, throw a TypeError exception.
Return desc.

6.2.6.6 CompletePropertyDescriptor ( `Desc` )

The abstract operation CompletePropertyDescriptor takes argument Desc (a Property Descriptor) and returns unused. It performs the following steps when called:

Let like be the Record { [[Value]]: undefined, [[Writable]]: false, [[Get]]: undefined, [[Set]]: undefined, [[Enumerable]]: false, [[Configurable]]: false }.
2. If IsGenericDescriptor(Desc) is true or IsDataDescriptor(Desc) is true, then
1. If Desc does not have a [[Value]] field, set Desc.[[Value]] to like.[[Value]].
2. If Desc does not have a [[Writable]] field, set Desc.[[Writable]] to like.[[Writable]].
3. Else,
1. If Desc does not have a [[Get]] field, set Desc.[[Get]] to like.[[Get]].
2. If Desc does not have a [[Set]] field, set Desc.[[Set]] to like.[[Set]].
If Desc does not have an [[Enumerable]] field, set Desc.[[Enumerable]] to like.[[Enumerable]].
If Desc does not have a [[Configurable]] field, set Desc.[[Configurable]] to like.[[Configurable]].
Return unused.

6.2.7 The Environment Record Specification Type

The Environment Record type is used to explain the behaviour of name resolution in nested functions and blocks. This type and the operations upon it are defined in 9.1.

6.2.8 The Abstract Closure Specification Type

The Abstract Closure specification type is used to refer to algorithm steps together with a collection of values. Abstract Closures are meta-values and are invoked using function application style such as closure(arg1, arg2). Like abstract operations, invocations perform the algorithm steps described by the Abstract Closure.

In algorithm steps that create an Abstract Closure, values are captured with the verb "capture" followed by a list of aliases. When an Abstract Closure is created, it captures the value that is associated with each alias at that time. In steps that specify the algorithm to be performed when an Abstract Closure is called, each captured value is referred to by the alias that was used to capture the value.

If an Abstract Closure returns a Completion Record, that Completion Record must be either a normal completion or a throw completion.

Abstract Closures are created inline as part of other algorithms, shown in the following example.

Let addend be 41.
2. Let closure be a new Abstract Closure with parameters (x) that captures addend and performs the following steps when called:
1. Return x + addend.
Let val be closure(1).
Assert: val is 42.

6.2.9 Data Blocks

The Data Block specification type is used to describe a distinct and mutable sequence of byte-sized (8 bit) numeric values. A byte value is an integer in the inclusive interval from 0 to 255. A Data Block value is created with a fixed number of bytes that each have the initial value 0.

For notational convenience within this specification, an array-like syntax can be used to access the individual bytes of a Data Block value. This notation presents a Data Block value as a 0-origined integer-indexed sequence of bytes. For example, if db is a 5 byte Data Block value then db[2] can be used to access its 3^rd byte.

A data block that resides in memory that can be referenced from multiple agents concurrently is designated a Shared Data Block. A Shared Data Block has an identity (for the purposes of equality testing Shared Data Block values) that is address-free: it is tied not to the virtual addresses the block is mapped to in any process, but to the set of locations in memory that the block represents. Two data blocks are equal only if the sets of the locations they contain are equal; otherwise, they are not equal and the intersection of the sets of locations they contain is empty. Finally, Shared Data Blocks can be distinguished from Data Blocks.

The semantics of Shared Data Blocks is defined using Shared Data Block events by the memory model. Abstract operations below introduce Shared Data Block events and act as the interface between evaluation semantics and the event semantics of the memory model. The events form a candidate execution, on which the memory model acts as a filter. Please consult the memory model for full semantics.

Shared Data Block events are modeled by Records, defined in the memory model.

The following abstract operations are used in this specification to operate upon Data Block values:

6.2.9.1 CreateByteDataBlock ( `size` )

The abstract operation CreateByteDataBlock takes argument size (a non-negative integer) and returns either a normal completion containing a Data Block or a throw completion. It performs the following steps when called:

If size > 2⁵³ - 1, throw a RangeError exception.
Let db be a new Data Block value consisting of size bytes. If it is impossible to create such a Data Block, throw a RangeError exception.
Set all of the bytes of db to 0.
Return db.

6.2.9.2 CreateSharedByteDataBlock ( `size` )

The abstract operation CreateSharedByteDataBlock takes argument size (a non-negative integer) and returns either a normal completion containing a Shared Data Block or a throw completion. It performs the following steps when called:

Let db be a new Shared Data Block value consisting of size bytes. If it is impossible to create such a Shared Data Block, throw a RangeError exception.
Let execution be the [[CandidateExecution]] field of the surrounding agent's Agent Record.
Let eventsRecord be the Agent Events Record of execution.[[EventsRecords]] whose [[AgentSignifier]] is AgentSignifier().
Let zero be « 0 ».
5. For each index i of db, do
1. Append WriteSharedMemory { [[Order]]: init, [[NoTear]]: true, [[Block]]: db, [[ByteIndex]]: i, [[ElementSize]]: 1, [[Payload]]: zero } to eventsRecord.[[EventList]].
Return db.

6.2.9.3 CopyDataBlockBytes ( `toBlock`, `toIndex`, `fromBlock`, `fromIndex`, `count` )

The abstract operation CopyDataBlockBytes takes arguments toBlock (a Data Block or a Shared Data Block), toIndex (a non-negative integer), fromBlock (a Data Block or a Shared Data Block), fromIndex (a non-negative integer), and count (a non-negative integer) and returns unused. It performs the following steps when called:

Assert: fromBlock and toBlock are distinct values.
Let fromSize be the number of bytes in fromBlock.
Assert: fromIndex + count ≤ fromSize.
Let toSize be the number of bytes in toBlock.
Assert: toIndex + count ≤ toSize.
6. Repeat, while count > 0,
1. a. If fromBlock is a Shared Data Block, then
  1. Let execution be the [[CandidateExecution]] field of the surrounding agent's Agent Record.
  2. Let eventsRecord be the Agent Events Record of execution.[[EventsRecords]] whose [[AgentSignifier]] is AgentSignifier().
  3. Let bytes be a List whose sole element is a nondeterministically chosen byte value.
  4. NOTE: In implementations, bytes is the result of a non-atomic read instruction on the underlying hardware. The nondeterminism is a semantic prescription of the memory model to describe observable behaviour of hardware with weak consistency.
  5. Let readEvent be ReadSharedMemory { [[Order]]: unordered, [[NoTear]]: true, [[Block]]: fromBlock, [[ByteIndex]]: fromIndex, [[ElementSize]]: 1 }.
  6. Append readEvent to eventsRecord.[[EventList]].
  7. Append Chosen Value Record { [[Event]]: readEvent, [[ChosenValue]]: bytes } to execution.[[ChosenValues]].
  8. viii. If toBlock is a Shared Data Block, then
    1. Append WriteSharedMemory { [[Order]]: unordered, [[NoTear]]: true, [[Block]]: toBlock, [[ByteIndex]]: toIndex, [[ElementSize]]: 1, [[Payload]]: bytes } to eventsRecord.[[EventList]].
  9. ix. Else,
    1. Set toBlock[toIndex] to bytes[0].
2. b. Else,
  1. Assert: toBlock is not a Shared Data Block.
  2. Set toBlock[toIndex] to fromBlock[fromIndex].
3. Set toIndex to toIndex + 1.
4. Set fromIndex to fromIndex + 1.
5. Set count to count - 1.
Return unused.

6.2.10 The PrivateElement Specification Type

The PrivateElement type is a Record used in the specification of private class fields, methods, and accessors. Although Property Descriptors are not used for private elements, private fields behave similarly to non-configurable, non-enumerable, writable data properties, private methods behave similarly to non-configurable, non-enumerable, non-writable data properties, and private accessors behave similarly to non-configurable, non-enumerable accessor properties.

Values of the PrivateElement type are Record values whose fields are defined by Table 9. Such values are referred to as PrivateElements.

Table 9: PrivateElement Fields

Field Name	Values of the `[[Kind]]` field for which it is present	Value	Meaning
`[[Key]]`	All	a Private Name	The name of the field, method, or accessor.
`[[Kind]]`	All	field, method, or accessor	The kind of the element.
`[[Value]]`	field and method	an ECMAScript language value	The value of the field.
`[[Get]]`	accessor	a function object or undefined	The getter for a private accessor.
`[[Set]]`	accessor	a function object or undefined	The setter for a private accessor.

6.2.11 The ClassFieldDefinition Record Specification Type

The ClassFieldDefinition type is a Record used in the specification of class fields.

Values of the ClassFieldDefinition type are Record values whose fields are defined by Table 10. Such values are referred to as ClassFieldDefinition Records.

Table 10: ClassFieldDefinition Record Fields

Field Name	Value	Meaning
`[[Name]]`	a Private Name, a String, or a Symbol	The name of the field.
`[[Initializer]]`	an ECMAScript function object or empty	The initializer of the field, if any.

6.2.12 Private Names

The Private Name specification type is used to describe a globally unique value (one which differs from any other Private Name, even if they are otherwise indistinguishable) which represents the key of a private class element (field, method, or accessor). Each Private Name has an associated immutable [[Description]] which is a String value. A Private Name may be installed on any ECMAScript object with PrivateFieldAdd or PrivateMethodOrAccessorAdd, and then read or written using PrivateGet and PrivateSet.

6.2.13 The ClassStaticBlockDefinition Record Specification Type

A ClassStaticBlockDefinition Record is a Record value used to encapsulate the executable code for a class static initialization block.

ClassStaticBlockDefinition Records have the fields listed in Table 11.

Table 11: ClassStaticBlockDefinition Record Fields

Field Name	Value	Meaning
`[[BodyFunction]]`	an ECMAScript function object	The function object to be called during static initialization of a class.

7 Abstract Operations

These operations are not a part of the ECMAScript language; they are defined here solely to aid the specification of the semantics of the ECMAScript language. Other, more specialized abstract operations are defined throughout this specification.

7.1 Type Conversion

The ECMAScript language implicitly performs automatic type conversion as needed. To clarify the semantics of certain constructs it is useful to define a set of conversion abstract operations. The conversion abstract operations are polymorphic; they can accept a value of any ECMAScript language type. But no other specification types are used with these operations.

The BigInt type has no implicit conversions in the ECMAScript language; programmers must call BigInt explicitly to convert values from other types.

7.1.1 ToPrimitive ( `input` [ , `preferredType` ] )

The abstract operation ToPrimitive takes argument input (an ECMAScript language value) and optional argument preferredType (string or number) and returns either a normal completion containing an ECMAScript language value or a throw completion. It converts its input argument to a non-Object type. If an object is capable of converting to more than one primitive type, it may use the optional hint preferredType to favour that type. It performs the following steps when called:

1. If input is an Object, then
1. Let exoticToPrim be ? GetMethod(input, @@toPrimitive).
2. b. If exoticToPrim is not undefined, then
  1. i. If preferredType is not present, then
    1. Let hint be "default".
  2. ii. Else if preferredType is string, then
    1. Let hint be "string".
  3. iii. Else,
    1. Assert: preferredType is number.
    2. Let hint be "number".
  4. Let result be ? Call(exoticToPrim, input, « hint »).
  5. If result is not an Object, return result.
  6. Throw a TypeError exception.
3. If preferredType is not present, let preferredType be number.
4. Return ? OrdinaryToPrimitive(input, preferredType).
Return input.

Note

When ToPrimitive is called without a hint, then it generally behaves as if the hint were number. However, objects may over-ride this behaviour by defining a @@toPrimitive method. Of the objects defined in this specification only Dates (see 21.4.4.45) and Symbol objects (see 20.4.3.5) over-ride the default ToPrimitive behaviour. Dates treat the absence of a hint as if the hint were string.

7.1.1.1 OrdinaryToPrimitive ( `O`, `hint` )

The abstract operation OrdinaryToPrimitive takes arguments O (an Object) and hint (string or number) and returns either a normal completion containing an ECMAScript language value or a throw completion. It performs the following steps when called:

1. If hint is string, then
1. Let methodNames be « "toString", "valueOf" ».
2. Else,
1. Let methodNames be « "valueOf", "toString" ».
3. For each element name of methodNames, do
1. Let method be ? Get(O, name).
2. b. If IsCallable(method) is true, then
  1. Let result be ? Call(method, O).
  2. If result is not an Object, return result.
Throw a TypeError exception.

7.1.2 ToBoolean ( `argument` )

The abstract operation ToBoolean takes argument argument (an ECMAScript language value) and returns a Boolean. It converts argument to a value of type Boolean. It performs the following steps when called:

If argument is a Boolean, return argument.
If argument is one of undefined, null, +0_𝔽, -0_𝔽, NaN, 0_ℤ, or the empty String, return false.
NOTE: This step is replaced in section B.3.6.1.
Return true.

7.1.3 ToNumeric ( `value` )

The abstract operation ToNumeric takes argument value (an ECMAScript language value) and returns either a normal completion containing either a Number or a BigInt, or a throw completion. It returns value converted to a Number or a BigInt. It performs the following steps when called:

Let primValue be ? ToPrimitive(value, number).
If primValue is a BigInt, return primValue.
Return ? ToNumber(primValue).

7.1.4 ToNumber ( `argument` )

The abstract operation ToNumber takes argument argument (an ECMAScript language value) and returns either a normal completion containing a Number or a throw completion. It converts argument to a value of type Number. It performs the following steps when called:

If argument is a Number, return argument.
If argument is either a Symbol or a BigInt, throw a TypeError exception.
If argument is undefined, return NaN.
If argument is either null or false, return +0_𝔽.
If argument is true, return 1_𝔽.
If argument is a String, return StringToNumber(argument).
Assert: argument is an Object.
Let primValue be ? ToPrimitive(argument, number).
Assert: primValue is not an Object.
Return ? ToNumber(primValue).

7.1.4.1 ToNumber Applied to the String Type

The abstract operation StringToNumber specifies how to convert a String value to a Number value, using the following grammar.

Syntax

:::

opt

opt

opt

:::

opt

:::

:::

NonDecimalIntegerLiteral

[~Sep]

StrDecimalLiteral

:::

StrUnsignedDecimalLiteral

:::

Infinity

DecimalDigits

[~Sep]

DecimalDigits

[~Sep]

opt

ExponentPart

[~Sep]

opt

DecimalDigits

[~Sep]

ExponentPart

[~Sep]

opt

DecimalDigits

[~Sep]

ExponentPart

[~Sep]

opt

All grammar symbols not explicitly defined above have the definitions used in the Lexical Grammar for numeric literals (12.9.3)

Note

Some differences should be noted between the syntax of a StringNumericLiteral and a NumericLiteral:

A StringNumericLiteral may include leading and/or trailing white space and/or line terminators.
A StringNumericLiteral that is decimal may have any number of leading 0 digits.
A StringNumericLiteral that is decimal may include a + or - to indicate its sign.
A StringNumericLiteral that is empty or contains only white space is converted to +0_𝔽.
Infinity and -Infinity are recognized as a StringNumericLiteral but not as a NumericLiteral.
A StringNumericLiteral cannot include a BigIntLiteralSuffix.
A StringNumericLiteral cannot include a NumericLiteralSeparator.

7.1.4.1.1 StringToNumber ( `str` )

The abstract operation StringToNumber takes argument str (a String) and returns a Number. It performs the following steps when called:

Let text be StringToCodePoints(str).
Let literal be ParseText(text, StringNumericLiteral).
If literal is a List of errors, return NaN.
Return StringNumericValue of literal.

7.1.4.1.2 Runtime Semantics: StringNumericValue

The syntax-directed operation StringNumericValue takes no arguments and returns a Number.

Note

The conversion of a StringNumericLiteral to a Number value is similar overall to the determination of the NumericValue of a NumericLiteral (see 12.9.3), but some of the details are different.

It is defined piecewise over the following productions:

StringNumericLiteral

:::

StrWhiteSpace

opt

Return +0_𝔽.

:::

opt

opt

Return StringNumericValue of StrNumericLiteral.

StrNumericLiteral

:::

NonDecimalIntegerLiteral

Return 𝔽(MV of NonDecimalIntegerLiteral).

StrDecimalLiteral

:::

StrUnsignedDecimalLiteral

Let a be StringNumericValue of StrUnsignedDecimalLiteral.
If a is +0_𝔽, return -0_𝔽.
Return -a.

StrUnsignedDecimalLiteral

:::

Infinity

Return +∞_𝔽.

StrUnsignedDecimalLiteral

:::

DecimalDigits

opt

ExponentPart

opt

Let a be MV of the first DecimalDigits.
2. If the second DecimalDigits is present, then
1. Let b be MV of the second DecimalDigits.
2. Let n be the number of code points in the second DecimalDigits.
3. Else,
1. Let b be 0.
2. Let n be 0.
If ExponentPart is present, let e be MV of ExponentPart. Otherwise, let e be 0.
Return RoundMVResult((a + (b × 10^-n)) × 10^e).

StrUnsignedDecimalLiteral

:::

DecimalDigits

ExponentPart

opt

Let b be MV of DecimalDigits.
If ExponentPart is present, let e be MV of ExponentPart. Otherwise, let e be 0.
Let n be the number of code points in DecimalDigits.
Return RoundMVResult(b × 10^{e - n}).

StrUnsignedDecimalLiteral

:::

DecimalDigits

ExponentPart

opt

Let a be MV of DecimalDigits.
If ExponentPart is present, let e be MV of ExponentPart. Otherwise, let e be 0.
Return RoundMVResult(a × 10^e).

7.1.4.1.3 RoundMVResult ( `n` )

The abstract operation RoundMVResult takes argument n (a mathematical value) and returns a Number. It converts n to a Number in an implementation-defined manner. For the purposes of this abstract operation, a digit is significant if it is not zero or there is a non-zero digit to its left and there is a non-zero digit to its right. For the purposes of this abstract operation, "the mathematical value denoted by" a representation of a mathematical value is the inverse of "the decimal representation of" a mathematical value. It performs the following steps when called:

If the decimal representation of n has 20 or fewer significant digits, return 𝔽(n).
Let option1 be the mathematical value denoted by the result of replacing each significant digit in the decimal representation of n after the 20th with a 0 digit.
Let option2 be the mathematical value denoted by the result of replacing each significant digit in the decimal representation of n after the 20th with a 0 digit and then incrementing it at the 20th position (with carrying as necessary).
Let chosen be an implementation-defined choice of either option1 or option2.
Return 𝔽(chosen).

7.1.5 ToIntegerOrInfinity ( `argument` )

The abstract operation ToIntegerOrInfinity takes argument argument (an ECMAScript language value) and returns either a normal completion containing either an integer, +∞, or -∞, or a throw completion. It converts argument to an integer representing its Number value with fractional part truncated, or to +∞ or -∞ when that Number value is infinite. It performs the following steps when called:

Let number be ? ToNumber(argument).
If number is one of NaN, +0_𝔽, or -0_𝔽, return 0.
If number is +∞_𝔽, return +∞.
If number is -∞_𝔽, return -∞.
Return truncate(ℝ(number)).

Note

𝔽(ToIntegerOrInfinity(x)) never returns -0_𝔽 for any value of x. The truncation of the fractional part is performed after converting x to a mathematical value.

7.1.6 ToInt32 ( `argument` )

The abstract operation ToInt32 takes argument argument (an ECMAScript language value) and returns either a normal completion containing an integral Number or a throw completion. It converts argument to one of 2³² integral Number values in the inclusive interval from 𝔽(-2³¹) to 𝔽(2³¹ - 1). It performs the following steps when called:

Let number be ? ToNumber(argument).
If number is not finite or number is either +0_𝔽 or -0_𝔽, return +0_𝔽.
Let int be truncate(ℝ(number)).
Let int32bit be int modulo 2³².
If int32bit ≥ 2³¹, return 𝔽(int32bit - 2³²); otherwise return 𝔽(int32bit).

Note

Given the above definition of ToInt32:

The ToInt32 abstract operation is idempotent: if applied to a result that it produced, the second application leaves that value unchanged.
ToInt32(ToUint32(x)) is the same value as ToInt32(x) for all values of x. (It is to preserve this latter property that +∞_𝔽 and -∞_𝔽 are mapped to +0_𝔽.)
ToInt32 maps -0_𝔽 to +0_𝔽.

7.1.7 ToUint32 ( `argument` )

The abstract operation ToUint32 takes argument argument (an ECMAScript language value) and returns either a normal completion containing an integral Number or a throw completion. It converts argument to one of 2³² integral Number values in the inclusive interval from +0_𝔽 to 𝔽(2³² - 1). It performs the following steps when called:

Let number be ? ToNumber(argument).
If number is not finite or number is either +0_𝔽 or -0_𝔽, return +0_𝔽.
Let int be truncate(ℝ(number)).
Let int32bit be int modulo 2³².
Return 𝔽(int32bit).

Note

Given the above definition of ToUint32:

Step 5 is the only difference between ToUint32 and ToInt32.
The ToUint32 abstract operation is idempotent: if applied to a result that it produced, the second application leaves that value unchanged.
ToUint32(ToInt32(x)) is the same value as ToUint32(x) for all values of x. (It is to preserve this latter property that +∞_𝔽 and -∞_𝔽 are mapped to +0_𝔽.)
ToUint32 maps -0_𝔽 to +0_𝔽.

7.1.8 ToInt16 ( `argument` )

The abstract operation ToInt16 takes argument argument (an ECMAScript language value) and returns either a normal completion containing an integral Number or a throw completion. It converts argument to one of 2¹⁶ integral Number values in the inclusive interval from 𝔽(-2¹⁵) to 𝔽(2¹⁵ - 1). It performs the following steps when called:

Let number be ? ToNumber(argument).
If number is not finite or number is either +0_𝔽 or -0_𝔽, return +0_𝔽.
Let int be truncate(ℝ(number)).
Let int16bit be int modulo 2¹⁶.
If int16bit ≥ 2¹⁵, return 𝔽(int16bit - 2¹⁶); otherwise return 𝔽(int16bit).

7.1.9 ToUint16 ( `argument` )

The abstract operation ToUint16 takes argument argument (an ECMAScript language value) and returns either a normal completion containing an integral Number or a throw completion. It converts argument to one of 2¹⁶ integral Number values in the inclusive interval from +0_𝔽 to 𝔽(2¹⁶ - 1). It performs the following steps when called:

Let number be ? ToNumber(argument).
If number is not finite or number is either +0_𝔽 or -0_𝔽, return +0_𝔽.
Let int be truncate(ℝ(number)).
Let int16bit be int modulo 2¹⁶.
Return 𝔽(int16bit).

Note

Given the above definition of ToUint16:

The substitution of 2¹⁶ for 2³² in step 4 is the only difference between ToUint32 and ToUint16.
ToUint16 maps -0_𝔽 to +0_𝔽.

7.1.10 ToInt8 ( `argument` )

The abstract operation ToInt8 takes argument argument (an ECMAScript language value) and returns either a normal completion containing an integral Number or a throw completion. It converts argument to one of 2⁸ integral Number values in the inclusive interval from -128_𝔽 to 127_𝔽. It performs the following steps when called:

Let number be ? ToNumber(argument).
If number is not finite or number is either +0_𝔽 or -0_𝔽, return +0_𝔽.
Let int be truncate(ℝ(number)).
Let int8bit be int modulo 2⁸.
If int8bit ≥ 2⁷, return 𝔽(int8bit - 2⁸); otherwise return 𝔽(int8bit).

7.1.11 ToUint8 ( `argument` )

The abstract operation ToUint8 takes argument argument (an ECMAScript language value) and returns either a normal completion containing an integral Number or a throw completion. It converts argument to one of 2⁸ integral Number values in the inclusive interval from +0_𝔽 to 255_𝔽. It performs the following steps when called:

Let number be ? ToNumber(argument).
If number is not finite or number is either +0_𝔽 or -0_𝔽, return +0_𝔽.
Let int be truncate(ℝ(number)).
Let int8bit be int modulo 2⁸.
Return 𝔽(int8bit).

7.1.12 ToUint8Clamp ( `argument` )

The abstract operation ToUint8Clamp takes argument argument (an ECMAScript language value) and returns either a normal completion containing an integral Number or a throw completion. It clamps and rounds argument to one of 2⁸ integral Number values in the inclusive interval from +0_𝔽 to 255_𝔽. It performs the following steps when called:

Let number be ? ToNumber(argument).
If number is NaN, return +0_𝔽.
Let mv be the extended mathematical value of number.
Let clamped be the result of clamping mv between 0 and 255.
Let f be floor(clamped).
If clamped < f + 0.5, return 𝔽(f).
If clamped > f + 0.5, return 𝔽(f + 1).
If f is even, return 𝔽(f). Otherwise, return 𝔽(f + 1).

Note

Unlike most other ECMAScript integer conversion operations, ToUint8Clamp rounds rather than truncates non-integral values. It also uses “round half to even” tie-breaking, which differs from the “round half up” tie-breaking of Math.round.

7.1.13 ToBigInt ( `argument` )

The abstract operation ToBigInt takes argument argument (an ECMAScript language value) and returns either a normal completion containing a BigInt or a throw completion. It converts argument to a BigInt value, or throws if an implicit conversion from Number would be required. It performs the following steps when called:

Let prim be ? ToPrimitive(argument, number).
Return the value that prim corresponds to in Table 12.

Table 12: BigInt Conversions

Argument Type	Result
Undefined	Throw a TypeError exception.
Null	Throw a TypeError exception.
Boolean	Return `1n` if `prim` is true and `0n` if `prim` is false.
BigInt	Return `prim`.
Number	Throw a TypeError exception.
String	Let `n` be StringToBigInt(`prim`). If `n` is undefined, throw a SyntaxError exception. Return `n`.
Symbol	Throw a TypeError exception.

7.1.14 StringToBigInt ( `str` )

The abstract operation StringToBigInt takes argument str (a String) and returns a BigInt or undefined. It performs the following steps when called:

Let text be StringToCodePoints(str).
Let literal be ParseText(text, StringIntegerLiteral).
If literal is a List of errors, return undefined.
Let mv be the MV of literal.
Assert: mv is an integer.
Return ℤ(mv).

7.1.14.1 StringIntegerLiteral Grammar

StringToBigInt uses the following grammar.

Syntax

:::

opt

opt

opt

:::

[~Sep]

NonDecimalIntegerLiteral

[~Sep]

7.1.14.2 Runtime Semantics: MV

The MV of StringIntegerLiteral ::: StrWhiteSpaceopt is 0.
The MV of StringIntegerLiteral ::: StrWhiteSpaceopt StrIntegerLiteral StrWhiteSpaceopt is the MV of StrIntegerLiteral.

7.1.15 ToBigInt64 ( `argument` )

The abstract operation ToBigInt64 takes argument argument (an ECMAScript language value) and returns either a normal completion containing a BigInt or a throw completion. It converts argument to one of 2⁶⁴ BigInt values in the inclusive interval from ℤ(-2⁶³) to ℤ(2⁶³ - 1). It performs the following steps when called:

Let n be ? ToBigInt(argument).
Let int64bit be ℝ(n) modulo 2⁶⁴.
If int64bit ≥ 2⁶³, return ℤ(int64bit - 2⁶⁴); otherwise return ℤ(int64bit).

7.1.16 ToBigUint64 ( `argument` )

The abstract operation ToBigUint64 takes argument argument (an ECMAScript language value) and returns either a normal completion containing a BigInt or a throw completion. It converts argument to one of 2⁶⁴ BigInt values in the inclusive interval from 0_ℤ to ℤ(2⁶⁴ - 1). It performs the following steps when called:

Let n be ? ToBigInt(argument).
Let int64bit be ℝ(n) modulo 2⁶⁴.
Return ℤ(int64bit).

7.1.17 ToString ( `argument` )

The abstract operation ToString takes argument argument (an ECMAScript language value) and returns either a normal completion containing a String or a throw completion. It converts argument to a value of type String. It performs the following steps when called:

If argument is a String, return argument.
If argument is a Symbol, throw a TypeError exception.
If argument is undefined, return "undefined".
If argument is null, return "null".
If argument is true, return "true".
If argument is false, return "false".
If argument is a Number, return Number::toString(argument, 10).
If argument is a BigInt, return BigInt::toString(argument, 10).
Assert: argument is an Object.
Let primValue be ? ToPrimitive(argument, string).
Assert: primValue is not an Object.
Return ? ToString(primValue).

7.1.18 ToObject ( `argument` )

The abstract operation ToObject takes argument argument (an ECMAScript language value) and returns either a normal completion containing an Object or a throw completion. It converts argument to a value of type Object according to Table 13:

Table 13: ToObject Conversions

Argument Type	Result
Undefined	Throw a TypeError exception.
Null	Throw a TypeError exception.
Boolean	Return a new Boolean object whose `[[BooleanData]]` internal slot is set to `argument`. See 20.3 for a description of Boolean objects.
Number	Return a new Number object whose `[[NumberData]]` internal slot is set to `argument`. See 21.1 for a description of Number objects.
String	Return a new String object whose `[[StringData]]` internal slot is set to `argument`. See 22.1 for a description of String objects.
Symbol	Return a new Symbol object whose `[[SymbolData]]` internal slot is set to `argument`. See 20.4 for a description of Symbol objects.
BigInt	Return a new BigInt object whose `[[BigIntData]]` internal slot is set to `argument`. See 21.2 for a description of BigInt objects.
Object	Return `argument`.

7.1.19 ToPropertyKey ( `argument` )

The abstract operation ToPropertyKey takes argument argument (an ECMAScript language value) and returns either a normal completion containing a property key or a throw completion. It converts argument to a value that can be used as a property key. It performs the following steps when called:

Let key be ? ToPrimitive(argument, string).
2. If key is a Symbol, then
1. Return key.
Return ! ToString(key).

7.1.20 ToLength ( `argument` )

The abstract operation ToLength takes argument argument (an ECMAScript language value) and returns either a normal completion containing an integral Number or a throw completion. It clamps and truncates argument to an integral Number suitable for use as the length of an array-like object. It performs the following steps when called:

Let len be ? ToIntegerOrInfinity(argument).
If len ≤ 0, return +0_𝔽.
Return 𝔽(min(len, 2⁵³ - 1)).

7.1.21 CanonicalNumericIndexString ( `argument` )

The abstract operation CanonicalNumericIndexString takes argument argument (a String) and returns a Number or undefined. If argument is either "-0" or exactly matches the result of ToString(n) for some Number value n, it returns the respective Number value. Otherwise, it returns undefined. It performs the following steps when called:

If argument is "-0", return -0_𝔽.
Let n be ! ToNumber(argument).
If ! ToString(n) is argument, return n.
Return undefined.

A canonical numeric string is any String value for which the CanonicalNumericIndexString abstract operation does not return undefined.

7.1.22 ToIndex ( `value` )

The abstract operation ToIndex takes argument value (an ECMAScript language value) and returns either a normal completion containing a non-negative integer or a throw completion. It converts value to an integer and returns that integer if it is non-negative and corresponds with an integer index. Otherwise, it throws an exception. It performs the following steps when called:

Let integer be ? ToIntegerOrInfinity(value).
If integer is not in the inclusive interval from 0 to 2⁵³ - 1, throw a RangeError exception.
Return integer.

7.2 Testing and Comparison Operations

7.2.1 RequireObjectCoercible ( `argument` )

The abstract operation RequireObjectCoercible takes argument argument (an ECMAScript language value) and returns either a normal completion containing an ECMAScript language value or a throw completion. It throws an error if argument is a value that cannot be converted to an Object using ToObject. It is defined by Table 14:

Table 14: RequireObjectCoercible Results

Argument Type	Result
Undefined	Throw a TypeError exception.
Null	Throw a TypeError exception.
Boolean	Return `argument`.
Number	Return `argument`.
String	Return `argument`.
Symbol	Return `argument`.
BigInt	Return `argument`.
Object	Return `argument`.

7.2.2 IsArray ( `argument` )

The abstract operation IsArray takes argument argument (an ECMAScript language value) and returns either a normal completion containing a Boolean or a throw completion. It performs the following steps when called:

If argument is not an Object, return false.
If argument is an Array exotic object, return true.
3. If argument is a Proxy exotic object, then
1. Perform ? ValidateNonRevokedProxy(argument).
2. Let proxyTarget be argument.[[ProxyTarget]].
3. Return ? IsArray(proxyTarget).
Return false.

7.2.3 IsCallable ( `argument` )

The abstract operation IsCallable takes argument argument (an ECMAScript language value) and returns a Boolean. It determines if argument is a callable function with a [[Call]] internal method. It performs the following steps when called:

If argument is not an Object, return false.
If argument has a [[Call]] internal method, return true.
Return false.

7.2.4 IsConstructor ( `argument` )

The abstract operation IsConstructor takes argument argument (an ECMAScript language value) and returns a Boolean. It determines if argument is a function object with a [[Construct]] internal method. It performs the following steps when called:

If argument is not an Object, return false.
If argument has a [[Construct]] internal method, return true.
Return false.

7.2.5 IsExtensible ( `O` )

The abstract operation IsExtensible takes argument O (an Object) and returns either a normal completion containing a Boolean or a throw completion. It is used to determine whether additional properties can be added to O. It performs the following steps when called:

Return ? O.[[IsExtensible]]().

7.2.6 IsIntegralNumber ( `argument` )

The abstract operation IsIntegralNumber takes argument argument (an ECMAScript language value) and returns a Boolean. It determines if argument is a finite integral Number value. It performs the following steps when called:

If argument is not a Number, return false.
If argument is not finite, return false.
If truncate(ℝ(argument)) ≠ ℝ(argument), return false.
Return true.

7.2.7 IsPropertyKey ( `argument` )

The abstract operation IsPropertyKey takes argument argument (an ECMAScript language value) and returns a Boolean. It determines if argument is a value that may be used as a property key. It performs the following steps when called:

If argument is a String, return true.
If argument is a Symbol, return true.
Return false.

7.2.8 IsRegExp ( `argument` )

The abstract operation IsRegExp takes argument argument (an ECMAScript language value) and returns either a normal completion containing a Boolean or a throw completion. It performs the following steps when called:

If argument is not an Object, return false.
Let matcher be ? Get(argument, @@match).
If matcher is not undefined, return ToBoolean(matcher).
If argument has a [[RegExpMatcher]] internal slot, return true.
Return false.

7.2.9 Static Semantics: IsStringWellFormedUnicode ( `string` )

The abstract operation IsStringWellFormedUnicode takes argument string (a String) and returns a Boolean. It interprets string as a sequence of UTF-16 encoded code points, as described in 6.1.4, and determines whether it is a well formed UTF-16 sequence. It performs the following steps when called:

Let len be the length of string.
Let k be 0.
3. Repeat, while k < len,
1. Let cp be CodePointAt(string, k).
2. If cp.[[IsUnpairedSurrogate]] is true, return false.
3. Set k to k + cp.[[CodeUnitCount]].
Return true.

7.2.10 SameValue ( `x`, `y` )

The abstract operation SameValue takes arguments x (an ECMAScript language value) and y (an ECMAScript language value) and returns a Boolean. It determines whether or not the two arguments are the same value. It performs the following steps when called:

If Type(x) is not Type(y), return false.
2. If x is a Number, then
1. Return Number::sameValue(x, y).
Return SameValueNonNumber(x, y).

Note

This algorithm differs from the IsStrictlyEqual Algorithm by treating all NaN values as equivalent and by differentiating +0_𝔽 from -0_𝔽.

7.2.11 SameValueZero ( `x`, `y` )

The abstract operation SameValueZero takes arguments x (an ECMAScript language value) and y (an ECMAScript language value) and returns a Boolean. It determines whether or not the two arguments are the same value (ignoring the difference between +0_𝔽 and -0_𝔽). It performs the following steps when called:

If Type(x) is not Type(y), return false.
2. If x is a Number, then
1. Return Number::sameValueZero(x, y).
Return SameValueNonNumber(x, y).

Note

SameValueZero differs from SameValue only in that it treats +0_𝔽 and -0_𝔽 as equivalent.

7.2.12 SameValueNonNumber ( `x`, `y` )

The abstract operation SameValueNonNumber takes arguments x (an ECMAScript language value, but not a Number) and y (an ECMAScript language value, but not a Number) and returns a Boolean. It performs the following steps when called:

Assert: Type(x) is Type(y).
If x is either null or undefined, return true.
3. If x is a BigInt, then
1. Return BigInt::equal(x, y).
4. If x is a String, then
1. If x and y have the same length and the same code units in the same positions, return true; otherwise, return false.
5. If x is a Boolean, then
1. If x and y are both true or both false, return true; otherwise, return false.
NOTE: All other ECMAScript language values are compared by identity.
If x is y, return true; otherwise, return false.

Note 1

For expository purposes, some cases are handled separately within this algorithm even if it is unnecessary to do so.

Note 2

The specifics of what "x is y" means are detailed in 5.2.7.

7.2.13 IsLessThan ( `x`, `y`, `LeftFirst` )

The abstract operation IsLessThan takes arguments x (an ECMAScript language value), y (an ECMAScript language value), and LeftFirst (a Boolean) and returns either a normal completion containing either a Boolean or undefined, or a throw completion. It provides the semantics for the comparison x < y, returning true, false, or undefined (which indicates that at least one operand is NaN). The LeftFirst flag is used to control the order in which operations with potentially visible side-effects are performed upon x and y. It is necessary because ECMAScript specifies left to right evaluation of expressions. If LeftFirst is true, the x parameter corresponds to an expression that occurs to the left of the y parameter's corresponding expression. If LeftFirst is false, the reverse is the case and operations must be performed upon y before x. It performs the following steps when called:

1. If LeftFirst is true, then
1. Let px be ? ToPrimitive(x, number).
2. Let py be ? ToPrimitive(y, number).
2. Else,
1. NOTE: The order of evaluation needs to be reversed to preserve left to right evaluation.
2. Let py be ? ToPrimitive(y, number).
3. Let px be ? ToPrimitive(x, number).
3. If px is a String and py is a String, then
1. Let lx be the length of px.
2. Let ly be the length of py.
3. c. For each integer i such that 0 ≤ i < min(lx, ly), in ascending order, do
  1. Let cx be the numeric value of the code unit at index i within px.
  2. Let cy be the numeric value of the code unit at index i within py.
  3. If cx < cy, return true.
  4. If cx > cy, return false.
4. If lx < ly, return true. Otherwise, return false.
4. Else,
1. a. If px is a BigInt and py is a String, then
  1. Let ny be StringToBigInt(py).
  2. If ny is undefined, return undefined.
  3. Return BigInt::lessThan(px, ny).
2. b. If px is a String and py is a BigInt, then
  1. Let nx be StringToBigInt(px).
  2. If nx is undefined, return undefined.
  3. Return BigInt::lessThan(nx, py).
3. NOTE: Because px and py are primitive values, evaluation order is not important.
4. Let nx be ? ToNumeric(px).
5. Let ny be ? ToNumeric(py).
6. f. If Type(nx) is Type(ny), then
  1. i. If nx is a Number, then
    1. Return Number::lessThan(nx, ny).
  2. ii. Else,
    1. Assert: nx is a BigInt.
    2. Return BigInt::lessThan(nx, ny).
7. Assert: nx is a BigInt and ny is a Number, or nx is a Number and ny is a BigInt.
8. If nx or ny is NaN, return undefined.
9. If nx is -∞_𝔽 or ny is +∞_𝔽, return true.
10. If nx is +∞_𝔽 or ny is -∞_𝔽, return false.
11. If ℝ(nx) < ℝ(ny), return true; otherwise return false.

Note 1

Step 3 differs from step 1.c in the algorithm that handles the addition operator + (13.15.3) by using the logical-and operation instead of the logical-or operation.

Note 2

The comparison of Strings uses a simple lexicographic ordering on sequences of UTF-16 code unit values. There is no attempt to use the more complex, semantically oriented definitions of character or string equality and collating order defined in the Unicode specification. Therefore String values that are canonically equal according to the Unicode Standard but not in the same normalization form could test as unequal. Also note that lexicographic ordering by code unit differs from ordering by code point for Strings containing surrogate pairs.

7.2.14 IsLooselyEqual ( `x`, `y` )

The abstract operation IsLooselyEqual takes arguments x (an ECMAScript language value) and y (an ECMAScript language value) and returns either a normal completion containing a Boolean or a throw completion. It provides the semantics for the == operator. It performs the following steps when called:

1. If Type(x) is Type(y), then
1. Return IsStrictlyEqual(x, y).
If x is null and y is undefined, return true.
If x is undefined and y is null, return true.
NOTE: This step is replaced in section B.3.6.2.
If x is a Number and y is a String, return ! IsLooselyEqual(x, ! ToNumber(y)).
If x is a String and y is a Number, return ! IsLooselyEqual(! ToNumber(x), y).
7. If x is a BigInt and y is a String, then
1. Let n be StringToBigInt(y).
2. If n is undefined, return false.
3. Return ! IsLooselyEqual(x, n).
If x is a String and y is a BigInt, return ! IsLooselyEqual(y, x).
If x is a Boolean, return ! IsLooselyEqual(! ToNumber(x), y).
If y is a Boolean, return ! IsLooselyEqual(x, ! ToNumber(y)).
If x is either a String, a Number, a BigInt, or a Symbol and y is an Object, return ! IsLooselyEqual(x, ? ToPrimitive(y)).
If x is an Object and y is either a String, a Number, a BigInt, or a Symbol, return ! IsLooselyEqual(? ToPrimitive(x), y).
13. If x is a BigInt and y is a Number, or if x is a Number and y is a BigInt, then
1. If x is not finite or y is not finite, return false.
2. If ℝ(x) = ℝ(y), return true; otherwise return false.
Return false.

7.2.15 IsStrictlyEqual ( `x`, `y` )

The abstract operation IsStrictlyEqual takes arguments x (an ECMAScript language value) and y (an ECMAScript language value) and returns a Boolean. It provides the semantics for the === operator. It performs the following steps when called:

If Type(x) is not Type(y), return false.
2. If x is a Number, then
1. Return Number::equal(x, y).
Return SameValueNonNumber(x, y).

Note

This algorithm differs from the SameValue Algorithm in its treatment of signed zeroes and NaNs.

7.3 Operations on Objects

7.3.1 MakeBasicObject ( `internalSlotsList` )

The abstract operation MakeBasicObject takes argument internalSlotsList (a List of internal slot names) and returns an Object. It is the source of all ECMAScript objects that are created algorithmically, including both ordinary objects and exotic objects. It factors out common steps used in creating all objects, and centralizes object creation. It performs the following steps when called:

Let obj be a newly created object with an internal slot for each name in internalSlotsList.
Set obj's essential internal methods to the default ordinary object definitions specified in 10.1.
Assert: If the caller will not be overriding both obj's [[GetPrototypeOf]] and [[SetPrototypeOf]] essential internal methods, then internalSlotsList contains [[Prototype]].
Assert: If the caller will not be overriding all of obj's [[SetPrototypeOf]], [[IsExtensible]], and [[PreventExtensions]] essential internal methods, then internalSlotsList contains [[Extensible]].
If internalSlotsList contains [[Extensible]], set obj.[[Extensible]] to true.
Return obj.

Note

Within this specification, exotic objects are created in abstract operations such as ArrayCreate and BoundFunctionCreate by first calling MakeBasicObject to obtain a basic, foundational object, and then overriding some or all of that object's internal methods. In order to encapsulate exotic object creation, the object's essential internal methods are never modified outside those operations.

7.3.2 Get ( `O`, `P` )

The abstract operation Get takes arguments O (an Object) and P (a property key) and returns either a normal completion containing an ECMAScript language value or a throw completion. It is used to retrieve the value of a specific property of an object. It performs the following steps when called:

Return ? O.[[Get]](P, O).

7.3.3 GetV ( `V`, `P` )

The abstract operation GetV takes arguments V (an ECMAScript language value) and P (a property key) and returns either a normal completion containing an ECMAScript language value or a throw completion. It is used to retrieve the value of a specific property of an ECMAScript language value. If the value is not an object, the property lookup is performed using a wrapper object appropriate for the type of the value. It performs the following steps when called:

Let O be ? ToObject(V).
Return ? O.[[Get]](P, V).

7.3.4 Set ( `O`, `P`, `V`, `Throw` )

The abstract operation Set takes arguments O (an Object), P (a property key), V (an ECMAScript language value), and Throw (a Boolean) and returns either a normal completion containing unused or a throw completion. It is used to set the value of a specific property of an object. V is the new value for the property. It performs the following steps when called:

Let success be ? O.[[Set]](P, V, O).
If success is false and Throw is true, throw a TypeError exception.
Return unused.

7.3.5 CreateDataProperty ( `O`, `P`, `V` )

The abstract operation CreateDataProperty takes arguments O (an Object), P (a property key), and V (an ECMAScript language value) and returns either a normal completion containing a Boolean or a throw completion. It is used to create a new own property of an object. It performs the following steps when called:

Let newDesc be the PropertyDescriptor { [[Value]]: V, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true }.
Return ? O.[[DefineOwnProperty]](P, newDesc).

Note

This abstract operation creates a property whose attributes are set to the same defaults used for properties created by the ECMAScript language assignment operator. Normally, the property will not already exist. If it does exist and is not configurable or if O is not extensible, [[DefineOwnProperty]] will return false.

7.3.6 CreateDataPropertyOrThrow ( `O`, `P`, `V` )

The abstract operation CreateDataPropertyOrThrow takes arguments O (an Object), P (a property key), and V (an ECMAScript language value) and returns either a normal completion containing unused or a throw completion. It is used to create a new own property of an object. It throws a TypeError exception if the requested property update cannot be performed. It performs the following steps when called:

Let success be ? CreateDataProperty(O, P, V).
If success is false, throw a TypeError exception.
Return unused.

Note

7.3.7 CreateNonEnumerableDataPropertyOrThrow ( `O`, `P`, `V` )

The abstract operation CreateNonEnumerableDataPropertyOrThrow takes arguments O (an Object), P (a property key), and V (an ECMAScript language value) and returns unused. It is used to create a new non-enumerable own property of an ordinary object. It performs the following steps when called:

Assert: O is an ordinary, extensible object with no non-configurable properties.
Let newDesc be the PropertyDescriptor { [[Value]]: V, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: true }.
Perform ! DefinePropertyOrThrow(O, P, newDesc).
Return unused.

Note

This abstract operation creates a property whose attributes are set to the same defaults used for properties created by the ECMAScript language assignment operator except it is not enumerable. Normally, the property will not already exist. If it does exist, DefinePropertyOrThrow is guaranteed to complete normally.

7.3.8 DefinePropertyOrThrow ( `O`, `P`, `desc` )

The abstract operation DefinePropertyOrThrow takes arguments O (an Object), P (a property key), and desc (a Property Descriptor) and returns either a normal completion containing unused or a throw completion. It is used to call the [[DefineOwnProperty]] internal method of an object in a manner that will throw a TypeError exception if the requested property update cannot be performed. It performs the following steps when called:

Let success be ? O.[[DefineOwnProperty]](P, desc).
If success is false, throw a TypeError exception.
Return unused.

7.3.9 DeletePropertyOrThrow ( `O`, `P` )

The abstract operation DeletePropertyOrThrow takes arguments O (an Object) and P (a property key) and returns either a normal completion containing unused or a throw completion. It is used to remove a specific own property of an object. It throws an exception if the property is not configurable. It performs the following steps when called:

Let success be ? O.[[Delete]](P).
If success is false, throw a TypeError exception.
Return unused.

7.3.10 GetMethod ( `V`, `P` )

The abstract operation GetMethod takes arguments V (an ECMAScript language value) and P (a property key) and returns either a normal completion containing either a function object or undefined, or a throw completion. It is used to get the value of a specific property of an ECMAScript language value when the value of the property is expected to be a function. It performs the following steps when called:

Let func be ? GetV(V, P).
If func is either undefined or null, return undefined.
If IsCallable(func) is false, throw a TypeError exception.
Return func.

7.3.11 HasProperty ( `O`, `P` )

The abstract operation HasProperty takes arguments O (an Object) and P (a property key) and returns either a normal completion containing a Boolean or a throw completion. It is used to determine whether an object has a property with the specified property key. The property may be either own or inherited. It performs the following steps when called:

Return ? O.[[HasProperty]](P).

7.3.12 HasOwnProperty ( `O`, `P` )

The abstract operation HasOwnProperty takes arguments O (an Object) and P (a property key) and returns either a normal completion containing a Boolean or a throw completion. It is used to determine whether an object has an own property with the specified property key. It performs the following steps when called:

Let desc be ? O.[[GetOwnProperty]](P).
If desc is undefined, return false.
Return true.

7.3.13 Call ( `F`, `V` [ , `argumentsList` ] )

The abstract operation Call takes arguments F (an ECMAScript language value) and V (an ECMAScript language value) and optional argument argumentsList (a List of ECMAScript language values) and returns either a normal completion containing an ECMAScript language value or a throw completion. It is used to call the [[Call]] internal method of a function object. F is the function object, V is an ECMAScript language value that is the this value of the [[Call]], and argumentsList is the value passed to the corresponding argument of the internal method. If argumentsList is not present, a new empty List is used as its value. It performs the following steps when called:

If argumentsList is not present, set argumentsList to a new empty List.
If IsCallable(F) is false, throw a TypeError exception.
Return ? F.[[Call]](V, argumentsList).

7.3.14 Construct ( `F` [ , `argumentsList` [ , `newTarget` ] ] )

The abstract operation Construct takes argument F (a constructor) and optional arguments argumentsList (a List of ECMAScript language values) and newTarget (a constructor) and returns either a normal completion containing an Object or a throw completion. It is used to call the [[Construct]] internal method of a function object. argumentsList and newTarget are the values to be passed as the corresponding arguments of the internal method. If argumentsList is not present, a new empty List is used as its value. If newTarget is not present, F is used as its value. It performs the following steps when called:

If newTarget is not present, set newTarget to F.
If argumentsList is not present, set argumentsList to a new empty List.
Return ? F.[[Construct]](argumentsList, newTarget).

Note

If newTarget is not present, this operation is equivalent to: new F(...argumentsList)

7.3.15 SetIntegrityLevel ( `O`, `level` )

The abstract operation SetIntegrityLevel takes arguments O (an Object) and level (sealed or frozen) and returns either a normal completion containing a Boolean or a throw completion. It is used to fix the set of own properties of an object. It performs the following steps when called:

Let status be ? O.[[PreventExtensions]]().
If status is false, return false.
Let keys be ? O.[[OwnPropertyKeys]]().
4. If level is sealed, then
1. a. For each element k of keys, do
  1. Perform ? DefinePropertyOrThrow(O, k, PropertyDescriptor { [[Configurable]]: false }).
5. Else,
1. Assert: level is frozen.
2. b. For each element k of keys, do
  1. Let currentDesc be ? O.[[GetOwnProperty]](k).
  2. ii. If currentDesc is not undefined, then
    1. 1. If IsAccessorDescriptor(currentDesc) is true, then
      1. Let desc be the PropertyDescriptor { [[Configurable]]: false }.
    2. 2. Else,
      1. Let desc be the PropertyDescriptor { [[Configurable]]: false, [[Writable]]: false }.
    3. Perform ? DefinePropertyOrThrow(O, k, desc).
Return true.

7.3.16 TestIntegrityLevel ( `O`, `level` )

The abstract operation TestIntegrityLevel takes arguments O (an Object) and level (sealed or frozen) and returns either a normal completion containing a Boolean or a throw completion. It is used to determine if the set of own properties of an object are fixed. It performs the following steps when called:

Let extensible be ? IsExtensible(O).
If extensible is true, return false.
NOTE: If the object is extensible, none of its properties are examined.
Let keys be ? O.[[OwnPropertyKeys]]().
5. For each element k of keys, do
1. Let currentDesc be ? O.[[GetOwnProperty]](k).
2. b. If currentDesc is not undefined, then
  1. If currentDesc.[[Configurable]] is true, return false.
  2. ii. If level is frozen and IsDataDescriptor(currentDesc) is true, then
    1. If currentDesc.[[Writable]] is true, return false.
Return true.

7.3.17 CreateArrayFromList ( `elements` )

The abstract operation CreateArrayFromList takes argument elements (a List of ECMAScript language values) and returns an Array. It is used to create an Array whose elements are provided by elements. It performs the following steps when called:

Let array be ! ArrayCreate(0).
Let n be 0.
3. For each element e of elements, do
1. Perform ! CreateDataPropertyOrThrow(array, ! ToString(𝔽(n)), e).
2. Set n to n + 1.
Return array.

7.3.18 LengthOfArrayLike ( `obj` )

The abstract operation LengthOfArrayLike takes argument obj (an Object) and returns either a normal completion containing a non-negative integer or a throw completion. It returns the value of the "length" property of an array-like object. It performs the following steps when called:

Return ℝ(? ToLength(? Get(obj, "length"))).

An array-like object is any object for which this operation returns a normal completion.

Note 1

Typically, an array-like object would also have some properties with integer index names. However, that is not a requirement of this definition.

Note 2

Arrays and String objects are examples of array-like objects.

7.3.19 CreateListFromArrayLike ( `obj` [ , `elementTypes` ] )

The abstract operation CreateListFromArrayLike takes argument obj (an ECMAScript language value) and optional argument elementTypes (a List of names of ECMAScript Language Types) and returns either a normal completion containing a List of ECMAScript language values or a throw completion. It is used to create a List value whose elements are provided by the indexed properties of obj. elementTypes contains the names of ECMAScript Language Types that are allowed for element values of the List that is created. It performs the following steps when called:

If elementTypes is not present, set elementTypes to « Undefined, Null, Boolean, String, Symbol, Number, BigInt, Object ».
If obj is not an Object, throw a TypeError exception.
Let len be ? LengthOfArrayLike(obj).
Let list be a new empty List.
Let index be 0.
6. Repeat, while index < len,
1. Let indexName be ! ToString(𝔽(index)).
2. Let next be ? Get(obj, indexName).
3. If elementTypes does not contain Type(next), throw a TypeError exception.
4. Append next to list.
5. Set index to index + 1.
Return list.

7.3.20 Invoke ( `V`, `P` [ , `argumentsList` ] )

The abstract operation Invoke takes arguments V (an ECMAScript language value) and P (a property key) and optional argument argumentsList (a List of ECMAScript language values) and returns either a normal completion containing an ECMAScript language value or a throw completion. It is used to call a method property of an ECMAScript language value. V serves as both the lookup point for the property and the this value of the call. argumentsList is the list of arguments values passed to the method. If argumentsList is not present, a new empty List is used as its value. It performs the following steps when called:

If argumentsList is not present, set argumentsList to a new empty List.
Let func be ? GetV(V, P).
Return ? Call(func, V, argumentsList).

7.3.21 OrdinaryHasInstance ( `C`, `O` )

The abstract operation OrdinaryHasInstance takes arguments C (an ECMAScript language value) and O (an ECMAScript language value) and returns either a normal completion containing a Boolean or a throw completion. It implements the default algorithm for determining if O inherits from the instance object inheritance path provided by C. It performs the following steps when called:

If IsCallable(C) is false, return false.
2. If C has a [[BoundTargetFunction]] internal slot, then
1. Let BC be C.[[BoundTargetFunction]].
2. Return ? InstanceofOperator(O, BC).
If O is not an Object, return false.
Let P be ? Get(C, "prototype").
If P is not an Object, throw a TypeError exception.
6. Repeat,
1. Set O to ? O.[[GetPrototypeOf]]().
2. If O is null, return false.
3. If SameValue(P, O) is true, return true.

7.3.22 SpeciesConstructor ( `O`, `defaultConstructor` )

The abstract operation SpeciesConstructor takes arguments O (an Object) and defaultConstructor (a constructor) and returns either a normal completion containing a constructor or a throw completion. It is used to retrieve the constructor that should be used to create new objects that are derived from O. defaultConstructor is the constructor to use if a constructor @@species property cannot be found starting from O. It performs the following steps when called:

Let C be ? Get(O, "constructor").
If C is undefined, return defaultConstructor.
If C is not an Object, throw a TypeError exception.
Let S be ? Get(C, @@species).
If S is either undefined or null, return defaultConstructor.
If IsConstructor(S) is true, return S.
Throw a TypeError exception.

7.3.23 EnumerableOwnProperties ( `O`, `kind` )

The abstract operation EnumerableOwnProperties takes arguments O (an Object) and kind (key, value, or key+value) and returns either a normal completion containing a List of ECMAScript language values or a throw completion. It performs the following steps when called:

Let ownKeys be ? O.[[OwnPropertyKeys]]().
Let results be a new empty List.
3. For each element key of ownKeys, do
1. a. If key is a String, then
  1. Let desc be ? O.[[GetOwnProperty]](key).
  2. ii. If desc is not undefined and desc.[[Enumerable]] is true, then
    1. 1. If kind is key, then
      1. Append key to results.
    2. 2. Else,
      1. Let value be ? Get(O, key).
      2. If kind is value, then
        Append value to results.
      3. Else,
        Assert: kind is key+value.
        Let entry be CreateArrayFromList(« key, value »).
        Append entry to results.
Return results.

7.3.24 GetFunctionRealm ( `obj` )

The abstract operation GetFunctionRealm takes argument obj (a function object) and returns either a normal completion containing a Realm Record or a throw completion. It performs the following steps when called:

1. If obj has a [[Realm]] internal slot, then
1. Return obj.[[Realm]].
2. If obj is a bound function exotic object, then
1. Let boundTargetFunction be obj.[[BoundTargetFunction]].
2. Return ? GetFunctionRealm(boundTargetFunction).
3. If obj is a Proxy exotic object, then
1. Perform ? ValidateNonRevokedProxy(obj).
2. Let proxyTarget be obj.[[ProxyTarget]].
3. Return ? GetFunctionRealm(proxyTarget).
Return the current Realm Record.

Note

Step 4 will only be reached if obj is a non-standard function exotic object that does not have a [[Realm]] internal slot.

7.3.25 CopyDataProperties ( `target`, `source`, `excludedItems` )

The abstract operation CopyDataProperties takes arguments target (an Object), source (an ECMAScript language value), and excludedItems (a List of property keys) and returns either a normal completion containing unused or a throw completion. It performs the following steps when called:

If source is either undefined or null, return unused.
Let from be ! ToObject(source).
Let keys be ? from.[[OwnPropertyKeys]]().
4. For each element nextKey of keys, do
1. Let excluded be false.
2. b. For each element e of excludedItems, do
  1. i. If SameValue(e, nextKey) is true, then
    1. Set excluded to true.
3. c. If excluded is false, then
  1. Let desc be ? from.[[GetOwnProperty]](nextKey).
  2. ii. If desc is not undefined and desc.[[Enumerable]] is true, then
    1. Let propValue be ? Get(from, nextKey).
    2. Perform ! CreateDataPropertyOrThrow(target, nextKey, propValue).
Return unused.

Note

The target passed in here is always a newly created object which is not directly accessible in case of an error being thrown.

7.3.26 PrivateElementFind ( `O`, `P` )

The abstract operation PrivateElementFind takes arguments O (an Object) and P (a Private Name) and returns a PrivateElement or empty. It performs the following steps when called:

1. If O.[[PrivateElements]] contains a PrivateElement pe such that pe.[[Key]] is P, then
1. Return pe.
Return empty.

7.3.27 PrivateFieldAdd ( `O`, `P`, `value` )

The abstract operation PrivateFieldAdd takes arguments O (an Object), P (a Private Name), and value (an ECMAScript language value) and returns either a normal completion containing unused or a throw completion. It performs the following steps when called:

1. If the host is a web browser, then
1. Perform ? HostEnsureCanAddPrivateElement(O).
Let entry be PrivateElementFind(O, P).
If entry is not empty, throw a TypeError exception.
Append PrivateElement { [[Key]]: P, [[Kind]]: field, [[Value]]: value } to O.[[PrivateElements]].
Return unused.

7.3.28 PrivateMethodOrAccessorAdd ( `O`, `method` )

The abstract operation PrivateMethodOrAccessorAdd takes arguments O (an Object) and method (a PrivateElement) and returns either a normal completion containing unused or a throw completion. It performs the following steps when called:

Assert: method.[[Kind]] is either method or accessor.
2. If the host is a web browser, then
1. Perform ? HostEnsureCanAddPrivateElement(O).
Let entry be PrivateElementFind(O, method.[[Key]]).
If entry is not empty, throw a TypeError exception.
Append method to O.[[PrivateElements]].
Return unused.

Note

The values for private methods and accessors are shared across instances. This operation does not create a new copy of the method or accessor.

7.3.29 HostEnsureCanAddPrivateElement ( `O` )

The host-defined abstract operation HostEnsureCanAddPrivateElement takes argument O (an Object) and returns either a normal completion containing unused or a throw completion. It allows host environments to prevent the addition of private elements to particular host-defined exotic objects.

An implementation of HostEnsureCanAddPrivateElement must conform to the following requirements:

If O is not a host-defined exotic object, this abstract operation must return NormalCompletion(unused) and perform no other steps.
Any two calls of this abstract operation with the same argument must return the same kind of Completion Record.

The default implementation of HostEnsureCanAddPrivateElement is to return NormalCompletion(unused).

This abstract operation is only invoked by ECMAScript hosts that are web browsers.

7.3.30 PrivateGet ( `O`, `P` )

The abstract operation PrivateGet takes arguments O (an Object) and P (a Private Name) and returns either a normal completion containing an ECMAScript language value or a throw completion. It performs the following steps when called:

Let entry be PrivateElementFind(O, P).
If entry is empty, throw a TypeError exception.
3. If entry.[[Kind]] is either field or method, then
1. Return entry.[[Value]].
Assert: entry.[[Kind]] is accessor.
If entry.[[Get]] is undefined, throw a TypeError exception.
Let getter be entry.[[Get]].
Return ? Call(getter, O).

7.3.31 PrivateSet ( `O`, `P`, `value` )

The abstract operation PrivateSet takes arguments O (an Object), P (a Private Name), and value (an ECMAScript language value) and returns either a normal completion containing unused or a throw completion. It performs the following steps when called:

Let entry be PrivateElementFind(O, P).
If entry is empty, throw a TypeError exception.
3. If entry.[[Kind]] is field, then
1. Set entry.[[Value]] to value.
4. Else if entry.[[Kind]] is method, then
1. Throw a TypeError exception.
5. Else,
1. Assert: entry.[[Kind]] is accessor.
2. If entry.[[Set]] is undefined, throw a TypeError exception.
3. Let setter be entry.[[Set]].
4. Perform ? Call(setter, O, « value »).
Return unused.

7.3.32 DefineField ( `receiver`, `fieldRecord` )

The abstract operation DefineField takes arguments receiver (an Object) and fieldRecord (a ClassFieldDefinition Record) and returns either a normal completion containing unused or a throw completion. It performs the following steps when called:

Let fieldName be fieldRecord.[[Name]].
Let initializer be fieldRecord.[[Initializer]].
3. If initializer is not empty, then
1. Let initValue be ? Call(initializer, receiver).
4. Else,
1. Let initValue be undefined.
5. If fieldName is a Private Name, then
1. Perform ? PrivateFieldAdd(receiver, fieldName, initValue).
6. Else,
1. Assert: IsPropertyKey(fieldName) is true.
2. Perform ? CreateDataPropertyOrThrow(receiver, fieldName, initValue).
Return unused.

7.3.33 InitializeInstanceElements ( `O`, `constructor` )

The abstract operation InitializeInstanceElements takes arguments O (an Object) and constructor (an ECMAScript function object) and returns either a normal completion containing unused or a throw completion. It performs the following steps when called:

Let methods be the value of constructor.[[PrivateMethods]].
2. For each PrivateElement method of methods, do
1. Perform ? PrivateMethodOrAccessorAdd(O, method).
Let fields be the value of constructor.[[Fields]].
4. For each element fieldRecord of fields, do
1. Perform ? DefineField(O, fieldRecord).
Return unused.

7.3.34 AddValueToKeyedGroup ( `groups`, `key`, `value` )

The abstract operation AddValueToKeyedGroup takes arguments groups (a List of Records with fields [[Key]] (an ECMAScript language value) and [[Elements]] (a List of ECMAScript language values)), key (an ECMAScript language value), and value (an ECMAScript language value) and returns unused. It performs the following steps when called:

1. For each Record { [[Key]], [[Elements]] } g of groups, do
1. a. If SameValue(g.[[Key]], key) is true, then
  1. Assert: Exactly one element of groups meets this criterion.
  2. Append value to g.[[Elements]].
  3. Return unused.
Let group be the Record { [[Key]]: key, [[Elements]]: « value » }.
Append group to groups.
Return unused.

7.3.35 GroupBy ( `items`, `callbackfn`, `keyCoercion` )

The abstract operation GroupBy takes arguments items (an ECMAScript language value), callbackfn (an ECMAScript language value), and keyCoercion (property or zero) and returns either a normal completion containing a List of Records with fields [[Key]] (an ECMAScript language value) and [[Elements]] (a List of ECMAScript language values), or a throw completion. It performs the following steps when called:

Perform ? RequireObjectCoercible(items).
If IsCallable(callbackfn) is false, throw a TypeError exception.
Let groups be a new empty List.
Let iteratorRecord be ? GetIterator(items, sync).
Let k be 0.
6. Repeat,
1. a. If k ≥ 2**⁵³ - 1, then
  1. Let error be ThrowCompletion(a newly created TypeError object).
  2. Return ? IteratorClose(iteratorRecord, error).
2. Let next be ? IteratorStepValue(iteratorRecord).
3. c. If next is done, then
  1. Return groups.
4. Let value be next.
5. Let key be Completion(Call(callbackfn, undefined, « value, 𝔽(k) »)).
6. IfAbruptCloseIterator(key, iteratorRecord).
7. g. If keyCoercion is property, then
  1. Set key to Completion(ToPropertyKey(key)).
  2. IfAbruptCloseIterator(key, iteratorRecord).
8. h. Else,
  1. Assert: keyCoercion is zero.
  2. If key is -0_𝔽, set key to +0_𝔽.
9. Perform AddValueToKeyedGroup(groups, key, value).
10. Set k to k + 1.

7.4 Operations on Iterator Objects

See Common Iteration Interfaces (27.1).

7.4.1 Iterator Records

An Iterator Record is a Record value used to encapsulate an Iterator or AsyncIterator along with the next method.

Iterator Records have the fields listed in Table 15.

Table 15: Iterator Record Fields

Field Name	Value	Meaning
`[[Iterator]]`	an Object	An object that conforms to the Iterator or AsyncIterator interface.
`[[NextMethod]]`	an ECMAScript language value	The `next` method of the `[[Iterator]]` object.
`[[Done]]`	a Boolean	Whether the iterator has been closed.

7.4.2 GetIteratorFromMethod ( `obj`, `method` )

The abstract operation GetIteratorFromMethod takes arguments obj (an ECMAScript language value) and method (a function object) and returns either a normal completion containing an Iterator Record or a throw completion. It performs the following steps when called:

Let iterator be ? Call(method, obj).
If iterator is not an Object, throw a TypeError exception.
Let nextMethod be ? Get(iterator, "next").
Let iteratorRecord be the Iterator Record { [[Iterator]]: iterator, [[NextMethod]]: nextMethod, [[Done]]: false }.
Return iteratorRecord.

7.4.3 GetIterator ( `obj`, `kind` )

The abstract operation GetIterator takes arguments obj (an ECMAScript language value) and kind (sync or async) and returns either a normal completion containing an Iterator Record or a throw completion. It performs the following steps when called:

1. If kind is async, then
1. Let method be ? GetMethod(obj, @@asyncIterator).
2. b. If method is undefined, then
  1. Let syncMethod be ? GetMethod(obj, @@iterator).
  2. If syncMethod is undefined, throw a TypeError exception.
  3. Let syncIteratorRecord be ? GetIteratorFromMethod(obj, syncMethod).
  4. Return CreateAsyncFromSyncIterator(syncIteratorRecord).
2. Else,
1. Let method be ? GetMethod(obj, @@iterator).
If method is undefined, throw a TypeError exception.
Return ? GetIteratorFromMethod(obj, method).

7.4.4 IteratorNext ( `iteratorRecord` [ , `value` ] )

The abstract operation IteratorNext takes argument iteratorRecord (an Iterator Record) and optional argument value (an ECMAScript language value) and returns either a normal completion containing an Object or a throw completion. It performs the following steps when called:

1. If value is not present, then
1. Let result be ? Call(iteratorRecord.[[NextMethod]], iteratorRecord.[[Iterator]]).
2. Else,
1. Let result be ? Call(iteratorRecord.[[NextMethod]], iteratorRecord.[[Iterator]], « value »).
If result is not an Object, throw a TypeError exception.
Return result.

7.4.5 IteratorComplete ( `iterResult` )

The abstract operation IteratorComplete takes argument iterResult (an Object) and returns either a normal completion containing a Boolean or a throw completion. It performs the following steps when called:

Return ToBoolean(? Get(iterResult, "done")).

7.4.6 IteratorValue ( `iterResult` )

The abstract operation IteratorValue takes argument iterResult (an Object) and returns either a normal completion containing an ECMAScript language value or a throw completion. It performs the following steps when called:

Return ? Get(iterResult, "value").

7.4.7 IteratorStep ( `iteratorRecord` )

The abstract operation IteratorStep takes argument iteratorRecord (an Iterator Record) and returns either a normal completion containing either an Object or false, or a throw completion. It requests the next value from iteratorRecord.[[Iterator]] by calling iteratorRecord.[[NextMethod]] and returns either false indicating that the iterator has reached its end or the IteratorResult object if a next value is available. It performs the following steps when called:

Let result be ? IteratorNext(iteratorRecord).
Let done be ? IteratorComplete(result).
If done is true, return false.
Return result.

7.4.8 IteratorStepValue ( `iteratorRecord` )

The abstract operation IteratorStepValue takes argument iteratorRecord (an Iterator Record) and returns either a normal completion containing either an ECMAScript language value or done, or a throw completion. It requests the next value from iteratorRecord.[[Iterator]] by calling iteratorRecord.[[NextMethod]] and returns either done indicating that the iterator has reached its end or the value from the IteratorResult object if a next value is available. It performs the following steps when called:

Let result be Completion(IteratorNext(iteratorRecord)).
2. If result is a throw completion, then
1. Set iteratorRecord.[[Done]] to true.
2. Return ? result.
Set result to ! result.
Let done be Completion(IteratorComplete(result)).
5. If done is a throw completion, then
1. Set iteratorRecord.[[Done]] to true.
2. Return ? done.
Set done to ! done.
7. If done is true, then
1. Set iteratorRecord.[[Done]] to true.
2. Return done.
Let value be Completion(Get(result, "value")).
9. If value is a throw completion, then
1. Set iteratorRecord.[[Done]] to true.
Return ? value.

7.4.9 IteratorClose ( `iteratorRecord`, `completion` )

The abstract operation IteratorClose takes arguments iteratorRecord (an Iterator Record) and completion (a Completion Record) and returns a Completion Record. It is used to notify an iterator that it should perform any actions it would normally perform when it has reached its completed state. It performs the following steps when called:

Assert: iteratorRecord.[[Iterator]] is an Object.
Let iterator be iteratorRecord.[[Iterator]].
Let innerResult be Completion(GetMethod(iterator, "return")).
4. If innerResult is a normal completion, then
1. Let return be innerResult.[[Value]].
2. If return is undefined, return ? completion.
3. Set innerResult to Completion(Call(return, iterator)).
If completion is a throw completion, return ? completion.
If innerResult is a throw completion, return ? innerResult.
If innerResult.[[Value]] is not an Object, throw a TypeError exception.
Return ? completion.

7.4.10 IfAbruptCloseIterator ( `value`, `iteratorRecord` )

IfAbruptCloseIterator is a shorthand for a sequence of algorithm steps that use an Iterator Record. An algorithm step of the form:

IfAbruptCloseIterator(value, iteratorRecord).

means the same thing as:

Assert: value is a Completion Record.
If value is an abrupt completion, return ? IteratorClose(iteratorRecord, value).
Else, set value to ! value.

7.4.11 AsyncIteratorClose ( `iteratorRecord`, `completion` )

The abstract operation AsyncIteratorClose takes arguments iteratorRecord (an Iterator Record) and completion (a Completion Record) and returns a Completion Record. It is used to notify an async iterator that it should perform any actions it would normally perform when it has reached its completed state. It performs the following steps when called:

Assert: iteratorRecord.[[Iterator]] is an Object.
Let iterator be iteratorRecord.[[Iterator]].
Let innerResult be Completion(GetMethod(iterator, "return")).
4. If innerResult is a normal completion, then
1. Let return be innerResult.[[Value]].
2. If return is undefined, return ? completion.
3. Set innerResult to Completion(Call(return, iterator)).
4. If innerResult is a normal completion, set innerResult to Completion(Await(innerResult.[[Value]])).
If completion is a throw completion, return ? completion.
If innerResult is a throw completion, return ? innerResult.
If innerResult.[[Value]] is not an Object, throw a TypeError exception.
Return ? completion.

7.4.12 CreateIterResultObject ( `value`, `done` )

The abstract operation CreateIterResultObject takes arguments value (an ECMAScript language value) and done (a Boolean) and returns an Object that conforms to the IteratorResult interface. It creates an object that conforms to the IteratorResult interface. It performs the following steps when called:

Let obj be OrdinaryObjectCreate(%Object.prototype%).
Perform ! CreateDataPropertyOrThrow(obj, "value", value).
Perform ! CreateDataPropertyOrThrow(obj, "done", done).
Return obj.

7.4.13 CreateListIteratorRecord ( `list` )

The abstract operation CreateListIteratorRecord takes argument list (a List of ECMAScript language values) and returns an Iterator Record. It creates an Iterator (27.1.1.2) object record whose next method returns the successive elements of list. It performs the following steps when called:

1. Let closure be a new Abstract Closure with no parameters that captures list and performs the following steps when called:
1. a. For each element E of list, do
  1. Perform ? GeneratorYield(CreateIterResultObject(E, false)).
2. Return NormalCompletion(undefined).
Let iterator be CreateIteratorFromClosure(closure, empty, %IteratorPrototype%).
Return the Iterator Record { [[Iterator]]: iterator, [[NextMethod]]: %GeneratorFunction.prototype.prototype.next%, [[Done]]: false }.

Note

The list iterator object is never directly accessible to ECMAScript code.

7.4.14 IteratorToList ( `iteratorRecord` )

The abstract operation IteratorToList takes argument iteratorRecord (an Iterator Record) and returns either a normal completion containing a List of ECMAScript language values or a throw completion. It performs the following steps when called:

Let values be a new empty List.
2. Repeat,
1. Let next be ? IteratorStepValue(iteratorRecord).
2. b. If next is done, then
  1. Return values.
3. Append next to values.

8 Syntax-Directed Operations

In addition to those defined in this section, specialized syntax-directed operations are defined throughout this specification.

8.1 Runtime Semantics: Evaluation

The syntax-directed operation Evaluation takes no arguments and returns a Completion Record.

Note

The definitions for this operation are distributed over the "ECMAScript Language" sections of this specification. Each definition appears after the defining occurrence of the relevant productions.

8.2 Scope Analysis

8.2.1 Static Semantics: BoundNames

The syntax-directed operation BoundNames takes no arguments and returns a List of Strings.

Note

"*default*" is used within this specification as a synthetic name for a module's default export when it does not have another name. An entry in the module's [[Environment]] is created with that name and holds the corresponding value, and resolving the export named "default" by calling ResolveExport ( exportName [ , resolveSet ] ) for the module will return a ResolvedBinding Record whose [[BindingName]] is "*default*", which will then resolve in the module's [[Environment]] to the above-mentioned value. This is done only for ease of specification, so that anonymous default exports can be resolved like any other export. This "*default*" string is never accessible to ECMAScript code or to the module linking algorithm.

It is defined piecewise over the following productions:

BindingIdentifier

Identifier

Return a List whose sole element is the StringValue of Identifier.

BindingIdentifier

yield

Return « "yield" ».

BindingIdentifier

await

Return « "await" ».

LexicalDeclaration

LetOrConst

BindingList

;

Return the BoundNames of BindingList.

BindingList

LexicalBinding

Let names1 be the BoundNames of BindingList.
Let names2 be the BoundNames of LexicalBinding.
Return the list-concatenation of names1 and names2.

LexicalBinding

BindingIdentifier

Initializer

opt

Return the BoundNames of BindingIdentifier.

LexicalBinding

BindingPattern

Initializer

Return the BoundNames of BindingPattern.

VariableDeclarationList

VariableDeclaration

Let names1 be BoundNames of VariableDeclarationList.
Let names2 be BoundNames of VariableDeclaration.
Return the list-concatenation of names1 and names2.

VariableDeclaration

BindingIdentifier

Initializer

opt

Return the BoundNames of BindingIdentifier.

VariableDeclaration

BindingPattern

Initializer

Return the BoundNames of BindingPattern.

ObjectBindingPattern

{

}

Return a new empty List.

ObjectBindingPattern

{

BindingPropertyList

BindingRestProperty

}

Let names1 be BoundNames of BindingPropertyList.
Let names2 be BoundNames of BindingRestProperty.
Return the list-concatenation of names1 and names2.

ArrayBindingPattern

[

Elision

opt

]

Return a new empty List.

ArrayBindingPattern

[

Elision

opt

BindingRestElement

]

Return the BoundNames of BindingRestElement.

ArrayBindingPattern

[

BindingElementList

Elision

opt

]

Return the BoundNames of BindingElementList.

[

opt

]

Let names1 be BoundNames of BindingElementList.
Let names2 be BoundNames of BindingRestElement.
Return the list-concatenation of names1 and names2.

BindingPropertyList

BindingProperty

Let names1 be BoundNames of BindingPropertyList.
Let names2 be BoundNames of BindingProperty.
Return the list-concatenation of names1 and names2.

BindingElementList

BindingElisionElement

Let names1 be BoundNames of BindingElementList.
Let names2 be BoundNames of BindingElisionElement.
Return the list-concatenation of names1 and names2.

BindingElisionElement

Elision

opt

BindingElement

Return BoundNames of BindingElement.

BindingProperty

PropertyName

BindingElement

Return the BoundNames of BindingElement.

SingleNameBinding

BindingIdentifier

Initializer

opt

Return the BoundNames of BindingIdentifier.

BindingElement

BindingPattern

Initializer

opt

Return the BoundNames of BindingPattern.

ForDeclaration

LetOrConst

ForBinding

Return the BoundNames of ForBinding.

FunctionDeclaration

function

BindingIdentifier

(

FormalParameters

)

{

FunctionBody

}

Return the BoundNames of BindingIdentifier.

FunctionDeclaration

function

(

FormalParameters

)

{

FunctionBody

}

Return « "*default*" ».

FormalParameters

[empty]

Return a new empty List.

FormalParameters

FormalParameterList

FunctionRestParameter

Let names1 be BoundNames of FormalParameterList.
Let names2 be BoundNames of FunctionRestParameter.
Return the list-concatenation of names1 and names2.

FormalParameterList

FormalParameter

Let names1 be BoundNames of FormalParameterList.
Let names2 be BoundNames of FormalParameter.
Return the list-concatenation of names1 and names2.

ArrowParameters

CoverParenthesizedExpressionAndArrowParameterList

Let formals be the ArrowFormalParameters that is covered by CoverParenthesizedExpressionAndArrowParameterList.
Return the BoundNames of formals.

GeneratorDeclaration

function

BindingIdentifier

(

FormalParameters

)

{

GeneratorBody

}

Return the BoundNames of BindingIdentifier.

GeneratorDeclaration

function

(

FormalParameters

)

{

GeneratorBody

}

Return « "*default*" ».

AsyncGeneratorDeclaration

async

function

BindingIdentifier

(

FormalParameters

)

{

AsyncGeneratorBody

}

Return the BoundNames of BindingIdentifier.

AsyncGeneratorDeclaration

async

function

(

FormalParameters

)

{

AsyncGeneratorBody

}

Return « "*default*" ».

ClassDeclaration

class

BindingIdentifier

ClassTail

Return the BoundNames of BindingIdentifier.

ClassDeclaration

class

ClassTail

Return « "*default*" ».

AsyncFunctionDeclaration

async

function

BindingIdentifier

(

FormalParameters

)

{

AsyncFunctionBody

}

Return the BoundNames of BindingIdentifier.

AsyncFunctionDeclaration

async

function

(

FormalParameters

)

{

AsyncFunctionBody

}

Return « "*default*" ».

CoverCallExpressionAndAsyncArrowHead

MemberExpression

Arguments

Let head be the AsyncArrowHead that is covered by CoverCallExpressionAndAsyncArrowHead.
Return the BoundNames of head.

ImportDeclaration

import

ImportClause

FromClause

;

Return the BoundNames of ImportClause.

ImportDeclaration

import

ModuleSpecifier

;

Return a new empty List.

ImportClause

ImportedDefaultBinding

NameSpaceImport

Let names1 be the BoundNames of ImportedDefaultBinding.
Let names2 be the BoundNames of NameSpaceImport.
Return the list-concatenation of names1 and names2.

ImportClause

ImportedDefaultBinding

NamedImports

Let names1 be the BoundNames of ImportedDefaultBinding.
Let names2 be the BoundNames of NamedImports.
Return the list-concatenation of names1 and names2.

NamedImports

{

}

Return a new empty List.

ImportsList

ImportSpecifier

Let names1 be the BoundNames of ImportsList.
Let names2 be the BoundNames of ImportSpecifier.
Return the list-concatenation of names1 and names2.

ImportSpecifier

ModuleExportName

ImportedBinding

Return the BoundNames of ImportedBinding.

ExportDeclaration

export

ExportFromClause

FromClause

;

export

NamedExports

;

Return a new empty List.

ExportDeclaration

export

VariableStatement

Return the BoundNames of VariableStatement.

ExportDeclaration

export

Declaration

Return the BoundNames of Declaration.

ExportDeclaration

export

default

HoistableDeclaration

Let declarationNames be the BoundNames of HoistableDeclaration.
If declarationNames does not include the element "*default*", append "*default*" to declarationNames.
Return declarationNames.

ExportDeclaration

export

default

ClassDeclaration

Let declarationNames be the BoundNames of ClassDeclaration.
If declarationNames does not include the element "*default*", append "*default*" to declarationNames.
Return declarationNames.

ExportDeclaration

export

default

AssignmentExpression

;

Return « "*default*" ».

8.2.2 Static Semantics: DeclarationPart

The syntax-directed operation DeclarationPart takes no arguments and returns a Parse Node. It is defined piecewise over the following productions:

HoistableDeclaration

FunctionDeclaration

Return FunctionDeclaration.

HoistableDeclaration

GeneratorDeclaration

Return GeneratorDeclaration.

HoistableDeclaration

AsyncFunctionDeclaration

Return AsyncFunctionDeclaration.

HoistableDeclaration

AsyncGeneratorDeclaration

Return AsyncGeneratorDeclaration.

Declaration

ClassDeclaration

Return ClassDeclaration.

Declaration

LexicalDeclaration

Return LexicalDeclaration.

8.2.3 Static Semantics: IsConstantDeclaration

The syntax-directed operation IsConstantDeclaration takes no arguments and returns a Boolean. It is defined piecewise over the following productions:

LexicalDeclaration

LetOrConst

BindingList

;

Return IsConstantDeclaration of LetOrConst.

LetOrConst

let

Return false.

LetOrConst

const

Return true.

FunctionDeclaration

function

BindingIdentifier

(

FormalParameters

)

{

FunctionBody

}

function

(

FormalParameters

)

{

}

function

(

)

{

GeneratorBody

}

function

(

FormalParameters

)

{

GeneratorBody

}

AsyncGeneratorDeclaration

async

function

BindingIdentifier

(

FormalParameters

)

{

AsyncGeneratorBody

}

async

function

(

FormalParameters

)

{

AsyncGeneratorBody

}

AsyncFunctionDeclaration

async

function

BindingIdentifier

(

FormalParameters

)

{

AsyncFunctionBody

}

async

function

(

FormalParameters

)

{

AsyncFunctionBody

}

Return false.

class

class

Return false.

ExportDeclaration

export

ExportFromClause

FromClause

;

export

NamedExports

;

export

default

AssignmentExpression

;

Return false.

Note

It is not necessary to treat export default AssignmentExpression as a constant declaration because there is no syntax that permits assignment to the internal bound name used to reference a module's default object.

8.2.4 Static Semantics: LexicallyDeclaredNames

The syntax-directed operation LexicallyDeclaredNames takes no arguments and returns a List of Strings. It is defined piecewise over the following productions:

Block

{

}

Return a new empty List.

StatementList

StatementListItem

Let names1 be LexicallyDeclaredNames of StatementList.
Let names2 be LexicallyDeclaredNames of StatementListItem.
Return the list-concatenation of names1 and names2.

StatementListItem

Statement

If Statement is Statement : LabelledStatement , return LexicallyDeclaredNames of LabelledStatement.
Return a new empty List.

StatementListItem

Declaration

Return the BoundNames of Declaration.

CaseBlock

{

}

Return a new empty List.

{

opt

opt

}

If the first CaseClauses is present, let names1 be the LexicallyDeclaredNames of the first CaseClauses.
Else, let names1 be a new empty List.
Let names2 be LexicallyDeclaredNames of DefaultClause.
If the second CaseClauses is present, let names3 be the LexicallyDeclaredNames of the second CaseClauses.
Else, let names3 be a new empty List.
Return the list-concatenation of names1, names2, and names3.

CaseClauses

CaseClause

Let names1 be LexicallyDeclaredNames of CaseClauses.
Let names2 be LexicallyDeclaredNames of CaseClause.
Return the list-concatenation of names1 and names2.

CaseClause

case

Expression

StatementList

opt

If the StatementList is present, return the LexicallyDeclaredNames of StatementList.
Return a new empty List.

DefaultClause

default

StatementList

opt

If the StatementList is present, return the LexicallyDeclaredNames of StatementList.
Return a new empty List.

LabelledStatement

LabelIdentifier

LabelledItem

Return the LexicallyDeclaredNames of LabelledItem.

LabelledItem

Statement

Return a new empty List.

LabelledItem

FunctionDeclaration

Return BoundNames of FunctionDeclaration.

FunctionStatementList

[empty]

Return a new empty List.

FunctionStatementList

StatementList

Return TopLevelLexicallyDeclaredNames of StatementList.

ClassStaticBlockStatementList

[empty]

Return a new empty List.

ClassStaticBlockStatementList

StatementList

Return the TopLevelLexicallyDeclaredNames of StatementList.

ConciseBody

ExpressionBody

Return a new empty List.

AsyncConciseBody

ExpressionBody

Return a new empty List.

Script

[empty]

Return a new empty List.

ScriptBody

StatementList

Return TopLevelLexicallyDeclaredNames of StatementList.

Note 1

At the top level of a Script, function declarations are treated like var declarations rather than like lexical declarations.

Note 2

The LexicallyDeclaredNames of a Module includes the names of all of its imported bindings.

ModuleItemList

ModuleItem

Let names1 be LexicallyDeclaredNames of ModuleItemList.
Let names2 be LexicallyDeclaredNames of ModuleItem.
Return the list-concatenation of names1 and names2.

ModuleItem

ImportDeclaration

Return the BoundNames of ImportDeclaration.

ModuleItem

ExportDeclaration

If ExportDeclaration is export VariableStatement, return a new empty List.
Return the BoundNames of ExportDeclaration.

ModuleItem

StatementListItem

Return LexicallyDeclaredNames of StatementListItem.

Note 3

At the top level of a Module, function declarations are treated like lexical declarations rather than like var declarations.

8.2.5 Static Semantics: LexicallyScopedDeclarations

The syntax-directed operation LexicallyScopedDeclarations takes no arguments and returns a List of Parse Nodes. It is defined piecewise over the following productions:

StatementList

StatementListItem

Let declarations1 be LexicallyScopedDeclarations of StatementList.
Let declarations2 be LexicallyScopedDeclarations of StatementListItem.
Return the list-concatenation of declarations1 and declarations2.

StatementListItem

Statement

If Statement is Statement : LabelledStatement , return LexicallyScopedDeclarations of LabelledStatement.
Return a new empty List.

StatementListItem

Declaration

Return a List whose sole element is DeclarationPart of Declaration.

CaseBlock

{

}

Return a new empty List.

{

opt

opt

}

If the first CaseClauses is present, let declarations1 be the LexicallyScopedDeclarations of the first CaseClauses.
Else, let declarations1 be a new empty List.
Let declarations2 be LexicallyScopedDeclarations of DefaultClause.
If the second CaseClauses is present, let declarations3 be the LexicallyScopedDeclarations of the second CaseClauses.
Else, let declarations3 be a new empty List.
Return the list-concatenation of declarations1, declarations2, and declarations3.

CaseClauses

CaseClause

Let declarations1 be LexicallyScopedDeclarations of CaseClauses.
Let declarations2 be LexicallyScopedDeclarations of CaseClause.
Return the list-concatenation of declarations1 and declarations2.

CaseClause

case

Expression

StatementList

opt

If the StatementList is present, return the LexicallyScopedDeclarations of StatementList.
Return a new empty List.

DefaultClause

default

StatementList

opt

If the StatementList is present, return the LexicallyScopedDeclarations of StatementList.
Return a new empty List.

LabelledStatement

LabelIdentifier

LabelledItem

Return the LexicallyScopedDeclarations of LabelledItem.

LabelledItem

Statement

Return a new empty List.

LabelledItem

FunctionDeclaration

Return « FunctionDeclaration ».

FunctionStatementList

[empty]

Return a new empty List.

FunctionStatementList

StatementList

Return the TopLevelLexicallyScopedDeclarations of StatementList.

ClassStaticBlockStatementList

[empty]

Return a new empty List.

ClassStaticBlockStatementList

StatementList

Return the TopLevelLexicallyScopedDeclarations of StatementList.

ConciseBody

ExpressionBody

Return a new empty List.

AsyncConciseBody

ExpressionBody

Return a new empty List.

Script

[empty]

Return a new empty List.

ScriptBody

StatementList

Return TopLevelLexicallyScopedDeclarations of StatementList.

Module

[empty]

Return a new empty List.

ModuleItemList

ModuleItem

Let declarations1 be LexicallyScopedDeclarations of ModuleItemList.
Let declarations2 be LexicallyScopedDeclarations of ModuleItem.
Return the list-concatenation of declarations1 and declarations2.

ModuleItem

ImportDeclaration

Return a new empty List.

ExportDeclaration

export

ExportFromClause

FromClause

;

export

NamedExports

;

export

VariableStatement

Return a new empty List.

ExportDeclaration

export

Declaration

Return a List whose sole element is DeclarationPart of Declaration.

ExportDeclaration

export

default

HoistableDeclaration

Return a List whose sole element is DeclarationPart of HoistableDeclaration.

ExportDeclaration

export

default

ClassDeclaration

Return a List whose sole element is ClassDeclaration.

ExportDeclaration

export

default

AssignmentExpression

;

Return a List whose sole element is this ExportDeclaration.

8.2.6 Static Semantics: VarDeclaredNames

The syntax-directed operation VarDeclaredNames takes no arguments and returns a List of Strings. It is defined piecewise over the following productions:

Return a new empty List.

Block

{

}

Return a new empty List.

StatementList

StatementListItem

Let names1 be VarDeclaredNames of StatementList.
Let names2 be VarDeclaredNames of StatementListItem.
Return the list-concatenation of names1 and names2.

StatementListItem

Declaration

Return a new empty List.

VariableStatement

var

VariableDeclarationList

;

Return BoundNames of VariableDeclarationList.

(

)

else

Let names1 be VarDeclaredNames of the first Statement.
Let names2 be VarDeclaredNames of the second Statement.
Return the list-concatenation of names1 and names2.

IfStatement

(

Expression

)

Statement

Return the VarDeclaredNames of Statement.

DoWhileStatement

Statement

while

(

Expression

)

;

Return the VarDeclaredNames of Statement.

WhileStatement

while

(

Expression

)

Statement

Return the VarDeclaredNames of Statement.

ForStatement

for

(

Expression

opt

;

Expression

opt

;

Expression

opt

)

Statement

Return the VarDeclaredNames of Statement.

ForStatement

for

(

var

VariableDeclarationList

;

Expression

opt

;

Expression

opt

)

Statement

Let names1 be BoundNames of VariableDeclarationList.
Let names2 be VarDeclaredNames of Statement.
Return the list-concatenation of names1 and names2.

ForStatement

for

(

LexicalDeclaration

Expression

opt

;

Expression

opt

)

Statement

Return the VarDeclaredNames of Statement.

ForInOfStatement

for

(

LeftHandSideExpression

Expression

)

Statement

for

(

ForDeclaration

Expression

)

Statement

for

(

LeftHandSideExpression

AssignmentExpression

)

Statement

for

(

ForDeclaration

AssignmentExpression

)

Statement

for

await

(

LeftHandSideExpression

AssignmentExpression

)

Statement

for

await

(

ForDeclaration

AssignmentExpression

)

Statement

Return the VarDeclaredNames of Statement.

ForInOfStatement

for

(

var

ForBinding

Expression

)

Statement

for

(

var

ForBinding

AssignmentExpression

)

Statement

for

await

(

var

ForBinding

AssignmentExpression

)

Statement

Let names1 be the BoundNames of ForBinding.
Let names2 be the VarDeclaredNames of Statement.
Return the list-concatenation of names1 and names2.

Note

This section is extended by Annex B.3.5.

WithStatement

with

(

Expression

)

Statement

Return the VarDeclaredNames of Statement.

SwitchStatement

switch

(

Expression

)

CaseBlock

Return the VarDeclaredNames of CaseBlock.

CaseBlock

{

}

Return a new empty List.

{

opt

opt

}

If the first CaseClauses is present, let names1 be the VarDeclaredNames of the first CaseClauses.
Else, let names1 be a new empty List.
Let names2 be VarDeclaredNames of DefaultClause.
If the second CaseClauses is present, let names3 be the VarDeclaredNames of the second CaseClauses.
Else, let names3 be a new empty List.
Return the list-concatenation of names1, names2, and names3.

CaseClauses

CaseClause

Let names1 be VarDeclaredNames of CaseClauses.
Let names2 be VarDeclaredNames of CaseClause.
Return the list-concatenation of names1 and names2.

CaseClause

case

Expression

StatementList

opt

If the StatementList is present, return the VarDeclaredNames of StatementList.
Return a new empty List.

DefaultClause

default

StatementList

opt

If the StatementList is present, return the VarDeclaredNames of StatementList.
Return a new empty List.

LabelledStatement

LabelIdentifier

LabelledItem

Return the VarDeclaredNames of LabelledItem.

LabelledItem

FunctionDeclaration

Return a new empty List.

TryStatement

try

Block

Catch

Let names1 be VarDeclaredNames of Block.
Let names2 be VarDeclaredNames of Catch.
Return the list-concatenation of names1 and names2.

TryStatement

try

Block

Finally

Let names1 be VarDeclaredNames of Block.
Let names2 be VarDeclaredNames of Finally.
Return the list-concatenation of names1 and names2.

try

Let names1 be VarDeclaredNames of Block.
Let names2 be VarDeclaredNames of Catch.
Let names3 be VarDeclaredNames of Finally.
Return the list-concatenation of names1, names2, and names3.

Catch

catch

(

CatchParameter

)

Block

Return the VarDeclaredNames of Block.

FunctionStatementList

[empty]

Return a new empty List.

FunctionStatementList

StatementList

Return TopLevelVarDeclaredNames of StatementList.

ClassStaticBlockStatementList

[empty]

Return a new empty List.

ClassStaticBlockStatementList

StatementList

Return the TopLevelVarDeclaredNames of StatementList.

ConciseBody

ExpressionBody

Return a new empty List.

AsyncConciseBody

ExpressionBody

Return a new empty List.

Script

[empty]

Return a new empty List.

ScriptBody

StatementList

Return TopLevelVarDeclaredNames of StatementList.

ModuleItemList

ModuleItem

Let names1 be VarDeclaredNames of ModuleItemList.
Let names2 be VarDeclaredNames of ModuleItem.
Return the list-concatenation of names1 and names2.

ModuleItem

ImportDeclaration

Return a new empty List.

ModuleItem

ExportDeclaration

If ExportDeclaration is export VariableStatement, return BoundNames of ExportDeclaration.
Return a new empty List.

8.2.7 Static Semantics: VarScopedDeclarations

The syntax-directed operation VarScopedDeclarations takes no arguments and returns a List of Parse Nodes. It is defined piecewise over the following productions:

Return a new empty List.

Block

{

}

Return a new empty List.

StatementList

StatementListItem

Let declarations1 be VarScopedDeclarations of StatementList.
Let declarations2 be VarScopedDeclarations of StatementListItem.
Return the list-concatenation of declarations1 and declarations2.

StatementListItem

Declaration

Return a new empty List.

VariableDeclarationList

VariableDeclaration

Return « VariableDeclaration ».

VariableDeclarationList

VariableDeclaration

Let declarations1 be VarScopedDeclarations of VariableDeclarationList.
Return the list-concatenation of declarations1 and « VariableDeclaration ».

(

)

else

Let declarations1 be VarScopedDeclarations of the first Statement.
Let declarations2 be VarScopedDeclarations of the second Statement.
Return the list-concatenation of declarations1 and declarations2.

IfStatement

(

Expression

)

Statement

Return the VarScopedDeclarations of Statement.

DoWhileStatement

Statement

while

(

Expression

)

;

Return the VarScopedDeclarations of Statement.

WhileStatement

while

(

Expression

)

Statement

Return the VarScopedDeclarations of Statement.

ForStatement

for

(

Expression

opt

;

Expression

opt

;

Expression

opt

)

Statement

Return the VarScopedDeclarations of Statement.

ForStatement

for

(

var

VariableDeclarationList

;

Expression

opt

;

Expression

opt

)

Statement

Let declarations1 be VarScopedDeclarations of VariableDeclarationList.
Let declarations2 be VarScopedDeclarations of Statement.
Return the list-concatenation of declarations1 and declarations2.

ForStatement

for

(

LexicalDeclaration

Expression

opt

;

Expression

opt

)

Statement

Return the VarScopedDeclarations of Statement.

ForInOfStatement

for

(

LeftHandSideExpression

Expression

)

Statement

for

(

ForDeclaration

Expression

)

Statement

for

(

LeftHandSideExpression

AssignmentExpression

)

Statement

for

(

ForDeclaration

AssignmentExpression

)

Statement

for

await

(

LeftHandSideExpression

AssignmentExpression

)

Statement

for

await

(

ForDeclaration

AssignmentExpression

)

Statement

Return the VarScopedDeclarations of Statement.

ForInOfStatement

for

(

var

ForBinding

Expression

)

Statement

for

(

var

ForBinding

AssignmentExpression

)

Statement

for

await

(

var

ForBinding

AssignmentExpression

)

Statement

Let declarations1 be « ForBinding ».
Let declarations2 be VarScopedDeclarations of Statement.
Return the list-concatenation of declarations1 and declarations2.

Note

This section is extended by Annex B.3.5.

WithStatement

with

(

Expression

)

Statement

Return the VarScopedDeclarations of Statement.

SwitchStatement

switch

(

Expression

)

CaseBlock

Return the VarScopedDeclarations of CaseBlock.

CaseBlock

{

}

Return a new empty List.

{

opt

opt

}

If the first CaseClauses is present, let declarations1 be the VarScopedDeclarations of the first CaseClauses.
Else, let declarations1 be a new empty List.
Let declarations2 be VarScopedDeclarations of DefaultClause.
If the second CaseClauses is present, let declarations3 be the VarScopedDeclarations of the second CaseClauses.
Else, let declarations3 be a new empty List.
Return the list-concatenation of declarations1, declarations2, and declarations3.

CaseClauses

CaseClause

Let declarations1 be VarScopedDeclarations of CaseClauses.
Let declarations2 be VarScopedDeclarations of CaseClause.
Return the list-concatenation of declarations1 and declarations2.

CaseClause

case

Expression

StatementList

opt

If the StatementList is present, return the VarScopedDeclarations of StatementList.
Return a new empty List.

DefaultClause

default

StatementList

opt

If the StatementList is present, return the VarScopedDeclarations of StatementList.
Return a new empty List.

LabelledStatement

LabelIdentifier

LabelledItem

Return the VarScopedDeclarations of LabelledItem.

LabelledItem

FunctionDeclaration

Return a new empty List.

TryStatement

try

Block

Catch

Let declarations1 be VarScopedDeclarations of Block.
Let declarations2 be VarScopedDeclarations of Catch.
Return the list-concatenation of declarations1 and declarations2.

TryStatement

try

Block

Finally

Let declarations1 be VarScopedDeclarations of Block.
Let declarations2 be VarScopedDeclarations of Finally.
Return the list-concatenation of declarations1 and declarations2.

try

Let declarations1 be VarScopedDeclarations of Block.
Let declarations2 be VarScopedDeclarations of Catch.
Let declarations3 be VarScopedDeclarations of Finally.
Return the list-concatenation of declarations1, declarations2, and declarations3.

Catch

catch

(

CatchParameter

)

Block

Return the VarScopedDeclarations of Block.

FunctionStatementList

[empty]

Return a new empty List.

FunctionStatementList

StatementList

Return the TopLevelVarScopedDeclarations of StatementList.

ClassStaticBlockStatementList

[empty]

Return a new empty List.

ClassStaticBlockStatementList

StatementList

Return the TopLevelVarScopedDeclarations of StatementList.

ConciseBody

ExpressionBody

Return a new empty List.

AsyncConciseBody

ExpressionBody

Return a new empty List.

Script

[empty]

Return a new empty List.

ScriptBody

StatementList

Return TopLevelVarScopedDeclarations of StatementList.

Module

[empty]

Return a new empty List.

ModuleItemList

ModuleItem

Let declarations1 be VarScopedDeclarations of ModuleItemList.
Let declarations2 be VarScopedDeclarations of ModuleItem.
Return the list-concatenation of declarations1 and declarations2.

ModuleItem

ImportDeclaration

Return a new empty List.

ModuleItem

ExportDeclaration

If ExportDeclaration is export VariableStatement, return VarScopedDeclarations of VariableStatement.
Return a new empty List.

8.2.8 Static Semantics: TopLevelLexicallyDeclaredNames

The syntax-directed operation TopLevelLexicallyDeclaredNames takes no arguments and returns a List of Strings. It is defined piecewise over the following productions:

StatementList

StatementListItem

Let names1 be TopLevelLexicallyDeclaredNames of StatementList.
Let names2 be TopLevelLexicallyDeclaredNames of StatementListItem.
Return the list-concatenation of names1 and names2.

StatementListItem

Statement

Return a new empty List.

StatementListItem

Declaration

1. If Declaration is Declaration : HoistableDeclaration , then
1. Return a new empty List.
Return the BoundNames of Declaration.

Note

At the top level of a function, or script, function declarations are treated like var declarations rather than like lexical declarations.

8.2.9 Static Semantics: TopLevelLexicallyScopedDeclarations

The syntax-directed operation TopLevelLexicallyScopedDeclarations takes no arguments and returns a List of Parse Nodes. It is defined piecewise over the following productions:

StatementList

StatementListItem

Let declarations1 be TopLevelLexicallyScopedDeclarations of StatementList.
Let declarations2 be TopLevelLexicallyScopedDeclarations of StatementListItem.
Return the list-concatenation of declarations1 and declarations2.

StatementListItem

Statement

Return a new empty List.

StatementListItem

Declaration

1. If Declaration is Declaration : HoistableDeclaration , then
1. Return a new empty List.
Return « Declaration ».

8.2.10 Static Semantics: TopLevelVarDeclaredNames

The syntax-directed operation TopLevelVarDeclaredNames takes no arguments and returns a List of Strings. It is defined piecewise over the following productions:

StatementList

StatementListItem

Let names1 be TopLevelVarDeclaredNames of StatementList.
Let names2 be TopLevelVarDeclaredNames of StatementListItem.
Return the list-concatenation of names1 and names2.

StatementListItem

Declaration

1. If Declaration is Declaration : HoistableDeclaration , then
1. Return the BoundNames of HoistableDeclaration.
Return a new empty List.

StatementListItem

Statement

If Statement is Statement : LabelledStatement , return TopLevelVarDeclaredNames of Statement.
Return VarDeclaredNames of Statement.

Note

At the top level of a function or script, inner function declarations are treated like var declarations.

LabelledStatement

LabelIdentifier

LabelledItem

Return the TopLevelVarDeclaredNames of LabelledItem.

LabelledItem

Statement

If Statement is Statement : LabelledStatement , return TopLevelVarDeclaredNames of Statement.
Return VarDeclaredNames of Statement.

LabelledItem

FunctionDeclaration

Return BoundNames of FunctionDeclaration.

8.2.11 Static Semantics: TopLevelVarScopedDeclarations

The syntax-directed operation TopLevelVarScopedDeclarations takes no arguments and returns a List of Parse Nodes. It is defined piecewise over the following productions:

StatementList

StatementListItem

Let declarations1 be TopLevelVarScopedDeclarations of StatementList.
Let declarations2 be TopLevelVarScopedDeclarations of StatementListItem.
Return the list-concatenation of declarations1 and declarations2.

StatementListItem

Statement

If Statement is Statement : LabelledStatement , return TopLevelVarScopedDeclarations of Statement.
Return VarScopedDeclarations of Statement.

StatementListItem

Declaration

1. If Declaration is Declaration : HoistableDeclaration , then
1. Let declaration be DeclarationPart of HoistableDeclaration.
2. Return « declaration ».
Return a new empty List.

LabelledStatement

LabelIdentifier

LabelledItem

Return the TopLevelVarScopedDeclarations of LabelledItem.

LabelledItem

Statement

If Statement is Statement : LabelledStatement , return TopLevelVarScopedDeclarations of Statement.
Return VarScopedDeclarations of Statement.

LabelledItem

FunctionDeclaration

Return « FunctionDeclaration ».

8.3 Labels

8.3.1 Static Semantics: ContainsDuplicateLabels

The syntax-directed operation ContainsDuplicateLabels takes argument labelSet (a List of Strings) and returns a Boolean. It is defined piecewise over the following productions:

{

}

StatementListItem

Declaration

Return false.

StatementList

StatementListItem

Let hasDuplicates be ContainsDuplicateLabels of StatementList with argument labelSet.
If hasDuplicates is true, return true.
Return ContainsDuplicateLabels of StatementListItem with argument labelSet.

(

)

else

Let hasDuplicate be ContainsDuplicateLabels of the first Statement with argument labelSet.
If hasDuplicate is true, return true.
Return ContainsDuplicateLabels of the second Statement with argument labelSet.

IfStatement

(

Expression

)

Statement

Return ContainsDuplicateLabels of Statement with argument labelSet.

DoWhileStatement

Statement

while

(

Expression

)

;

Return ContainsDuplicateLabels of Statement with argument labelSet.

WhileStatement

while

(

Expression

)

Statement

Return ContainsDuplicateLabels of Statement with argument labelSet.

ForStatement

for

(

Expression

opt

;

Expression

opt

;

Expression

opt

)

Statement

for

(

var

VariableDeclarationList

;

Expression

opt

;

Expression

opt

)

Statement

for

(

LexicalDeclaration

Expression

opt

;

Expression

opt

)

Statement

Return ContainsDuplicateLabels of Statement with argument labelSet.

ForInOfStatement

for

(

LeftHandSideExpression

Expression

)

Statement

for

(

var

ForBinding

Expression

)

Statement

for

(

ForDeclaration

Expression

)

Statement

for

(

LeftHandSideExpression

AssignmentExpression

)

Statement

for

(

var

ForBinding

AssignmentExpression

)

Statement

for

(

ForDeclaration

AssignmentExpression

)

Statement

for

await

(

LeftHandSideExpression

AssignmentExpression

)

Statement

for

await

(

var

ForBinding

AssignmentExpression

)

Statement

for

await

(

ForDeclaration

AssignmentExpression

)

Statement

Return ContainsDuplicateLabels of Statement with argument labelSet.

Note

This section is extended by Annex B.3.5.

WithStatement

with

(

Expression

)

Statement

Return ContainsDuplicateLabels of Statement with argument labelSet.

SwitchStatement

switch

(

Expression

)

CaseBlock

Return ContainsDuplicateLabels of CaseBlock with argument labelSet.

CaseBlock

{

}

Return false.

{

opt

opt

}

1. If the first CaseClauses is present, then
1. If ContainsDuplicateLabels of the first CaseClauses with argument labelSet is true, return true.
If ContainsDuplicateLabels of DefaultClause with argument labelSet is true, return true.
If the second CaseClauses is not present, return false.
Return ContainsDuplicateLabels of the second CaseClauses with argument labelSet.

CaseClauses

CaseClause

Let hasDuplicates be ContainsDuplicateLabels of CaseClauses with argument labelSet.
If hasDuplicates is true, return true.
Return ContainsDuplicateLabels of CaseClause with argument labelSet.

CaseClause

case

Expression

StatementList

opt

If the StatementList is present, return ContainsDuplicateLabels of StatementList with argument labelSet.
Return false.

DefaultClause

default

StatementList

opt

If the StatementList is present, return ContainsDuplicateLabels of StatementList with argument labelSet.
Return false.

LabelledStatement

LabelIdentifier

LabelledItem

Let label be the StringValue of LabelIdentifier.
If labelSet contains label, return true.
Let newLabelSet be the list-concatenation of labelSet and « label ».
Return ContainsDuplicateLabels of LabelledItem with argument newLabelSet.

LabelledItem

FunctionDeclaration

Return false.

TryStatement

try

Block

Catch

Let hasDuplicates be ContainsDuplicateLabels of Block with argument labelSet.
If hasDuplicates is true, return true.
Return ContainsDuplicateLabels of Catch with argument labelSet.

TryStatement

try

Block

Finally

Let hasDuplicates be ContainsDuplicateLabels of Block with argument labelSet.
If hasDuplicates is true, return true.
Return ContainsDuplicateLabels of Finally with argument labelSet.

try

If ContainsDuplicateLabels of Block with argument labelSet is true, return true.
If ContainsDuplicateLabels of Catch with argument labelSet is true, return true.
Return ContainsDuplicateLabels of Finally with argument labelSet.

Catch

catch

(

CatchParameter

)

Block

Return ContainsDuplicateLabels of Block with argument labelSet.

FunctionStatementList

[empty]

Return false.

ClassStaticBlockStatementList

[empty]

Return false.

ModuleItemList

ModuleItem

Let hasDuplicates be ContainsDuplicateLabels of ModuleItemList with argument labelSet.
If hasDuplicates is true, return true.
Return ContainsDuplicateLabels of ModuleItem with argument labelSet.

ModuleItem

ImportDeclaration

ExportDeclaration

Return false.

8.3.2 Static Semantics: ContainsUndefinedBreakTarget

The syntax-directed operation ContainsUndefinedBreakTarget takes argument labelSet (a List of Strings) and returns a Boolean. It is defined piecewise over the following productions:

{

}

StatementListItem

Declaration

Return false.

StatementList

StatementListItem

Let hasUndefinedLabels be ContainsUndefinedBreakTarget of StatementList with argument labelSet.
If hasUndefinedLabels is true, return true.
Return ContainsUndefinedBreakTarget of StatementListItem with argument labelSet.

(

)

else

Let hasUndefinedLabels be ContainsUndefinedBreakTarget of the first Statement with argument labelSet.
If hasUndefinedLabels is true, return true.
Return ContainsUndefinedBreakTarget of the second Statement with argument labelSet.

IfStatement

(

Expression

)

Statement

Return ContainsUndefinedBreakTarget of Statement with argument labelSet.

DoWhileStatement

Statement

while

(

Expression

)

;

Return ContainsUndefinedBreakTarget of Statement with argument labelSet.

WhileStatement

while

(

Expression

)

Statement

Return ContainsUndefinedBreakTarget of Statement with argument labelSet.

ForStatement

for

(

Expression

opt

;

Expression

opt

;

Expression

opt

)

Statement

for

(

var

VariableDeclarationList

;

Expression

opt

;

Expression

opt

)

Statement

for

(

LexicalDeclaration

Expression

opt

;

Expression

opt

)

Statement

Return ContainsUndefinedBreakTarget of Statement with argument labelSet.

ForInOfStatement

for

(

LeftHandSideExpression

Expression

)

Statement

for

(

var

ForBinding

Expression

)

Statement

for

(

ForDeclaration

Expression

)

Statement

for

(

LeftHandSideExpression

AssignmentExpression

)

Statement

for

(

var

ForBinding

AssignmentExpression

)

Statement

for

(

ForDeclaration

AssignmentExpression

)

Statement

for

await

(

LeftHandSideExpression

AssignmentExpression

)

Statement

for

await

(

var

ForBinding

AssignmentExpression

)

Statement

for

await

(

ForDeclaration

AssignmentExpression

)

Statement

Return ContainsUndefinedBreakTarget of Statement with argument labelSet.

Note

This section is extended by Annex B.3.5.

BreakStatement

break

;

Return false.

BreakStatement

break

LabelIdentifier

;

If labelSet does not contain the StringValue of LabelIdentifier, return true.
Return false.

WithStatement

with

(

Expression

)

Statement

Return ContainsUndefinedBreakTarget of Statement with argument labelSet.

SwitchStatement

switch

(

Expression

)

CaseBlock

Return ContainsUndefinedBreakTarget of CaseBlock with argument labelSet.

CaseBlock

{

}

Return false.

{

opt

opt

}

1. If the first CaseClauses is present, then
1. If ContainsUndefinedBreakTarget of the first CaseClauses with argument labelSet is true, return true.
If ContainsUndefinedBreakTarget of DefaultClause with argument labelSet is true, return true.
If the second CaseClauses is not present, return false.
Return ContainsUndefinedBreakTarget of the second CaseClauses with argument labelSet.

CaseClauses

CaseClause

Let hasUndefinedLabels be ContainsUndefinedBreakTarget of CaseClauses with argument labelSet.
If hasUndefinedLabels is true, return true.
Return ContainsUndefinedBreakTarget of CaseClause with argument labelSet.

CaseClause

case

Expression

StatementList

opt

If the StatementList is present, return ContainsUndefinedBreakTarget of StatementList with argument labelSet.
Return false.

DefaultClause

default

StatementList

opt

If the StatementList is present, return ContainsUndefinedBreakTarget of StatementList with argument labelSet.
Return false.

LabelledStatement

LabelIdentifier

LabelledItem

Let label be the StringValue of LabelIdentifier.
Let newLabelSet be the list-concatenation of labelSet and « label ».
Return ContainsUndefinedBreakTarget of LabelledItem with argument newLabelSet.

LabelledItem

FunctionDeclaration

Return false.

TryStatement

try

Block

Catch

Let hasUndefinedLabels be ContainsUndefinedBreakTarget of Block with argument labelSet.
If hasUndefinedLabels is true, return true.
Return ContainsUndefinedBreakTarget of Catch with argument labelSet.

TryStatement

try

Block

Finally

Let hasUndefinedLabels be ContainsUndefinedBreakTarget of Block with argument labelSet.
If hasUndefinedLabels is true, return true.
Return ContainsUndefinedBreakTarget of Finally with argument labelSet.

try

If ContainsUndefinedBreakTarget of Block with argument labelSet is true, return true.
If ContainsUndefinedBreakTarget of Catch with argument labelSet is true, return true.
Return ContainsUndefinedBreakTarget of Finally with argument labelSet.

Catch

catch

(

CatchParameter

)

Block

Return ContainsUndefinedBreakTarget of Block with argument labelSet.

FunctionStatementList

[empty]

Return false.

ClassStaticBlockStatementList

[empty]

Return false.

ModuleItemList

ModuleItem

Let hasUndefinedLabels be ContainsUndefinedBreakTarget of ModuleItemList with argument labelSet.
If hasUndefinedLabels is true, return true.
Return ContainsUndefinedBreakTarget of ModuleItem with argument labelSet.

ModuleItem

ImportDeclaration

ExportDeclaration

Return false.

8.3.3 Static Semantics: ContainsUndefinedContinueTarget

The syntax-directed operation ContainsUndefinedContinueTarget takes arguments iterationSet (a List of Strings) and labelSet (a List of Strings) and returns a Boolean. It is defined piecewise over the following productions:

{

}

StatementListItem

Declaration

Return false.

Statement

BlockStatement

Return ContainsUndefinedContinueTarget of BlockStatement with arguments iterationSet and « ».

BreakableStatement

IterationStatement

Let newIterationSet be the list-concatenation of iterationSet and labelSet.
Return ContainsUndefinedContinueTarget of IterationStatement with arguments newIterationSet and « ».

StatementList

StatementListItem

Let hasUndefinedLabels be ContainsUndefinedContinueTarget of StatementList with arguments iterationSet and « ».
If hasUndefinedLabels is true, return true.
Return ContainsUndefinedContinueTarget of StatementListItem with arguments iterationSet and « ».

(

)

else

Let hasUndefinedLabels be ContainsUndefinedContinueTarget of the first Statement with arguments iterationSet and « ».
If hasUndefinedLabels is true, return true.
Return ContainsUndefinedContinueTarget of the second Statement with arguments iterationSet and « ».

IfStatement

(

Expression

)

Statement

Return ContainsUndefinedContinueTarget of Statement with arguments iterationSet and « ».

DoWhileStatement

Statement

while

(

Expression

)

;

Return ContainsUndefinedContinueTarget of Statement with arguments iterationSet and « ».

WhileStatement

while

(

Expression

)

Statement

Return ContainsUndefinedContinueTarget of Statement with arguments iterationSet and « ».

ForStatement

for

(

Expression

opt

;

Expression

opt

;

Expression

opt

)

Statement

for

(

var

VariableDeclarationList

;

Expression

opt

;

Expression

opt

)

Statement

for

(

LexicalDeclaration

Expression

opt

;

Expression

opt

)

Statement

Return ContainsUndefinedContinueTarget of Statement with arguments iterationSet and « ».

ForInOfStatement

for

(

LeftHandSideExpression

Expression

)

Statement

for

(

var

ForBinding

Expression

)

Statement

for

(

ForDeclaration

Expression

)

Statement

for

(

LeftHandSideExpression

AssignmentExpression

)

Statement

for

(

var

ForBinding

AssignmentExpression

)

Statement

for

(

ForDeclaration

AssignmentExpression

)

Statement

for

await

(

LeftHandSideExpression

AssignmentExpression

)

Statement

for

await

(

var

ForBinding

AssignmentExpression

)

Statement

for

await

(

ForDeclaration

AssignmentExpression

)

Statement

Return ContainsUndefinedContinueTarget of Statement with arguments iterationSet and « ».

Note

This section is extended by Annex B.3.5.

ContinueStatement

continue

;

Return false.

ContinueStatement

continue

LabelIdentifier

;

If iterationSet does not contain the StringValue of LabelIdentifier, return true.
Return false.

WithStatement

with

(

Expression

)

Statement

Return ContainsUndefinedContinueTarget of Statement with arguments iterationSet and « ».

SwitchStatement

switch

(

Expression

)

CaseBlock

Return ContainsUndefinedContinueTarget of CaseBlock with arguments iterationSet and « ».

CaseBlock

{

}

Return false.

{

opt

opt

}

1. If the first CaseClauses is present, then
1. If ContainsUndefinedContinueTarget of the first CaseClauses with arguments iterationSet and « » is true, return true.
If ContainsUndefinedContinueTarget of DefaultClause with arguments iterationSet and « » is true, return true.
If the second CaseClauses is not present, return false.
Return ContainsUndefinedContinueTarget of the second CaseClauses with arguments iterationSet and « ».

CaseClauses

CaseClause

Let hasUndefinedLabels be ContainsUndefinedContinueTarget of CaseClauses with arguments iterationSet and « ».
If hasUndefinedLabels is true, return true.
Return ContainsUndefinedContinueTarget of CaseClause with arguments iterationSet and « ».

CaseClause

case

Expression

StatementList

opt

If the StatementList is present, return ContainsUndefinedContinueTarget of StatementList with arguments iterationSet and « ».
Return false.

DefaultClause

default

StatementList

opt

If the StatementList is present, return ContainsUndefinedContinueTarget of StatementList with arguments iterationSet and « ».
Return false.

LabelledStatement

LabelIdentifier

LabelledItem

Let label be the StringValue of LabelIdentifier.
Let newLabelSet be the list-concatenation of labelSet and « label ».
Return ContainsUndefinedContinueTarget of LabelledItem with arguments iterationSet and newLabelSet.

LabelledItem

FunctionDeclaration

Return false.

TryStatement

try

Block

Catch

Let hasUndefinedLabels be ContainsUndefinedContinueTarget of Block with arguments iterationSet and « ».
If hasUndefinedLabels is true, return true.
Return ContainsUndefinedContinueTarget of Catch with arguments iterationSet and « ».

TryStatement

try

Block

Finally

Let hasUndefinedLabels be ContainsUndefinedContinueTarget of Block with arguments iterationSet and « ».
If hasUndefinedLabels is true, return true.
Return ContainsUndefinedContinueTarget of Finally with arguments iterationSet and « ».

try

If ContainsUndefinedContinueTarget of Block with arguments iterationSet and « » is true, return true.
If ContainsUndefinedContinueTarget of Catch with arguments iterationSet and « » is true, return true.
Return ContainsUndefinedContinueTarget of Finally with arguments iterationSet and « ».

Catch

catch

(

CatchParameter

)

Block

Return ContainsUndefinedContinueTarget of Block with arguments iterationSet and « ».

FunctionStatementList

[empty]

Return false.

ClassStaticBlockStatementList

[empty]

Return false.

ModuleItemList

ModuleItem

Let hasUndefinedLabels be ContainsUndefinedContinueTarget of ModuleItemList with arguments iterationSet and « ».
If hasUndefinedLabels is true, return true.
Return ContainsUndefinedContinueTarget of ModuleItem with arguments iterationSet and « ».

ModuleItem

ImportDeclaration

ExportDeclaration

Return false.

8.4 Function Name Inference

8.4.1 Static Semantics: HasName

The syntax-directed operation HasName takes no arguments and returns a Boolean. It is defined piecewise over the following productions:

PrimaryExpression

CoverParenthesizedExpressionAndArrowParameterList

Let expr be the ParenthesizedExpression that is covered by CoverParenthesizedExpressionAndArrowParameterList.
If IsFunctionDefinition of expr is false, return false.
Return HasName of expr.

FunctionExpression

function

(

FormalParameters

)

{

FunctionBody

}

GeneratorExpression

function

(

FormalParameters

)

{

GeneratorBody

}

AsyncGeneratorExpression

async

function

(

FormalParameters

)

{

AsyncGeneratorBody

}

AsyncFunctionExpression

async

function

(

FormalParameters

)

{

}

async

AsyncArrowBindingIdentifier

AsyncConciseBody

CoverCallExpressionAndAsyncArrowHead

AsyncConciseBody

ClassExpression

class

ClassTail

Return false.

FunctionExpression

function

BindingIdentifier

(

FormalParameters

)

{

}

function

(

)

{

GeneratorBody

}

AsyncGeneratorExpression

async

function

BindingIdentifier

(

FormalParameters

)

{

AsyncGeneratorBody

}

AsyncFunctionExpression

async

function

BindingIdentifier

(

FormalParameters

)

{

}

class

Return true.

8.4.2 Static Semantics: IsFunctionDefinition

The syntax-directed operation IsFunctionDefinition takes no arguments and returns a Boolean. It is defined piecewise over the following productions:

PrimaryExpression

CoverParenthesizedExpressionAndArrowParameterList

Let expr be the ParenthesizedExpression that is covered by CoverParenthesizedExpressionAndArrowParameterList.
Return IsFunctionDefinition of expr.

this

RegularExpressionLiteral

[

]

new

new

LeftHandSideExpression

CallExpression

OptionalExpression

UpdateExpression

LeftHandSideExpression

delete

void

typeof

ExponentiationExpression

UpdateExpression

ExponentiationExpression

MultiplicativeExpression

MultiplicativeOperator

ExponentiationExpression

AdditiveExpression

MultiplicativeExpression

AdditiveExpression

MultiplicativeExpression

>>>

instanceof

===

!==

CoalesceExpressionHead

BitwiseORExpression

ConditionalExpression

ShortCircuitExpression

AssignmentExpression

YieldExpression

LeftHandSideExpression

AssignmentExpression

LeftHandSideExpression

AssignmentOperator

AssignmentExpression

LeftHandSideExpression

&&=

AssignmentExpression

LeftHandSideExpression

||=

AssignmentExpression

LeftHandSideExpression

??=

AssignmentExpression

Expression

AssignmentExpression

Return false.

function

opt

(

FormalParameters

)

{

FunctionBody

}

GeneratorExpression

function

BindingIdentifier

opt

(

FormalParameters

)

{

GeneratorBody

}

AsyncGeneratorExpression

async

function

BindingIdentifier

opt

(

FormalParameters

)

{

AsyncGeneratorBody

}

AsyncFunctionExpression

async

function

BindingIdentifier

opt

(

FormalParameters

)

{

}

class

opt

Return true.

8.4.3 Static Semantics: IsAnonymousFunctionDefinition ( `expr` )

The abstract operation IsAnonymousFunctionDefinition takes argument expr (an AssignmentExpression Parse Node, an Initializer Parse Node, or an Expression Parse Node) and returns a Boolean. It determines if its argument is a function definition that does not bind a name. It performs the following steps when called:

If IsFunctionDefinition of expr is false, return false.
Let hasName be HasName of expr.
If hasName is true, return false.
Return true.

8.4.4 Static Semantics: IsIdentifierRef

The syntax-directed operation IsIdentifierRef takes no arguments and returns a Boolean. It is defined piecewise over the following productions:

PrimaryExpression

IdentifierReference

Return true.

this

AsyncFunctionExpression

AsyncGeneratorExpression

RegularExpressionLiteral

TemplateLiteral

CoverParenthesizedExpressionAndArrowParameterList

[

]

new

new

LeftHandSideExpression

CallExpression

OptionalExpression

Return false.

8.4.5 Runtime Semantics: NamedEvaluation

The syntax-directed operation NamedEvaluation takes argument name (a property key or a Private Name) and returns either a normal completion containing a function object or an abrupt completion. It is defined piecewise over the following productions:

PrimaryExpression

CoverParenthesizedExpressionAndArrowParameterList

Let expr be the ParenthesizedExpression that is covered by CoverParenthesizedExpressionAndArrowParameterList.
Return ? NamedEvaluation of expr with argument name.

ParenthesizedExpression

(

Expression

)

Assert: IsAnonymousFunctionDefinition(Expression) is true.
Return ? NamedEvaluation of Expression with argument name.

FunctionExpression

function

(

FormalParameters

)

{

FunctionBody

}

Return InstantiateOrdinaryFunctionExpression of FunctionExpression with argument name.

GeneratorExpression

function

(

FormalParameters

)

{

GeneratorBody

}

Return InstantiateGeneratorFunctionExpression of GeneratorExpression with argument name.

AsyncGeneratorExpression

async

function

(

FormalParameters

)

{

AsyncGeneratorBody

}

Return InstantiateAsyncGeneratorFunctionExpression of AsyncGeneratorExpression with argument name.

AsyncFunctionExpression

async

function

(

FormalParameters

)

{

AsyncFunctionBody

}

Return InstantiateAsyncFunctionExpression of AsyncFunctionExpression with argument name.

ArrowFunction

ArrowParameters

ConciseBody

Return InstantiateArrowFunctionExpression of ArrowFunction with argument name.

AsyncArrowFunction

async

AsyncArrowBindingIdentifier

AsyncConciseBody

CoverCallExpressionAndAsyncArrowHead

AsyncConciseBody

Return InstantiateAsyncArrowFunctionExpression of AsyncArrowFunction with argument name.

ClassExpression

class

ClassTail

Let value be ? ClassDefinitionEvaluation of ClassTail with arguments undefined and name.
Set value.[[SourceText]] to the source text matched by ClassExpression.
Return value.

8.5 Contains

8.5.1 Static Semantics: Contains

The syntax-directed operation Contains takes argument symbol (a grammar symbol) and returns a Boolean.

Every grammar production alternative in this specification which is not listed below implicitly has the following default definition of Contains:

1. For each child node child of this Parse Node, do
1. If child is an instance of symbol, return true.
2. b. If child is an instance of a nonterminal, then
  1. Let contained be the result of child Contains symbol.
  2. If contained is true, return true.
Return false.

FunctionDeclaration

function

BindingIdentifier

(

FormalParameters

)

{

FunctionBody

}

function

(

FormalParameters

)

{

FunctionBody

}

FunctionExpression

function

BindingIdentifier

opt

(

FormalParameters

)

{

}

function

(

)

{

GeneratorBody

}

function

(

FormalParameters

)

{

GeneratorBody

}

GeneratorExpression

function

BindingIdentifier

opt

(

FormalParameters

)

{

GeneratorBody

}

AsyncGeneratorDeclaration

async

function

BindingIdentifier

(

FormalParameters

)

{

AsyncGeneratorBody

}

async

function

(

FormalParameters

)

{

AsyncGeneratorBody

}

AsyncGeneratorExpression

async

function

BindingIdentifier

opt

(

FormalParameters

)

{

AsyncGeneratorBody

}

AsyncFunctionDeclaration

async

function

BindingIdentifier

(

FormalParameters

)

{

AsyncFunctionBody

}

async

function

(

FormalParameters

)

{

AsyncFunctionBody

}

AsyncFunctionExpression

async

function

BindingIdentifier

opt

(

FormalParameters

)

{

AsyncFunctionBody

}

Return false.

Note 1

Static semantic rules that depend upon substructure generally do not look into function definitions.

ClassTail

ClassHeritage

opt

{

ClassBody

}

If symbol is ClassBody, return true.
2. If symbol is ClassHeritage, then
1. If ClassHeritage is present, return true; otherwise return false.
3. If ClassHeritage is present, then
1. If ClassHeritage Contains symbol is true, return true.
Return the result of ComputedPropertyContains of ClassBody with argument symbol.

Note 2

Static semantic rules that depend upon substructure generally do not look into class bodies except for PropertyNames.

ClassStaticBlock

static

{

ClassStaticBlockBody

}

Return false.

Note 3

Static semantic rules that depend upon substructure generally do not look into static initialization blocks.

ArrowFunction

ArrowParameters

ConciseBody

If symbol is not one of NewTarget, SuperProperty, SuperCall, super, or this, return false.
If ArrowParameters Contains symbol is true, return true.
Return ConciseBody Contains symbol.

ArrowParameters

CoverParenthesizedExpressionAndArrowParameterList

Let formals be the ArrowFormalParameters that is covered by CoverParenthesizedExpressionAndArrowParameterList.
Return formals Contains symbol.

AsyncArrowFunction

async

AsyncArrowBindingIdentifier

AsyncConciseBody

If symbol is not one of NewTarget, SuperProperty, SuperCall, super, or this, return false.
Return AsyncConciseBody Contains symbol.

AsyncArrowFunction

CoverCallExpressionAndAsyncArrowHead

AsyncConciseBody

If symbol is not one of NewTarget, SuperProperty, SuperCall, super, or this, return false.
Let head be the AsyncArrowHead that is covered by CoverCallExpressionAndAsyncArrowHead.
If head Contains symbol is true, return true.
Return AsyncConciseBody Contains symbol.

Note 4

Contains is used to detect new.target, this, and super usage within an ArrowFunction or AsyncArrowFunction.

PropertyDefinition

MethodDefinition

If symbol is MethodDefinition, return true.
Return the result of ComputedPropertyContains of MethodDefinition with argument symbol.

LiteralPropertyName

IdentifierName

Return false.

MemberExpression

IdentifierName

If MemberExpression Contains symbol is true, return true.
Return false.

SuperProperty

super

IdentifierName

If symbol is the ReservedWord super, return true.
Return false.

CallExpression

IdentifierName

If CallExpression Contains symbol is true, return true.
Return false.

OptionalChain

IdentifierName

Return false.

OptionalChain

IdentifierName

If OptionalChain Contains symbol is true, return true.
Return false.

8.5.2 Static Semantics: ComputedPropertyContains

The syntax-directed operation ComputedPropertyContains takes argument symbol (a grammar symbol) and returns a Boolean. It is defined piecewise over the following productions:

Return false.

PropertyName

ComputedPropertyName

Return the result of ComputedPropertyName Contains symbol.

MethodDefinition

ClassElementName

(

UniqueFormalParameters

)

{

FunctionBody

}

get

ClassElementName

(

)

{

FunctionBody

}

set

ClassElementName

(

PropertySetParameterList

)

{

FunctionBody

}

Return the result of ComputedPropertyContains of ClassElementName with argument symbol.

GeneratorMethod

ClassElementName

(

UniqueFormalParameters

)

{

GeneratorBody

}

Return the result of ComputedPropertyContains of ClassElementName with argument symbol.

AsyncGeneratorMethod

async

ClassElementName

(

UniqueFormalParameters

)

{

AsyncGeneratorBody

}

Return the result of ComputedPropertyContains of ClassElementName with argument symbol.

ClassElementList

ClassElement

Let inList be ComputedPropertyContains of ClassElementList with argument symbol.
If inList is true, return true.
Return the result of ComputedPropertyContains of ClassElement with argument symbol.

ClassElement

ClassStaticBlock

Return false.

ClassElement

;

Return false.

AsyncMethod

async

ClassElementName

(

UniqueFormalParameters

)

{

AsyncFunctionBody

}

Return the result of ComputedPropertyContains of ClassElementName with argument symbol.

FieldDefinition

ClassElementName

Initializer

opt

Return the result of ComputedPropertyContains of ClassElementName with argument symbol.

8.6 Miscellaneous

These operations are used in multiple places throughout the specification.

8.6.1 Runtime Semantics: InstantiateFunctionObject

The syntax-directed operation InstantiateFunctionObject takes arguments env (an Environment Record) and privateEnv (a PrivateEnvironment Record or null) and returns an ECMAScript function object. It is defined piecewise over the following productions:

FunctionDeclaration

function

BindingIdentifier

(

FormalParameters

)

{

FunctionBody

}

function

(

FormalParameters

)

{

FunctionBody

}

Return InstantiateOrdinaryFunctionObject of FunctionDeclaration with arguments env and privateEnv.

GeneratorDeclaration

function

BindingIdentifier

(

FormalParameters

)

{

GeneratorBody

}

function

(

FormalParameters

)

{

GeneratorBody

}

Return InstantiateGeneratorFunctionObject of GeneratorDeclaration with arguments env and privateEnv.

AsyncGeneratorDeclaration

async

function

BindingIdentifier

(

FormalParameters

)

{

AsyncGeneratorBody

}

async

function

(

FormalParameters

)

{

AsyncGeneratorBody

}

Return InstantiateAsyncGeneratorFunctionObject of AsyncGeneratorDeclaration with arguments env and privateEnv.

AsyncFunctionDeclaration

async

function

BindingIdentifier

(

FormalParameters

)

{

AsyncFunctionBody

}

async

function

(

FormalParameters

)

{

AsyncFunctionBody

}

Return InstantiateAsyncFunctionObject of AsyncFunctionDeclaration with arguments env and privateEnv.

8.6.2 Runtime Semantics: BindingInitialization

The syntax-directed operation BindingInitialization takes arguments value (an ECMAScript language value) and environment (an Environment Record or undefined) and returns either a normal completion containing unused or an abrupt completion.

Note

undefined is passed for environment to indicate that a PutValue operation should be used to assign the initialization value. This is the case for var statements and formal parameter lists of some non-strict functions (See 10.2.11). In those cases a lexical binding is hoisted and preinitialized prior to evaluation of its initializer.

It is defined piecewise over the following productions:

BindingIdentifier

Identifier

Let name be StringValue of Identifier.
Return ? InitializeBoundName(name, value, environment).

BindingIdentifier

yield

Return ? InitializeBoundName("yield", value, environment).

BindingIdentifier

await

Return ? InitializeBoundName("await", value, environment).

BindingPattern

ObjectBindingPattern

Perform ? RequireObjectCoercible(value).
Return ? BindingInitialization of ObjectBindingPattern with arguments value and environment.

BindingPattern

ArrayBindingPattern

Let iteratorRecord be ? GetIterator(value, sync).
Let result be Completion(IteratorBindingInitialization of ArrayBindingPattern with arguments iteratorRecord and environment).
If iteratorRecord.[[Done]] is false, return ? IteratorClose(iteratorRecord, result).
Return ? result.

ObjectBindingPattern

{

}

Return unused.

ObjectBindingPattern

{

BindingPropertyList

}

{

BindingPropertyList

}

Perform ? PropertyBindingInitialization of BindingPropertyList with arguments value and environment.
Return unused.

ObjectBindingPattern

{

BindingRestProperty

}

Let excludedNames be a new empty List.
Return ? RestBindingInitialization of BindingRestProperty with arguments value, environment, and excludedNames.

ObjectBindingPattern

{

BindingPropertyList

BindingRestProperty

}

Let excludedNames be ? PropertyBindingInitialization of BindingPropertyList with arguments value and environment.
Return ? RestBindingInitialization of BindingRestProperty with arguments value, environment, and excludedNames.

8.6.2.1 InitializeBoundName ( `name`, `value`, `environment` )

The abstract operation InitializeBoundName takes arguments name (a String), value (an ECMAScript language value), and environment (an Environment Record or undefined) and returns either a normal completion containing unused or an abrupt completion. It performs the following steps when called:

1. If environment is not undefined, then
1. Perform ! environment.InitializeBinding(name, value).
2. Return unused.
2. Else,
1. Let lhs be ? ResolveBinding(name).
2. Return ? PutValue(lhs, value).

8.6.3 Runtime Semantics: IteratorBindingInitialization

The syntax-directed operation IteratorBindingInitialization takes arguments iteratorRecord (an Iterator Record) and environment (an Environment Record or undefined) and returns either a normal completion containing unused or an abrupt completion.

Note

When undefined is passed for environment it indicates that a PutValue operation should be used to assign the initialization value. This is the case for formal parameter lists of non-strict functions. In that case the formal parameter bindings are preinitialized in order to deal with the possibility of multiple parameters with the same name.

It is defined piecewise over the following productions:

ArrayBindingPattern

[

]

Return unused.

ArrayBindingPattern

[

Elision

]

Return ? IteratorDestructuringAssignmentEvaluation of Elision with argument iteratorRecord.

ArrayBindingPattern

[

Elision

opt

BindingRestElement

]

1. If Elision is present, then
1. Perform ? IteratorDestructuringAssignmentEvaluation of Elision with argument iteratorRecord.
Return ? IteratorBindingInitialization of BindingRestElement with arguments iteratorRecord and environment.

ArrayBindingPattern

[

BindingElementList

Elision

]

Perform ? IteratorBindingInitialization of BindingElementList with arguments iteratorRecord and environment.
Return ? IteratorDestructuringAssignmentEvaluation of Elision with argument iteratorRecord.

[

opt

]

Perform ? IteratorBindingInitialization of BindingElementList with arguments iteratorRecord and environment.
2. If Elision is present, then
1. Perform ? IteratorDestructuringAssignmentEvaluation of Elision with argument iteratorRecord.
Return ? IteratorBindingInitialization of BindingRestElement with arguments iteratorRecord and environment.

BindingElementList

BindingElisionElement

Perform ? IteratorBindingInitialization of BindingElementList with arguments iteratorRecord and environment.
Return ? IteratorBindingInitialization of BindingElisionElement with arguments iteratorRecord and environment.

BindingElisionElement

Elision

BindingElement

Perform ? IteratorDestructuringAssignmentEvaluation of Elision with argument iteratorRecord.
Return ? IteratorBindingInitialization of BindingElement with arguments iteratorRecord and environment.

SingleNameBinding

BindingIdentifier

Initializer

opt

Let bindingId be StringValue of BindingIdentifier.
Let lhs be ? ResolveBinding(bindingId, environment).
Let v be undefined.
4. If iteratorRecord.[[Done]] is false, then
1. Let next be ? IteratorStepValue(iteratorRecord).
2. b. If next is not done, then
  1. Set v to next.
5. If Initializer is present and v is undefined, then
1. a. If IsAnonymousFunctionDefinition(Initializer) is true, then
  1. Set v to ? NamedEvaluation of Initializer with argument bindingId.
2. b. Else,
  1. Let defaultValue be ? Evaluation of Initializer.
  2. Set v to ? GetValue(defaultValue).
If environment is undefined, return ? PutValue(lhs, v).
Return ? InitializeReferencedBinding(lhs, v).

BindingElement

BindingPattern

Initializer

opt

Let v be undefined.
2. If iteratorRecord.[[Done]] is false, then
1. Let next be ? IteratorStepValue(iteratorRecord).
2. b. If next is not done, then
  1. Set v to next.
3. If Initializer is present and v is undefined, then
1. Let defaultValue be ? Evaluation of Initializer.
2. Set v to ? GetValue(defaultValue).
Return ? BindingInitialization of BindingPattern with arguments v and environment.

BindingRestElement

...

BindingIdentifier

Let lhs be ? ResolveBinding(StringValue of BindingIdentifier, environment).
Let A be ! ArrayCreate(0).
Let n be 0.
4. Repeat,
1. Let next be done.
2. b. If iteratorRecord.[[Done]] is false, then
  1. Set next to ? IteratorStepValue(iteratorRecord).
3. c. If next is done, then
  1. If environment is undefined, return ? PutValue(lhs, A).
  2. Return ? InitializeReferencedBinding(lhs, A).
4. Perform ! CreateDataPropertyOrThrow(A, ! ToString(𝔽(n)), next).
5. Set n to n + 1.

BindingRestElement

...

BindingPattern

Let A be ! ArrayCreate(0).
Let n be 0.
3. Repeat,
1. Let next be done.
2. b. If iteratorRecord.[[Done]] is false, then
  1. Set next to ? IteratorStepValue(iteratorRecord).
3. c. If next is done, then
  1. Return ? BindingInitialization of BindingPattern with arguments A and environment.
4. Perform ! CreateDataPropertyOrThrow(A, ! ToString(𝔽(n)), next).
5. Set n to n + 1.

FormalParameters

[empty]

Return unused.

FormalParameters

FormalParameterList

FunctionRestParameter

Perform ? IteratorBindingInitialization of FormalParameterList with arguments iteratorRecord and environment.
Return ? IteratorBindingInitialization of FunctionRestParameter with arguments iteratorRecord and environment.

FormalParameterList

FormalParameter

Perform ? IteratorBindingInitialization of FormalParameterList with arguments iteratorRecord and environment.
Return ? IteratorBindingInitialization of FormalParameter with arguments iteratorRecord and environment.

ArrowParameters

BindingIdentifier

Let v be undefined.
Assert: iteratorRecord.[[Done]] is false.
Let next be ? IteratorStepValue(iteratorRecord).
4. If next is not done, then
1. Set v to next.
Return ? BindingInitialization of BindingIdentifier with arguments v and environment.

ArrowParameters

CoverParenthesizedExpressionAndArrowParameterList

Let formals be the ArrowFormalParameters that is covered by CoverParenthesizedExpressionAndArrowParameterList.
Return ? IteratorBindingInitialization of formals with arguments iteratorRecord and environment.

AsyncArrowBindingIdentifier

BindingIdentifier

Let v be undefined.
Assert: iteratorRecord.[[Done]] is false.
Let next be ? IteratorStepValue(iteratorRecord).
4. If next is not done, then
1. Set v to next.
Return ? BindingInitialization of BindingIdentifier with arguments v and environment.

8.6.4 Static Semantics: AssignmentTargetType

The syntax-directed operation AssignmentTargetType takes no arguments and returns simple or invalid. It is defined piecewise over the following productions:

IdentifierReference

Identifier

If this IdentifierReference is contained in strict mode code and StringValue of Identifier is either "eval" or "arguments", return invalid.
Return simple.

IdentifierReference

yield

await

[

]

[

]

Return simple.

PrimaryExpression

CoverParenthesizedExpressionAndArrowParameterList

Let expr be the ParenthesizedExpression that is covered by CoverParenthesizedExpressionAndArrowParameterList.
Return AssignmentTargetType of expr.

this

AsyncFunctionExpression

AsyncGeneratorExpression

RegularExpressionLiteral

TemplateLiteral

CallExpression

CoverCallExpressionAndAsyncArrowHead

new

new

new

target

ImportMeta

import

meta

LeftHandSideExpression

OptionalExpression

UpdateExpression

LeftHandSideExpression

delete

void

typeof

ExponentiationExpression

UpdateExpression

ExponentiationExpression

MultiplicativeExpression

MultiplicativeOperator

ExponentiationExpression

AdditiveExpression

MultiplicativeExpression

AdditiveExpression

MultiplicativeExpression

>>>

instanceof

===

!==

CoalesceExpressionHead

BitwiseORExpression

ConditionalExpression

ShortCircuitExpression

LeftHandSideExpression

AssignmentExpression

LeftHandSideExpression

AssignmentOperator

AssignmentExpression

LeftHandSideExpression

&&=

AssignmentExpression

LeftHandSideExpression

||=

AssignmentExpression

LeftHandSideExpression

??=

AssignmentExpression

Expression

AssignmentExpression

Return invalid.

8.6.5 Static Semantics: PropName

The syntax-directed operation PropName takes no arguments and returns a String or empty. It is defined piecewise over the following productions:

PropertyDefinition

IdentifierReference

Return StringValue of IdentifierReference.

PropertyDefinition

...

AssignmentExpression

Return empty.

PropertyDefinition

PropertyName

AssignmentExpression

Return PropName of PropertyName.

LiteralPropertyName

IdentifierName

Return StringValue of IdentifierName.

LiteralPropertyName

StringLiteral

Return the SV of StringLiteral.

LiteralPropertyName

NumericLiteral

Let nbr be the NumericValue of NumericLiteral.
Return ! ToString(nbr).

ComputedPropertyName

[

AssignmentExpression

]

Return empty.

MethodDefinition

ClassElementName

(

UniqueFormalParameters

)

{

FunctionBody

}

get

ClassElementName

(

)

{

FunctionBody

}

set

ClassElementName

(

PropertySetParameterList

)

{

FunctionBody

}

Return PropName of ClassElementName.

GeneratorMethod

ClassElementName

(

UniqueFormalParameters

)

{

GeneratorBody

}

Return PropName of ClassElementName.

AsyncGeneratorMethod

async

ClassElementName

(

UniqueFormalParameters

)

{

AsyncGeneratorBody

}

Return PropName of ClassElementName.

ClassElement

ClassStaticBlock

Return empty.

ClassElement

;

Return empty.

AsyncMethod

async

ClassElementName

(

UniqueFormalParameters

)

{

AsyncFunctionBody

}

Return PropName of ClassElementName.

FieldDefinition

ClassElementName

Initializer

opt

Return PropName of ClassElementName.

ClassElementName

PrivateIdentifier

Return empty.

9 Executable Code and Execution Contexts

9.1 Environment Records

Environment Record is a specification type used to define the association of Identifiers to specific variables and functions, based upon the lexical nesting structure of ECMAScript code. Usually an Environment Record is associated with some specific syntactic structure of ECMAScript code such as a FunctionDeclaration, a BlockStatement, or a Catch clause of a TryStatement. Each time such code is evaluated, a new Environment Record is created to record the identifier bindings that are created by that code.

Every Environment Record has an [[OuterEnv]] field, which is either null or a reference to an outer Environment Record. This is used to model the logical nesting of Environment Record values. The outer reference of an (inner) Environment Record is a reference to the Environment Record that logically surrounds the inner Environment Record. An outer Environment Record may, of course, have its own outer Environment Record. An Environment Record may serve as the outer environment for multiple inner Environment Records. For example, if a FunctionDeclaration contains two nested FunctionDeclarations then the Environment Records of each of the nested functions will have as their outer Environment Record the Environment Record of the current evaluation of the surrounding function.

Environment Records are purely specification mechanisms and need not correspond to any specific artefact of an ECMAScript implementation. It is impossible for an ECMAScript program to directly access or manipulate such values.

9.1.1 The Environment Record Type Hierarchy

Environment Records can be thought of as existing in a simple object-oriented hierarchy where Environment Record is an abstract class with three concrete subclasses: Declarative Environment Record, Object Environment Record, and Global Environment Record. Function Environment Records and Module Environment Records are subclasses of Declarative Environment Record.

Environment Record (abstract)
- A Declarative Environment Record is used to define the effect of ECMAScript language syntactic elements such as FunctionDeclarations, VariableDeclarations, and Catch clauses that directly associate identifier bindings with ECMAScript language values.
  - A Function Environment Record corresponds to the invocation of an ECMAScript function object, and contains bindings for the top-level declarations within that function. It may establish a new this binding. It also captures the state necessary to support super method invocations.
  - A Module Environment Record contains the bindings for the top-level declarations of a Module. It also contains the bindings that are explicitly imported by the Module. Its [[OuterEnv]] is a Global Environment Record.
- An Object Environment Record is used to define the effect of ECMAScript elements such as WithStatement that associate identifier bindings with the properties of some object.
- A Global Environment Record is used for Script global declarations. It does not have an outer environment; its [[OuterEnv]] is null. It may be prepopulated with identifier bindings and it includes an associated global object whose properties provide some of the global environment's identifier bindings. As ECMAScript code is executed, additional properties may be added to the global object and the initial properties may be modified.

The Environment Record abstract class includes the abstract specification methods defined in Table 16. These abstract methods have distinct concrete algorithms for each of the concrete subclasses.

Table 16: Abstract Methods of Environment Records

Method	Purpose
HasBinding(N)	Determine if an Environment Record has a binding for the String value `N`. Return true if it does and false if it does not.
CreateMutableBinding(N, D)	Create a new but uninitialized mutable binding in an Environment Record. The String value `N` is the text of the bound name. If the Boolean argument `D` is true the binding may be subsequently deleted.
CreateImmutableBinding(N, S)	Create a new but uninitialized immutable binding in an Environment Record. The String value `N` is the text of the bound name. If `S` is true then attempts to set it after it has been initialized will always throw an exception, regardless of the strict mode setting of operations that reference that binding.
InitializeBinding(N, V)	Set the value of an already existing but uninitialized binding in an Environment Record. The String value `N` is the text of the bound name. `V` is the value for the binding and is a value of any ECMAScript language type.
SetMutableBinding(N, V, S)	Set the value of an already existing mutable binding in an Environment Record. The String value `N` is the text of the bound name. `V` is the value for the binding and may be a value of any ECMAScript language type. `S` is a Boolean flag. If `S` is true and the binding cannot be set throw a TypeError exception.
GetBindingValue(N, S)	Returns the value of an already existing binding from an Environment Record. The String value `N` is the text of the bound name. `S` is used to identify references originating in strict mode code or that otherwise require strict mode reference semantics. If `S` is true and the binding does not exist throw a ReferenceError exception. If the binding exists but is uninitialized a ReferenceError is thrown, regardless of the value of `S`.
DeleteBinding(N)	Delete a binding from an Environment Record. The String value `N` is the text of the bound name. If a binding for `N` exists, remove the binding and return true. If the binding exists but cannot be removed return false. If the binding does not exist return true.
HasThisBinding()	Determine if an Environment Record establishes a `this` binding. Return true if it does and false if it does not.
HasSuperBinding()	Determine if an Environment Record establishes a `super` method binding. Return true if it does and false if it does not.
WithBaseObject()	If this Environment Record is associated with a `with` statement, return the with object. Otherwise, return undefined.

9.1.1.1 Declarative Environment Records

Each Declarative Environment Record is associated with an ECMAScript program scope containing variable, constant, let, class, module, import, and/or function declarations. A Declarative Environment Record binds the set of identifiers defined by the declarations contained within its scope.

The behaviour of the concrete specification methods for Declarative Environment Records is defined by the following algorithms.

9.1.1.1.1 HasBinding ( `N` )

The HasBinding concrete method of a Declarative Environment Record envRec takes argument N (a String) and returns a normal completion containing a Boolean. It determines if the argument identifier is one of the identifiers bound by the record. It performs the following steps when called:

If envRec has a binding for N, return true.
Return false.

9.1.1.1.2 CreateMutableBinding ( `N`, `D` )

The CreateMutableBinding concrete method of a Declarative Environment Record envRec takes arguments N (a String) and D (a Boolean) and returns a normal completion containing unused. It creates a new mutable binding for the name N that is uninitialized. A binding must not already exist in this Environment Record for N. If D is true, the new binding is marked as being subject to deletion. It performs the following steps when called:

Assert: envRec does not already have a binding for N.
Create a mutable binding in envRec for N and record that it is uninitialized. If D is true, record that the newly created binding may be deleted by a subsequent DeleteBinding call.
Return unused.

9.1.1.1.3 CreateImmutableBinding ( `N`, `S` )

The CreateImmutableBinding concrete method of a Declarative Environment Record envRec takes arguments N (a String) and S (a Boolean) and returns a normal completion containing unused. It creates a new immutable binding for the name N that is uninitialized. A binding must not already exist in this Environment Record for N. If S is true, the new binding is marked as a strict binding. It performs the following steps when called:

Assert: envRec does not already have a binding for N.
Create an immutable binding in envRec for N and record that it is uninitialized. If S is true, record that the newly created binding is a strict binding.
Return unused.

9.1.1.1.4 InitializeBinding ( `N`, `V` )

The InitializeBinding concrete method of a Declarative Environment Record envRec takes arguments N (a String) and V (an ECMAScript language value) and returns a normal completion containing unused. It is used to set the bound value of the current binding of the identifier whose name is N to the value V. An uninitialized binding for N must already exist. It performs the following steps when called:

Assert: envRec must have an uninitialized binding for N.
Set the bound value for N in envRec to V.
Record that the binding for N in envRec has been initialized.
Return unused.

9.1.1.1.5 SetMutableBinding ( `N`, `V`, `S` )

The SetMutableBinding concrete method of a Declarative Environment Record envRec takes arguments N (a String), V (an ECMAScript language value), and S (a Boolean) and returns either a normal completion containing unused or a throw completion. It attempts to change the bound value of the current binding of the identifier whose name is N to the value V. A binding for N normally already exists, but in rare cases it may not. If the binding is an immutable binding, a TypeError is thrown if S is true. It performs the following steps when called:

1. If envRec does not have a binding for N, then
1. If S is true, throw a ReferenceError exception.
2. Perform ! envRec.CreateMutableBinding(N, true).
3. Perform ! envRec.InitializeBinding(N, V).
4. Return unused.
If the binding for N in envRec is a strict binding, set S to true.
3. If the binding for N in envRec has not yet been initialized, then
1. Throw a ReferenceError exception.
4. Else if the binding for N in envRec is a mutable binding, then
1. Change its bound value to V.
5. Else,
1. Assert: This is an attempt to change the value of an immutable binding.
2. If S is true, throw a TypeError exception.
Return unused.

Note

An example of ECMAScript code that results in a missing binding at step 1 is:

function f() { eval("var x; x = (delete x, 0);"); }

9.1.1.1.6 GetBindingValue ( `N`, `S` )

The GetBindingValue concrete method of a Declarative Environment Record envRec takes arguments N (a String) and S (a Boolean) and returns either a normal completion containing an ECMAScript language value or a throw completion. It returns the value of its bound identifier whose name is N. If the binding exists but is uninitialized a ReferenceError is thrown, regardless of the value of S. It performs the following steps when called:

Assert: envRec has a binding for N.
If the binding for N in envRec is an uninitialized binding, throw a ReferenceError exception.
Return the value currently bound to N in envRec.

9.1.1.1.7 DeleteBinding ( `N` )

The DeleteBinding concrete method of a Declarative Environment Record envRec takes argument N (a String) and returns a normal completion containing a Boolean. It can only delete bindings that have been explicitly designated as being subject to deletion. It performs the following steps when called:

Assert: envRec has a binding for N.
If the binding for N in envRec cannot be deleted, return false.
Remove the binding for N from envRec.
Return true.

9.1.1.1.8 HasThisBinding ( )

The HasThisBinding concrete method of a Declarative Environment Record envRec takes no arguments and returns false. It performs the following steps when called:

Return false.

Note

A regular Declarative Environment Record (i.e., one that is neither a Function Environment Record nor a Module Environment Record) does not provide a this binding.

9.1.1.1.9 HasSuperBinding ( )

The HasSuperBinding concrete method of a Declarative Environment Record envRec takes no arguments and returns false. It performs the following steps when called:

Return false.

Note

A regular Declarative Environment Record (i.e., one that is neither a Function Environment Record nor a Module Environment Record) does not provide a super binding.

9.1.1.1.10 WithBaseObject ( )

The WithBaseObject concrete method of a Declarative Environment Record envRec takes no arguments and returns undefined. It performs the following steps when called:

Return undefined.

9.1.1.2 Object Environment Records

Each Object Environment Record is associated with an object called its binding object. An Object Environment Record binds the set of string identifier names that directly correspond to the property names of its binding object. Property keys that are not strings in the form of an IdentifierName are not included in the set of bound identifiers. Both own and inherited properties are included in the set regardless of the setting of their [[Enumerable]] attribute. Because properties can be dynamically added and deleted from objects, the set of identifiers bound by an Object Environment Record may potentially change as a side-effect of any operation that adds or deletes properties. Any bindings that are created as a result of such a side-effect are considered to be a mutable binding even if the Writable attribute of the corresponding property is false. Immutable bindings do not exist for Object Environment Records.

Object Environment Records created for with statements (14.11) can provide their binding object as an implicit this value for use in function calls. The capability is controlled by a Boolean [[IsWithEnvironment]] field.

Object Environment Records have the additional state fields listed in Table 17.

Table 17: Additional Fields of Object Environment Records

Field Name	Value	Meaning
`[[BindingObject]]`	an Object	The binding object of this Environment Record.
`[[IsWithEnvironment]]`	a Boolean	Indicates whether this Environment Record is created for a `with` statement.

The behaviour of the concrete specification methods for Object Environment Records is defined by the following algorithms.

9.1.1.2.1 HasBinding ( `N` )

The HasBinding concrete method of an Object Environment Record envRec takes argument N (a String) and returns either a normal completion containing a Boolean or a throw completion. It determines if its associated binding object has a property whose name is N. It performs the following steps when called:

Let bindingObject be envRec.[[BindingObject]].
Let foundBinding be ? HasProperty(bindingObject, N).
If foundBinding is false, return false.
If envRec.[[IsWithEnvironment]] is false, return true.
Let unscopables be ? Get(bindingObject, @@unscopables).
6. If unscopables is an Object, then
1. Let blocked be ToBoolean(? Get(unscopables, N)).
2. If blocked is true, return false.
Return true.

9.1.1.2.2 CreateMutableBinding ( `N`, `D` )

The CreateMutableBinding concrete method of an Object Environment Record envRec takes arguments N (a String) and D (a Boolean) and returns either a normal completion containing unused or a throw completion. It creates in an Environment Record's associated binding object a property whose name is N and initializes it to the value undefined. If D is true, the new property's [[Configurable]] attribute is set to true; otherwise it is set to false. It performs the following steps when called:

Let bindingObject be envRec.[[BindingObject]].
Perform ? DefinePropertyOrThrow(bindingObject, N, PropertyDescriptor { [[Value]]: undefined, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: D }).
Return unused.

Note

Normally envRec will not have a binding for N but if it does, the semantics of DefinePropertyOrThrow may result in an existing binding being replaced or shadowed or cause an abrupt completion to be returned.

9.1.1.2.3 CreateImmutableBinding ( `N`, `S` )

The CreateImmutableBinding concrete method of an Object Environment Record is never used within this specification.

9.1.1.2.4 InitializeBinding ( `N`, `V` )

The InitializeBinding concrete method of an Object Environment Record envRec takes arguments N (a String) and V (an ECMAScript language value) and returns either a normal completion containing unused or a throw completion. It is used to set the bound value of the current binding of the identifier whose name is N to the value V. It performs the following steps when called:

Perform ? envRec.SetMutableBinding(N, V, false).
Return unused.

Note

In this specification, all uses of CreateMutableBinding for Object Environment Records are immediately followed by a call to InitializeBinding for the same name. Hence, this specification does not explicitly track the initialization state of bindings in Object Environment Records.

9.1.1.2.5 SetMutableBinding ( `N`, `V`, `S` )

The SetMutableBinding concrete method of an Object Environment Record envRec takes arguments N (a String), V (an ECMAScript language value), and S (a Boolean) and returns either a normal completion containing unused or a throw completion. It attempts to set the value of the Environment Record's associated binding object's property whose name is N to the value V. A property named N normally already exists but if it does not or is not currently writable, error handling is determined by S. It performs the following steps when called:

Let bindingObject be envRec.[[BindingObject]].
Let stillExists be ? HasProperty(bindingObject, N).
If stillExists is false and S is true, throw a ReferenceError exception.
Perform ? Set(bindingObject, N, V, S).
Return unused.

9.1.1.2.6 GetBindingValue ( `N`, `S` )

The GetBindingValue concrete method of an Object Environment Record envRec takes arguments N (a String) and S (a Boolean) and returns either a normal completion containing an ECMAScript language value or a throw completion. It returns the value of its associated binding object's property whose name is N. The property should already exist but if it does not the result depends upon S. It performs the following steps when called:

Let bindingObject be envRec.[[BindingObject]].
Let value be ? HasProperty(bindingObject, N).
3. If value is false, then
1. If S is false, return undefined; otherwise throw a ReferenceError exception.
Return ? Get(bindingObject, N).

9.1.1.2.7 DeleteBinding ( `N` )

The DeleteBinding concrete method of an Object Environment Record envRec takes argument N (a String) and returns either a normal completion containing a Boolean or a throw completion. It can only delete bindings that correspond to properties of the environment object whose [[Configurable]] attribute have the value true. It performs the following steps when called:

Let bindingObject be envRec.[[BindingObject]].
Return ? bindingObject.[[Delete]](N).

9.1.1.2.8 HasThisBinding ( )

The HasThisBinding concrete method of an Object Environment Record envRec takes no arguments and returns false. It performs the following steps when called:

Return false.

Note

Object Environment Records do not provide a this binding.

9.1.1.2.9 HasSuperBinding ( )

The HasSuperBinding concrete method of an Object Environment Record envRec takes no arguments and returns false. It performs the following steps when called:

Return false.

Note

Object Environment Records do not provide a super binding.

9.1.1.2.10 WithBaseObject ( )

The WithBaseObject concrete method of an Object Environment Record envRec takes no arguments and returns an Object or undefined. It performs the following steps when called:

If envRec.[[IsWithEnvironment]] is true, return envRec.[[BindingObject]].
Otherwise, return undefined.

9.1.1.3 Function Environment Records

A Function Environment Record is a Declarative Environment Record that is used to represent the top-level scope of a function and, if the function is not an ArrowFunction, provides a this binding. If a function is not an ArrowFunction function and references super, its Function Environment Record also contains the state that is used to perform super method invocations from within the function.

Function Environment Records have the additional state fields listed in Table 18.

Table 18: Additional Fields of Function Environment Records

Field Name	Value	Meaning
`[[ThisValue]]`	an ECMAScript language value	This is the this value used for this invocation of the function.
`[[ThisBindingStatus]]`	lexical, initialized, or uninitialized	If the value is lexical, this is an ArrowFunction and does not have a local this value.
`[[FunctionObject]]`	an ECMAScript function object	The function object whose invocation caused this Environment Record to be created.
`[[NewTarget]]`	an Object or undefined	If this Environment Record was created by the `[[Construct]]` internal method, `[[NewTarget]]` is the value of the `[[Construct]]` `newTarget` parameter. Otherwise, its value is undefined.

Function Environment Records support all of the Declarative Environment Record methods listed in Table 16 and share the same specifications for all of those methods except for HasThisBinding and HasSuperBinding. In addition, Function Environment Records support the methods listed in Table 19:

Table 19: Additional Methods of Function Environment Records

Method	Purpose
BindThisValue(V)	Set the `[[ThisValue]]` and record that it has been initialized.
GetThisBinding()	Return the value of this Environment Record's `this` binding. Throws a ReferenceError if the `this` binding has not been initialized.
GetSuperBase()	Return the object that is the base for `super` property accesses bound in this Environment Record. The value undefined indicates that such accesses will produce runtime errors.

The behaviour of the additional concrete specification methods for Function Environment Records is defined by the following algorithms:

9.1.1.3.1 BindThisValue ( `V` )

The BindThisValue concrete method of a Function Environment Record envRec takes argument V (an ECMAScript language value) and returns either a normal completion containing an ECMAScript language value or a throw completion. It performs the following steps when called:

Assert: envRec.[[ThisBindingStatus]] is not lexical.
If envRec.[[ThisBindingStatus]] is initialized, throw a ReferenceError exception.
Set envRec.[[ThisValue]] to V.
Set envRec.[[ThisBindingStatus]] to initialized.
Return V.

9.1.1.3.2 HasThisBinding ( )

The HasThisBinding concrete method of a Function Environment Record envRec takes no arguments and returns a Boolean. It performs the following steps when called:

If envRec.[[ThisBindingStatus]] is lexical, return false; otherwise, return true.

9.1.1.3.3 HasSuperBinding ( )

The HasSuperBinding concrete method of a Function Environment Record envRec takes no arguments and returns a Boolean. It performs the following steps when called:

If envRec.[[ThisBindingStatus]] is lexical, return false.
If envRec.[[FunctionObject]].[[HomeObject]] is undefined, return false; otherwise, return true.

9.1.1.3.4 GetThisBinding ( )

The GetThisBinding concrete method of a Function Environment Record envRec takes no arguments and returns either a normal completion containing an ECMAScript language value or a throw completion. It performs the following steps when called:

Assert: envRec.[[ThisBindingStatus]] is not lexical.
If envRec.[[ThisBindingStatus]] is uninitialized, throw a ReferenceError exception.
Return envRec.[[ThisValue]].

9.1.1.3.5 GetSuperBase ( )

The GetSuperBase concrete method of a Function Environment Record envRec takes no arguments and returns either a normal completion containing either an Object, null, or undefined, or a throw completion. It performs the following steps when called:

Let home be envRec.[[FunctionObject]].[[HomeObject]].
If home is undefined, return undefined.
Assert: home is an Object.
Return ? home.[[GetPrototypeOf]]().

9.1.1.4 Global Environment Records

A Global Environment Record is used to represent the outer most scope that is shared by all of the ECMAScript Script elements that are processed in a common realm. A Global Environment Record provides the bindings for built-in globals (clause 19), properties of the global object, and for all top-level declarations (8.2.9, 8.2.11) that occur within a Script.

A Global Environment Record is logically a single record but it is specified as a composite encapsulating an Object Environment Record and a Declarative Environment Record. The Object Environment Record has as its base object the global object of the associated Realm Record. This global object is the value returned by the Global Environment Record's GetThisBinding concrete method. The Object Environment Record component of a Global Environment Record contains the bindings for all built-in globals (clause 19) and all bindings introduced by a FunctionDeclaration, GeneratorDeclaration, AsyncFunctionDeclaration, AsyncGeneratorDeclaration, or VariableStatement contained in global code. The bindings for all other ECMAScript declarations in global code are contained in the Declarative Environment Record component of the Global Environment Record.

Properties may be created directly on a global object. Hence, the Object Environment Record component of a Global Environment Record may contain both bindings created explicitly by FunctionDeclaration, GeneratorDeclaration, AsyncFunctionDeclaration, AsyncGeneratorDeclaration, or VariableDeclaration declarations and bindings created implicitly as properties of the global object. In order to identify which bindings were explicitly created using declarations, a Global Environment Record maintains a list of the names bound using its CreateGlobalVarBinding and CreateGlobalFunctionBinding concrete methods.

Global Environment Records have the additional fields listed in Table 20 and the additional methods listed in Table 21.

Table 20: Additional Fields of Global Environment Records

Field Name	Value	Meaning
`[[ObjectRecord]]`	an Object Environment Record	Binding object is the global object. It contains global built-in bindings as well as FunctionDeclaration, GeneratorDeclaration, AsyncFunctionDeclaration, AsyncGeneratorDeclaration, and VariableDeclaration bindings in global code for the associated realm.
`[[GlobalThisValue]]`	an Object	The value returned by `this` in global scope. Hosts may provide any ECMAScript Object value.
`[[DeclarativeRecord]]`	a Declarative Environment Record	Contains bindings for all declarations in global code for the associated realm code except for FunctionDeclaration, GeneratorDeclaration, AsyncFunctionDeclaration, AsyncGeneratorDeclaration, and VariableDeclaration bindings.
`[[VarNames]]`	a List of Strings	The string names bound by FunctionDeclaration, GeneratorDeclaration, AsyncFunctionDeclaration, AsyncGeneratorDeclaration, and VariableDeclaration declarations in global code for the associated realm.

Table 21: Additional Methods of Global Environment Records

Method	Purpose
GetThisBinding()	Return the value of this Environment Record's `this` binding.
HasVarDeclaration (N)	Determines if the argument identifier has a binding in this Environment Record that was created using a VariableDeclaration, FunctionDeclaration, GeneratorDeclaration, AsyncFunctionDeclaration, or AsyncGeneratorDeclaration.
HasLexicalDeclaration (N)	Determines if the argument identifier has a binding in this Environment Record that was created using a lexical declaration such as a LexicalDeclaration or a ClassDeclaration.
HasRestrictedGlobalProperty (N)	Determines if the argument is the name of a global object property that may not be shadowed by a global lexical binding.
CanDeclareGlobalVar (N)	Determines if a corresponding CreateGlobalVarBinding call would succeed if called for the same argument `N`.
CanDeclareGlobalFunction (N)	Determines if a corresponding CreateGlobalFunctionBinding call would succeed if called for the same argument `N`.
CreateGlobalVarBinding(N, D)	Used to create and initialize to undefined a global `var` binding in the `[[ObjectRecord]]` component of a Global Environment Record. The binding will be a mutable binding. The corresponding global object property will have attribute values appropriate for a `var`. The String value `N` is the bound name. If `D` is true, the binding may be deleted. Logically equivalent to CreateMutableBinding followed by a SetMutableBinding but it allows var declarations to receive special treatment.
CreateGlobalFunctionBinding(N, V, D)	Create and initialize a global `function` binding in the `[[ObjectRecord]]` component of a Global Environment Record. The binding will be a mutable binding. The corresponding global object property will have attribute values appropriate for a `function`. The String value `N` is the bound name. `V` is the initialization value. If the Boolean argument `D` is true, the binding may be deleted. Logically equivalent to CreateMutableBinding followed by a SetMutableBinding but it allows function declarations to receive special treatment.

The behaviour of the concrete specification methods for Global Environment Records is defined by the following algorithms.

9.1.1.4.1 HasBinding ( `N` )

The HasBinding concrete method of a Global Environment Record envRec takes argument N (a String) and returns either a normal completion containing a Boolean or a throw completion. It determines if the argument identifier is one of the identifiers bound by the record. It performs the following steps when called:

Let DclRec be envRec.[[DeclarativeRecord]].
If ! DclRec.HasBinding(N) is true, return true.
Let ObjRec be envRec.[[ObjectRecord]].
Return ? ObjRec.HasBinding(N).

9.1.1.4.2 CreateMutableBinding ( `N`, `D` )

The CreateMutableBinding concrete method of a Global Environment Record envRec takes arguments N (a String) and D (a Boolean) and returns either a normal completion containing unused or a throw completion. It creates a new mutable binding for the name N that is uninitialized. The binding is created in the associated DeclarativeRecord. A binding for N must not already exist in the DeclarativeRecord. If D is true, the new binding is marked as being subject to deletion. It performs the following steps when called:

Let DclRec be envRec.[[DeclarativeRecord]].
If ! DclRec.HasBinding(N) is true, throw a TypeError exception.
Return ! DclRec.CreateMutableBinding(N, D).

9.1.1.4.3 CreateImmutableBinding ( `N`, `S` )

The CreateImmutableBinding concrete method of a Global Environment Record envRec takes arguments N (a String) and S (a Boolean) and returns either a normal completion containing unused or a throw completion. It creates a new immutable binding for the name N that is uninitialized. A binding must not already exist in this Environment Record for N. If S is true, the new binding is marked as a strict binding. It performs the following steps when called:

Let DclRec be envRec.[[DeclarativeRecord]].
If ! DclRec.HasBinding(N) is true, throw a TypeError exception.
Return ! DclRec.CreateImmutableBinding(N, S).

9.1.1.4.4 InitializeBinding ( `N`, `V` )

The InitializeBinding concrete method of a Global Environment Record envRec takes arguments N (a String) and V (an ECMAScript language value) and returns either a normal completion containing unused or a throw completion. It is used to set the bound value of the current binding of the identifier whose name is N to the value V. An uninitialized binding for N must already exist. It performs the following steps when called:

Let DclRec be envRec.[[DeclarativeRecord]].
2. If ! DclRec.HasBinding(N) is true, then
1. Return ! DclRec.InitializeBinding(N, V).
Assert: If the binding exists, it must be in the Object Environment Record.
Let ObjRec be envRec.[[ObjectRecord]].
Return ? ObjRec.InitializeBinding(N, V).

9.1.1.4.5 SetMutableBinding ( `N`, `V`, `S` )

The SetMutableBinding concrete method of a Global Environment Record envRec takes arguments N (a String), V (an ECMAScript language value), and S (a Boolean) and returns either a normal completion containing unused or a throw completion. It attempts to change the bound value of the current binding of the identifier whose name is N to the value V. If the binding is an immutable binding and S is true, a TypeError is thrown. A property named N normally already exists but if it does not or is not currently writable, error handling is determined by S. It performs the following steps when called:

Let DclRec be envRec.[[DeclarativeRecord]].
2. If ! DclRec.HasBinding(N) is true, then
1. Return ? DclRec.SetMutableBinding(N, V, S).
Let ObjRec be envRec.[[ObjectRecord]].
Return ? ObjRec.SetMutableBinding(N, V, S).

9.1.1.4.6 GetBindingValue ( `N`, `S` )

The GetBindingValue concrete method of a Global Environment Record envRec takes arguments N (a String) and S (a Boolean) and returns either a normal completion containing an ECMAScript language value or a throw completion. It returns the value of its bound identifier whose name is N. If the binding is an uninitialized binding throw a ReferenceError exception. A property named N normally already exists but if it does not or is not currently writable, error handling is determined by S. It performs the following steps when called:

Let DclRec be envRec.[[DeclarativeRecord]].
2. If ! DclRec.HasBinding(N) is true, then
1. Return ? DclRec.GetBindingValue(N, S).
Let ObjRec be envRec.[[ObjectRecord]].
Return ? ObjRec.GetBindingValue(N, S).

9.1.1.4.7 DeleteBinding ( `N` )

The DeleteBinding concrete method of a Global Environment Record envRec takes argument N (a String) and returns either a normal completion containing a Boolean or a throw completion. It can only delete bindings that have been explicitly designated as being subject to deletion. It performs the following steps when called:

Let DclRec be envRec.[[DeclarativeRecord]].
2. If ! DclRec.HasBinding(N) is true, then
1. Return ! DclRec.DeleteBinding(N).
Let ObjRec be envRec.[[ObjectRecord]].
Let globalObject be ObjRec.[[BindingObject]].
Let existingProp be ? HasOwnProperty(globalObject, N).
6. If existingProp is true, then
1. Let status be ? ObjRec.DeleteBinding(N).
2. b. If status is true and envRec.[[VarNames]] contains N, then
  1. Remove N from envRec.[[VarNames]].
3. Return status.
Return true.

9.1.1.4.8 HasThisBinding ( )

The HasThisBinding concrete method of a Global Environment Record envRec takes no arguments and returns true. It performs the following steps when called:

Return true.

Note

Global Environment Records always provide a this binding.

9.1.1.4.9 HasSuperBinding ( )

The HasSuperBinding concrete method of a Global Environment Record envRec takes no arguments and returns false. It performs the following steps when called:

Return false.

Note

Global Environment Records do not provide a super binding.

9.1.1.4.10 WithBaseObject ( )

The WithBaseObject concrete method of a Global Environment Record envRec takes no arguments and returns undefined. It performs the following steps when called:

Return undefined.

9.1.1.4.11 GetThisBinding ( )

The GetThisBinding concrete method of a Global Environment Record envRec takes no arguments and returns a normal completion containing an Object. It performs the following steps when called:

Return envRec.[[GlobalThisValue]].

9.1.1.4.12 HasVarDeclaration ( `N` )

The HasVarDeclaration concrete method of a Global Environment Record envRec takes argument N (a String) and returns a Boolean. It determines if the argument identifier has a binding in this record that was created using a VariableStatement or a FunctionDeclaration. It performs the following steps when called:

Let varDeclaredNames be envRec.[[VarNames]].
If varDeclaredNames contains N, return true.
Return false.

9.1.1.4.13 HasLexicalDeclaration ( `N` )

The HasLexicalDeclaration concrete method of a Global Environment Record envRec takes argument N (a String) and returns a Boolean. It determines if the argument identifier has a binding in this record that was created using a lexical declaration such as a LexicalDeclaration or a ClassDeclaration. It performs the following steps when called:

Let DclRec be envRec.[[DeclarativeRecord]].
Return ! DclRec.HasBinding(N).

9.1.1.4.14 HasRestrictedGlobalProperty ( `N` )

The HasRestrictedGlobalProperty concrete method of a Global Environment Record envRec takes argument N (a String) and returns either a normal completion containing a Boolean or a throw completion. It determines if the argument identifier is the name of a property of the global object that must not be shadowed by a global lexical binding. It performs the following steps when called:

Let ObjRec be envRec.[[ObjectRecord]].
Let globalObject be ObjRec.[[BindingObject]].
Let existingProp be ? globalObject.[[GetOwnProperty]](N).
If existingProp is undefined, return false.
If existingProp.[[Configurable]] is true, return false.
Return true.

Note

Properties may exist upon a global object that were directly created rather than being declared using a var or function declaration. A global lexical binding may not be created that has the same name as a non-configurable property of the global object. The global property "undefined" is an example of such a property.

9.1.1.4.15 CanDeclareGlobalVar ( `N` )

The CanDeclareGlobalVar concrete method of a Global Environment Record envRec takes argument N (a String) and returns either a normal completion containing a Boolean or a throw completion. It determines if a corresponding CreateGlobalVarBinding call would succeed if called for the same argument N. Redundant var declarations and var declarations for pre-existing global object properties are allowed. It performs the following steps when called:

Let ObjRec be envRec.[[ObjectRecord]].
Let globalObject be ObjRec.[[BindingObject]].
Let hasProperty be ? HasOwnProperty(globalObject, N).
If hasProperty is true, return true.
Return ? IsExtensible(globalObject).

9.1.1.4.16 CanDeclareGlobalFunction ( `N` )

The CanDeclareGlobalFunction concrete method of a Global Environment Record envRec takes argument N (a String) and returns either a normal completion containing a Boolean or a throw completion. It determines if a corresponding CreateGlobalFunctionBinding call would succeed if called for the same argument N. It performs the following steps when called:

Let ObjRec be envRec.[[ObjectRecord]].
Let globalObject be ObjRec.[[BindingObject]].
Let existingProp be ? globalObject.[[GetOwnProperty]](N).
If existingProp is undefined, return ? IsExtensible(globalObject).
If existingProp.[[Configurable]] is true, return true.
If IsDataDescriptor(existingProp) is true and existingProp has attribute values { [[Writable]]: true, [[Enumerable]]: true }, return true.
Return false.

9.1.1.4.17 CreateGlobalVarBinding ( `N`, `D` )

The CreateGlobalVarBinding concrete method of a Global Environment Record envRec takes arguments N (a String) and D (a Boolean) and returns either a normal completion containing unused or a throw completion. It creates and initializes a mutable binding in the associated Object Environment Record and records the bound name in the associated [[VarNames]] List. If a binding already exists, it is reused and assumed to be initialized. It performs the following steps when called:

Let ObjRec be envRec.[[ObjectRecord]].
Let globalObject be ObjRec.[[BindingObject]].
Let hasProperty be ? HasOwnProperty(globalObject, N).
Let extensible be ? IsExtensible(globalObject).
5. If hasProperty is false and extensible is true, then
1. Perform ? ObjRec.CreateMutableBinding(N, D).
2. Perform ? ObjRec.InitializeBinding(N, undefined).
6. If envRec.[[VarNames]] does not contain N, then
1. Append N to envRec.[[VarNames]].
Return unused.

9.1.1.4.18 CreateGlobalFunctionBinding ( `N`, `V`, `D` )

The CreateGlobalFunctionBinding concrete method of a Global Environment Record envRec takes arguments N (a String), V (an ECMAScript language value), and D (a Boolean) and returns either a normal completion containing unused or a throw completion. It creates and initializes a mutable binding in the associated Object Environment Record and records the bound name in the associated [[VarNames]] List. If a binding already exists, it is replaced. It performs the following steps when called:

Let ObjRec be envRec.[[ObjectRecord]].
Let globalObject be ObjRec.[[BindingObject]].
Let existingProp be ? globalObject.[[GetOwnProperty]](N).
4. If existingProp is undefined or existingProp.[[Configurable]] is true, then
1. Let desc be the PropertyDescriptor { [[Value]]: V, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: D }.
5. Else,
1. Let desc be the PropertyDescriptor { [[Value]]: V }.
Perform ? DefinePropertyOrThrow(globalObject, N, desc).
Perform ? Set(globalObject, N, V, false).
8. If envRec.[[VarNames]] does not contain N, then
1. Append N to envRec.[[VarNames]].
Return unused.

Note

Global function declarations are always represented as own properties of the global object. If possible, an existing own property is reconfigured to have a standard set of attribute values. Step 7 is equivalent to what calling the InitializeBinding concrete method would do and if globalObject is a Proxy will produce the same sequence of Proxy trap calls.

9.1.1.5 Module Environment Records

A Module Environment Record is a Declarative Environment Record that is used to represent the outer scope of an ECMAScript Module. In additional to normal mutable and immutable bindings, Module Environment Records also provide immutable import bindings which are bindings that provide indirect access to a target binding that exists in another Environment Record.

Module Environment Records support all of the Declarative Environment Record methods listed in Table 16 and share the same specifications for all of those methods except for GetBindingValue, DeleteBinding, HasThisBinding and GetThisBinding. In addition, Module Environment Records support the methods listed in Table 22:

Table 22: Additional Methods of Module Environment Records

Method	Purpose
CreateImportBinding(N, M, N2)	Create an immutable indirect binding in a Module Environment Record. The String value `N` is the text of the bound name. `M` is a Module Record, and `N2` is a binding that exists in `M`'s Module Environment Record.
GetThisBinding()	Return the value of this Environment Record's `this` binding.

The behaviour of the additional concrete specification methods for Module Environment Records are defined by the following algorithms:

9.1.1.5.1 GetBindingValue ( `N`, `S` )

The GetBindingValue concrete method of a Module Environment Record envRec takes arguments N (a String) and S (a Boolean) and returns either a normal completion containing an ECMAScript language value or a throw completion. It returns the value of its bound identifier whose name is N. However, if the binding is an indirect binding the value of the target binding is returned. If the binding exists but is uninitialized a ReferenceError is thrown. It performs the following steps when called:

Assert: S is true.
Assert: envRec has a binding for N.
3. If the binding for N is an indirect binding, then
1. Let M and N2 be the indirection values provided when this binding for N was created.
2. Let targetEnv be M.[[Environment]].
3. If targetEnv is empty, throw a ReferenceError exception.
4. Return ? targetEnv.GetBindingValue(N2, true).
If the binding for N in envRec is an uninitialized binding, throw a ReferenceError exception.
Return the value currently bound to N in envRec.

Note

S will always be true because a Module is always strict mode code.

9.1.1.5.2 DeleteBinding ( `N` )

The DeleteBinding concrete method of a Module Environment Record is never used within this specification.

Note

Module Environment Records are only used within strict code and an early error rule prevents the delete operator, in strict code, from being applied to a Reference Record that would resolve to a Module Environment Record binding. See 13.5.1.1.

9.1.1.5.3 HasThisBinding ( )

The HasThisBinding concrete method of a Module Environment Record envRec takes no arguments and returns true. It performs the following steps when called:

Return true.

Note

Module Environment Records always provide a this binding.

9.1.1.5.4 GetThisBinding ( )

The GetThisBinding concrete method of a Module Environment Record envRec takes no arguments and returns a normal completion containing undefined. It performs the following steps when called:

Return undefined.

9.1.1.5.5 CreateImportBinding ( `N`, `M`, `N2` )

The CreateImportBinding concrete method of a Module Environment Record envRec takes arguments N (a String), M (a Module Record), and N2 (a String) and returns unused. It creates a new initialized immutable indirect binding for the name N. A binding must not already exist in this Environment Record for N. N2 is the name of a binding that exists in M's Module Environment Record. Accesses to the value of the new binding will indirectly access the bound value of the target binding. It performs the following steps when called:

Assert: envRec does not already have a binding for N.
Assert: When M.[[Environment]] is instantiated, it will have a direct binding for N2.
Create an immutable indirect binding in envRec for N that references M and N2 as its target binding and record that the binding is initialized.
Return unused.

9.1.2 Environment Record Operations

The following abstract operations are used in this specification to operate upon Environment Records:

9.1.2.1 GetIdentifierReference ( `env`, `name`, `strict` )

The abstract operation GetIdentifierReference takes arguments env (an Environment Record or null), name (a String), and strict (a Boolean) and returns either a normal completion containing a Reference Record or a throw completion. It performs the following steps when called:

1. If env is null, then
1. Return the Reference Record { [[Base]]: unresolvable, [[ReferencedName]]: name, [[Strict]]: strict, [[ThisValue]]: empty }.
Let exists be ? env.HasBinding(name).
3. If exists is true, then
1. Return the Reference Record { [[Base]]: env, [[ReferencedName]]: name, [[Strict]]: strict, [[ThisValue]]: empty }.
4. Else,
1. Let outer be env.[[OuterEnv]].
2. Return ? GetIdentifierReference(outer, name, strict).

9.1.2.2 NewDeclarativeEnvironment ( `E` )

The abstract operation NewDeclarativeEnvironment takes argument E (an Environment Record or null) and returns a Declarative Environment Record. It performs the following steps when called:

Let env be a new Declarative Environment Record containing no bindings.
Set env.[[OuterEnv]] to E.
Return env.

9.1.2.3 NewObjectEnvironment ( `O`, `W`, `E` )

The abstract operation NewObjectEnvironment takes arguments O (an Object), W (a Boolean), and E (an Environment Record or null) and returns an Object Environment Record. It performs the following steps when called:

Let env be a new Object Environment Record.
Set env.[[BindingObject]] to O.
Set env.[[IsWithEnvironment]] to W.
Set env.[[OuterEnv]] to E.
Return env.

9.1.2.4 NewFunctionEnvironment ( `F`, `newTarget` )

The abstract operation NewFunctionEnvironment takes arguments F (an ECMAScript function object) and newTarget (an Object or undefined) and returns a Function Environment Record. It performs the following steps when called:

Let env be a new Function Environment Record containing no bindings.
Set env.[[FunctionObject]] to F.
If F.[[ThisMode]] is lexical, set env.[[ThisBindingStatus]] to lexical.
Else, set env.[[ThisBindingStatus]] to uninitialized.
Set env.[[NewTarget]] to newTarget.
Set env.[[OuterEnv]] to F.[[Environment]].
Return env.

9.1.2.5 NewGlobalEnvironment ( `G`, `thisValue` )

The abstract operation NewGlobalEnvironment takes arguments G (an Object) and thisValue (an Object) and returns a Global Environment Record. It performs the following steps when called:

Let objRec be NewObjectEnvironment(G, false, null).
Let dclRec be NewDeclarativeEnvironment(null).
Let env be a new Global Environment Record.
Set env.[[ObjectRecord]] to objRec.
Set env.[[GlobalThisValue]] to thisValue.
Set env.[[DeclarativeRecord]] to dclRec.
Set env.[[VarNames]] to a new empty List.
Set env.[[OuterEnv]] to null.
Return env.

9.1.2.6 NewModuleEnvironment ( `E` )

The abstract operation NewModuleEnvironment takes argument E (an Environment Record) and returns a Module Environment Record. It performs the following steps when called:

Let env be a new Module Environment Record containing no bindings.
Set env.[[OuterEnv]] to E.
Return env.

9.2 PrivateEnvironment Records

A PrivateEnvironment Record is a specification mechanism used to track Private Names based upon the lexical nesting structure of ClassDeclarations and ClassExpressions in ECMAScript code. They are similar to, but distinct from, Environment Records. Each PrivateEnvironment Record is associated with a ClassDeclaration or ClassExpression. Each time such a class is evaluated, a new PrivateEnvironment Record is created to record the Private Names declared by that class.

Each PrivateEnvironment Record has the fields defined in Table 23.

Table 23: PrivateEnvironment Record Fields

Field Name	Value Type	Meaning
`[[OuterPrivateEnvironment]]`	a PrivateEnvironment Record or null	The PrivateEnvironment Record of the nearest containing class. null if the class with which this PrivateEnvironment Record is associated is not contained in any other class.
`[[Names]]`	a List of Private Names	The Private Names declared by this class.

9.2.1 PrivateEnvironment Record Operations

The following abstract operations are used in this specification to operate upon PrivateEnvironment Records:

9.2.1.1 NewPrivateEnvironment ( `outerPrivEnv` )

The abstract operation NewPrivateEnvironment takes argument outerPrivEnv (a PrivateEnvironment Record or null) and returns a PrivateEnvironment Record. It performs the following steps when called:

Let names be a new empty List.
Return the PrivateEnvironment Record { [[OuterPrivateEnvironment]]: outerPrivEnv, [[Names]]: names }.

9.2.1.2 ResolvePrivateIdentifier ( `privEnv`, `identifier` )

The abstract operation ResolvePrivateIdentifier takes arguments privEnv (a PrivateEnvironment Record) and identifier (a String) and returns a Private Name. It performs the following steps when called:

Let names be privEnv.[[Names]].
2. For each Private Name pn of names, do
1. a. If pn.[[Description]] is identifier, then
  1. Return pn.
Let outerPrivEnv be privEnv.[[OuterPrivateEnvironment]].
Assert: outerPrivEnv is not null.
Return ResolvePrivateIdentifier(outerPrivEnv, identifier).

9.3 Realms

Before it is evaluated, all ECMAScript code must be associated with a realm. Conceptually, a realm consists of a set of intrinsic objects, an ECMAScript global environment, all of the ECMAScript code that is loaded within the scope of that global environment, and other associated state and resources.

A realm is represented in this specification as a Realm Record with the fields specified in Table 24:

Table 24: Realm Record Fields

Field Name	Value	Meaning
`[[AgentSignifier]]`	an agent signifier	The agent that owns this realm
`[[Intrinsics]]`	a Record whose field names are intrinsic keys and whose values are objects	The intrinsic values used by code associated with this realm
`[[GlobalObject]]`	an Object or undefined	The global object for this realm
`[[GlobalEnv]]`	a Global Environment Record	The global environment for this realm
`[[TemplateMap]]`	a List of Records with fields `[[Site]]` (a TemplateLiteral Parse Node) and `[[Array]]` (an Array)	Template objects are canonicalized separately for each realm using its Realm Record's `[[TemplateMap]]`. Each `[[Site]]` value is a Parse Node that is a TemplateLiteral. The associated `[[Array]]` value is the corresponding template object that is passed to a tag function. Note 1 Once a Parse Node becomes unreachable, the corresponding `[[Array]]` is also unreachable, and it would be unobservable if an implementation removed the pair from the `[[TemplateMap]]` list.
`[[LoadedModules]]`	a List of Records with fields `[[Specifier]]` (a String) and `[[Module]]` (a Module Record)	A map from the specifier strings imported by this realm to the resolved Module Record. The list does not contain two different Records with the same `[[Specifier]]`. Note 2 As mentioned in HostLoadImportedModule (16.2.1.8 Note 1), `[[LoadedModules]]` in Realm Records is only used when running an `import()` expression in a context where there is no active script or module.
`[[HostDefined]]`	anything (default value is undefined)	Field reserved for use by hosts that need to associate additional information with a Realm Record.

9.3.1 CreateRealm ( )

The abstract operation CreateRealm takes no arguments and returns a Realm Record. It performs the following steps when called:

Let realmRec be a new Realm Record.
Perform CreateIntrinsics(realmRec).
Set realmRec.[[AgentSignifier]] to AgentSignifier().
Set realmRec.[[GlobalObject]] to undefined.
Set realmRec.[[GlobalEnv]] to undefined.
Set realmRec.[[TemplateMap]] to a new empty List.
Return realmRec.

9.3.2 CreateIntrinsics ( `realmRec` )

The abstract operation CreateIntrinsics takes argument realmRec (a Realm Record) and returns unused. It performs the following steps when called:

Set realmRec.[[Intrinsics]] to a new Record.
Set fields of realmRec.[[Intrinsics]] with the values listed in Table 6. The field names are the names listed in column one of the table. The value of each field is a new object value fully and recursively populated with property values as defined by the specification of each object in clauses 19 through 28. All object property values are newly created object values. All values that are built-in function objects are created by performing CreateBuiltinFunction(steps, length, name, slots, realmRec, prototype) where steps is the definition of that function provided by this specification, name is the initial value of the function's "name" property, length is the initial value of the function's "length" property, slots is a list of the names, if any, of the function's specified internal slots, and prototype is the specified value of the function's [[Prototype]] internal slot. The creation of the intrinsics and their properties must be ordered to avoid any dependencies upon objects that have not yet been created.
Perform AddRestrictedFunctionProperties(realmRec.[[Intrinsics]].[[%Function.prototype%]], realmRec).
Return unused.

9.3.3 SetRealmGlobalObject ( `realmRec`, `globalObj`, `thisValue` )

The abstract operation SetRealmGlobalObject takes arguments realmRec (a Realm Record), globalObj (an Object or undefined), and thisValue (an Object or undefined) and returns unused. It performs the following steps when called:

1. If globalObj is undefined, then
1. Let intrinsics be realmRec.[[Intrinsics]].
2. Set globalObj to OrdinaryObjectCreate(intrinsics.[[%Object.prototype%]]).
Assert: globalObj is an Object.
If thisValue is undefined, set thisValue to globalObj.
Set realmRec.[[GlobalObject]] to globalObj.
Let newGlobalEnv be NewGlobalEnvironment(globalObj, thisValue).
Set realmRec.[[GlobalEnv]] to newGlobalEnv.
Return unused.

9.3.4 SetDefaultGlobalBindings ( `realmRec` )

The abstract operation SetDefaultGlobalBindings takes argument realmRec (a Realm Record) and returns either a normal completion containing an Object or a throw completion. It performs the following steps when called:

Let global be realmRec.[[GlobalObject]].
2. For each property of the Global Object specified in clause 19, do
1. Let name be the String value of the property name.
2. Let desc be the fully populated data Property Descriptor for the property, containing the specified attributes for the property. For properties listed in 19.2, 19.3, or 19.4 the value of the [[Value]] attribute is the corresponding intrinsic object from realmRec.
3. Perform ? DefinePropertyOrThrow(global, name, desc).
Return global.

9.4 Execution Contexts

An execution context is a specification device that is used to track the runtime evaluation of code by an ECMAScript implementation. At any point in time, there is at most one execution context per agent that is actually executing code. This is known as the agent's running execution context. All references to the running execution context in this specification denote the running execution context of the surrounding agent.

The execution context stack is used to track execution contexts. The running execution context is always the top element of this stack. A new execution context is created whenever control is transferred from the executable code associated with the currently running execution context to executable code that is not associated with that execution context. The newly created execution context is pushed onto the stack and becomes the running execution context.

An execution context contains whatever implementation specific state is necessary to track the execution progress of its associated code. Each execution context has at least the state components listed in Table 25.

Table 25: State Components for All Execution Contexts

Component	Purpose
code evaluation state	Any state needed to perform, suspend, and resume evaluation of the code associated with this execution context.
Function	If this execution context is evaluating the code of a function object, then the value of this component is that function object. If the context is evaluating the code of a Script or Module, the value is null.
Realm	The Realm Record from which associated code accesses ECMAScript resources.
ScriptOrModule	The Module Record or Script Record from which associated code originates. If there is no originating script or module, as is the case for the original execution context created in InitializeHostDefinedRealm, the value is null.

Evaluation of code by the running execution context may be suspended at various points defined within this specification. Once the running execution context has been suspended a different execution context may become the running execution context and commence evaluating its code. At some later time a suspended execution context may again become the running execution context and continue evaluating its code at the point where it had previously been suspended. Transition of the running execution context status among execution contexts usually occurs in stack-like last-in/first-out manner. However, some ECMAScript features require non-LIFO transitions of the running execution context.

The value of the Realm component of the running execution context is also called the current Realm Record. The value of the Function component of the running execution context is also called the active function object.

ECMAScript code execution contexts have the additional state components listed in Table 26.

Table 26: Additional State Components for ECMAScript Code Execution Contexts

Component	Purpose
LexicalEnvironment	Identifies the Environment Record used to resolve identifier references made by code within this execution context.
VariableEnvironment	Identifies the Environment Record that holds bindings created by VariableStatements within this execution context.
PrivateEnvironment	Identifies the PrivateEnvironment Record that holds Private Names created by ClassElements in the nearest containing class. null if there is no containing class.

The LexicalEnvironment and VariableEnvironment components of an execution context are always Environment Records.

Execution contexts representing the evaluation of Generators have the additional state components listed in Table 27.

Table 27: Additional State Components for Generator Execution Contexts

Component	Purpose
Generator	The Generator that this execution context is evaluating.

In most situations only the running execution context (the top of the execution context stack) is directly manipulated by algorithms within this specification. Hence when the terms “LexicalEnvironment”, and “VariableEnvironment” are used without qualification they are in reference to those components of the running execution context.

An execution context is purely a specification mechanism and need not correspond to any particular artefact of an ECMAScript implementation. It is impossible for ECMAScript code to directly access or observe an execution context.

9.4.1 GetActiveScriptOrModule ( )

The abstract operation GetActiveScriptOrModule takes no arguments and returns a Script Record, a Module Record, or null. It is used to determine the running script or module, based on the running execution context. It performs the following steps when called:

If the execution context stack is empty, return null.
Let ec be the topmost execution context on the execution context stack whose ScriptOrModule component is not null.
If no such execution context exists, return null. Otherwise, return ec's ScriptOrModule.

9.4.2 ResolveBinding ( `name` [ , `env` ] )

The abstract operation ResolveBinding takes argument name (a String) and optional argument env (an Environment Record or undefined) and returns either a normal completion containing a Reference Record or a throw completion. It is used to determine the binding of name. env can be used to explicitly provide the Environment Record that is to be searched for the binding. It performs the following steps when called:

1. If env is not present or env is undefined, then
1. Set env to the running execution context's LexicalEnvironment.
Assert: env is an Environment Record.
If the source text matched by the syntactic production that is being evaluated is contained in strict mode code, let strict be true; else let strict be false.
Return ? GetIdentifierReference(env, name, strict).

Note

The result of ResolveBinding is always a Reference Record whose [[ReferencedName]] field is name.

9.4.3 GetThisEnvironment ( )

The abstract operation GetThisEnvironment takes no arguments and returns an Environment Record. It finds the Environment Record that currently supplies the binding of the keyword this. It performs the following steps when called:

Let env be the running execution context's LexicalEnvironment.
2. Repeat,
1. Let exists be env.HasThisBinding().
2. If exists is true, return env.
3. Let outer be env.[[OuterEnv]].
4. Assert: outer is not null.
5. Set env to outer.

Note

The loop in step 2 will always terminate because the list of environments always ends with the global environment which has a this binding.

9.4.4 ResolveThisBinding ( )

The abstract operation ResolveThisBinding takes no arguments and returns either a normal completion containing an ECMAScript language value or a throw completion. It determines the binding of the keyword this using the LexicalEnvironment of the running execution context. It performs the following steps when called:

Let envRec be GetThisEnvironment().
Return ? envRec.GetThisBinding().

9.4.5 GetNewTarget ( )

The abstract operation GetNewTarget takes no arguments and returns an Object or undefined. It determines the NewTarget value using the LexicalEnvironment of the running execution context. It performs the following steps when called:

Let envRec be GetThisEnvironment().
Assert: envRec has a [[NewTarget]] field.
Return envRec.[[NewTarget]].

9.4.6 GetGlobalObject ( )

The abstract operation GetGlobalObject takes no arguments and returns an Object. It returns the global object used by the currently running execution context. It performs the following steps when called:

Let currentRealm be the current Realm Record.
Return currentRealm.[[GlobalObject]].

9.5 Jobs and Host Operations to Enqueue Jobs

A Job is an Abstract Closure with no parameters that initiates an ECMAScript computation when no other ECMAScript computation is currently in progress.

Jobs are scheduled for execution by ECMAScript host environments in a particular agent. This specification describes the host hooks HostEnqueueGenericJob, HostEnqueueFinalizationRegistryCleanupJob, HostEnqueuePromiseJob, and HostEnqueueTimeoutJob to schedule jobs. The host hooks in this specification are organized by the additional constraints imposed on the scheduling of jobs. Hosts may define additional abstract operations which schedule jobs. Such operations accept a Job Abstract Closure and a realm (a Realm Record or null) as parameters. If a Realm Record is provided, these operations schedule the job to be performed at some future time in the provided realm, in the agent that owns the realm. If null is provided instead for the realm, then the job does not evaluate ECMAScript code. Their implementations must conform to the following requirements:

At some future point in time, when there is no running context in the agent for which the job is scheduled and that agent's execution context stack is empty, the implementation must:
1. Perform any host-defined preparation steps.
2. Invoke the Job Abstract Closure.
3. Perform any host-defined cleanup steps, after which the execution context stack must be empty.
Only one Job may be actively undergoing evaluation at any point in time in an agent.
Once evaluation of a Job starts, it must run to completion before evaluation of any other Job starts in an agent.
The Abstract Closure must return a normal completion, implementing its own handling of errors.

Note 1

Host environments are not required to treat Jobs uniformly with respect to scheduling. For example, web browsers and Node.js treat Promise-handling Jobs as a higher priority than other work; future features may add Jobs that are not treated at such a high priority.

At any particular time, scriptOrModule (a Script Record, a Module Record, or null) is the active script or module if all of the following conditions are true:

GetActiveScriptOrModule() is scriptOrModule.
If scriptOrModule is a Script Record or Module Record, let ec be the topmost execution context on the execution context stack whose ScriptOrModule component is scriptOrModule. The Realm component of ec is scriptOrModule.[[Realm]].

At any particular time, an execution is prepared to evaluate ECMAScript code if all of the following conditions are true:

The execution context stack is not empty.
The Realm component of the topmost execution context on the execution context stack is a Realm Record.

Note 2

Host environments may prepare an execution to evaluate code by pushing execution contexts onto the execution context stack. The specific steps are implementation-defined.

The specific choice of Realm is up to the host environment. This initial execution context and Realm is only in use before any callback function is invoked. When a callback function related to a Job, like a Promise handler, is invoked, the invocation pushes its own execution context and Realm.

Particular kinds of Jobs have additional conformance requirements.

9.5.1 JobCallback Records

A JobCallback Record is a Record value used to store a function object and a host-defined value. Function objects that are invoked via a Job enqueued by the host may have additional host-defined context. To propagate the state, Job Abstract Closures should not capture and call function objects directly. Instead, use HostMakeJobCallback and HostCallJobCallback.

Note

The WHATWG HTML specification (https://html.spec.whatwg.org/), for example, uses the host-defined value to propagate the incumbent settings object for Promise callbacks.

JobCallback Records have the fields listed in Table 28.

Table 28: JobCallback Record Fields

Field Name	Value	Meaning
`[[Callback]]`	a function object	The function to invoke when the Job is invoked.
`[[HostDefined]]`	anything (default value is empty)	Field reserved for use by hosts.

9.5.2 HostMakeJobCallback ( `callback` )

The host-defined abstract operation HostMakeJobCallback takes argument callback (a function object) and returns a JobCallback Record.

An implementation of HostMakeJobCallback must conform to the following requirements:

It must return a JobCallback Record whose [[Callback]] field is callback.

The default implementation of HostMakeJobCallback performs the following steps when called:

Return the JobCallback Record { [[Callback]]: callback, [[HostDefined]]: empty }.

ECMAScript hosts that are not web browsers must use the default implementation of HostMakeJobCallback.

Note

This is called at the time that the callback is passed to the function that is responsible for its being eventually scheduled and run. For example, promise.then(thenAction) calls MakeJobCallback on thenAction at the time of invoking Promise.prototype.then, not at the time of scheduling the reaction Job.

9.5.3 HostCallJobCallback ( `jobCallback`, `V`, `argumentsList` )

The host-defined abstract operation HostCallJobCallback takes arguments jobCallback (a JobCallback Record), V (an ECMAScript language value), and argumentsList (a List of ECMAScript language values) and returns either a normal completion containing an ECMAScript language value or a throw completion.

An implementation of HostCallJobCallback must conform to the following requirements:

It must perform and return the result of Call(jobCallback.[[Callback]], V, argumentsList).

Note

This requirement means that hosts cannot change the [[Call]] behaviour of function objects defined in this specification.

The default implementation of HostCallJobCallback performs the following steps when called:

Assert: IsCallable(jobCallback.[[Callback]]) is true.
Return ? Call(jobCallback.[[Callback]], V, argumentsList).

ECMAScript hosts that are not web browsers must use the default implementation of HostCallJobCallback.

9.5.4 HostEnqueueGenericJob ( `job`, `realm` )

The host-defined abstract operation HostEnqueueGenericJob takes arguments job (a Job Abstract Closure) and realm (a Realm Record) and returns unused. It schedules job in the realm realm in the agent signified by realm.[[AgentSignifier]] to be performed at some future time. The Abstract Closures used with this algorithm are intended to be scheduled without additional constraints, such as priority and ordering.

An implementation of HostEnqueueGenericJob must conform to the requirements in 9.5.

9.5.5 HostEnqueuePromiseJob ( `job`, `realm` )

The host-defined abstract operation HostEnqueuePromiseJob takes arguments job (a Job Abstract Closure) and realm (a Realm Record or null) and returns unused. It schedules job to be performed at some future time. The Abstract Closures used with this algorithm are intended to be related to the handling of Promises, or otherwise, to be scheduled with equal priority to Promise handling operations.

An implementation of HostEnqueuePromiseJob must conform to the requirements in 9.5 as well as the following:

If realm is not null, each time job is invoked the implementation must perform implementation-defined steps such that execution is prepared to evaluate ECMAScript code at the time of job's invocation.
Let scriptOrModule be GetActiveScriptOrModule() at the time HostEnqueuePromiseJob is invoked. If realm is not null, each time job is invoked the implementation must perform implementation-defined steps such that scriptOrModule is the active script or module at the time of job's invocation.
Jobs must run in the same order as the HostEnqueuePromiseJob invocations that scheduled them.

Note

The realm for Jobs returned by NewPromiseResolveThenableJob is usually the result of calling GetFunctionRealm on the then function object. The realm for Jobs returned by NewPromiseReactionJob is usually the result of calling GetFunctionRealm on the handler if the handler is not undefined. If the handler is undefined, realm is null. For both kinds of Jobs, when GetFunctionRealm completes abnormally (i.e. called on a revoked Proxy), realm is the current Realm Record at the time of the GetFunctionRealm call. When the realm is null, no user ECMAScript code will be evaluated and no new ECMAScript objects (e.g. Error objects) will be created. The WHATWG HTML specification (https://html.spec.whatwg.org/), for example, uses realm to check for the ability to run script and for the entry concept.

9.5.6 HostEnqueueTimeoutJob ( `timeoutJob`, `realm`, `milliseconds` )

The host-defined abstract operation HostEnqueueTimeoutJob takes arguments timeoutJob (a Job Abstract Closure), realm (a Realm Record), and milliseconds (a non-negative finite Number) and returns unused. It schedules timeoutJob in the realm realm in the agent signified by realm.[[AgentSignifier]] to be performed after at least milliseconds milliseconds.

An implementation of HostEnqueueTimeoutJob must conform to the requirements in 9.5.

9.6 InitializeHostDefinedRealm ( )

The abstract operation InitializeHostDefinedRealm takes no arguments and returns either a normal completion containing unused or a throw completion. It performs the following steps when called:

Let realm be CreateRealm().
Let newContext be a new execution context.
Set the Function of newContext to null.
Set the Realm of newContext to realm.
Set the ScriptOrModule of newContext to null.
Push newContext onto the execution context stack; newContext is now the running execution context.
If the host requires use of an exotic object to serve as realm's global object, let global be such an object created in a host-defined manner. Otherwise, let global be undefined, indicating that an ordinary object should be created as the global object.
If the host requires that the this binding in realm's global scope return an object other than the global object, let thisValue be such an object created in a host-defined manner. Otherwise, let thisValue be undefined, indicating that realm's global this binding should be the global object.
Perform SetRealmGlobalObject(realm, global, thisValue).
Let globalObj be ? SetDefaultGlobalBindings(realm).
Create any host-defined global object properties on globalObj.
Return unused.

9.7 Agents

An agent comprises a set of ECMAScript execution contexts, an execution context stack, a running execution context, an Agent Record, and an executing thread. Except for the executing thread, the constituents of an agent belong exclusively to that agent.

An agent's executing thread executes algorithmic steps on the agent's execution contexts independently of other agents, except that an executing thread may be used as the executing thread by multiple agents, provided none of the agents sharing the thread have an Agent Record whose [[CanBlock]] field is true.

Note 1

Some web browsers share a single executing thread across multiple unrelated tabs of a browser window, for example.

While an agent's executing thread is executing algorithmic steps, the agent is the surrounding agent for those steps. The steps use the surrounding agent to access the specification-level execution objects held within the agent: the running execution context, the execution context stack, and the Agent Record's fields.

An agent signifier is a globally-unique opaque value used to identify an Agent.

Table 29: Agent Record Fields

Field Name	Value	Meaning
`[[LittleEndian]]`	a Boolean	The default value computed for the isLittleEndian parameter when it is needed by the algorithms GetValueFromBuffer and SetValueInBuffer. The choice is implementation-defined and should be the alternative that is most efficient for the implementation. Once the value has been observed it cannot change.
`[[CanBlock]]`	a Boolean	Determines whether the agent can block or not.
`[[Signifier]]`	an agent signifier	Uniquely identifies the agent within its agent cluster.
`[[IsLockFree1]]`	a Boolean	true if atomic operations on one-byte values are lock-free, false otherwise.
`[[IsLockFree2]]`	a Boolean	true if atomic operations on two-byte values are lock-free, false otherwise.
`[[IsLockFree8]]`	a Boolean	true if atomic operations on eight-byte values are lock-free, false otherwise.
`[[CandidateExecution]]`	a candidate execution Record	See the memory model.
`[[KeptAlive]]`	a List of either Objects or Symbols	Initially a new empty List, representing the list of objects and/or symbols to be kept alive until the end of the current Job

Once the values of [[Signifier]], [[IsLockFree1]], and [[IsLockFree2]] have been observed by any agent in the agent cluster they cannot change.

Note 2

The values of [[IsLockFree1]] and [[IsLockFree2]] are not necessarily determined by the hardware, but may also reflect implementation choices that can vary over time and between ECMAScript implementations.

There is no [[IsLockFree4]] field: 4-byte atomic operations are always lock-free.

In practice, if an atomic operation is implemented with any type of lock the operation is not lock-free. Lock-free does not imply wait-free: there is no upper bound on how many machine steps may be required to complete a lock-free atomic operation.

That an atomic access of size n is lock-free does not imply anything about the (perceived) atomicity of non-atomic accesses of size n, specifically, non-atomic accesses may still be performed as a sequence of several separate memory accesses. See ReadSharedMemory and WriteSharedMemory for details.

Note 3

An agent is a specification mechanism and need not correspond to any particular artefact of an ECMAScript implementation.

9.7.1 AgentSignifier ( )

The abstract operation AgentSignifier takes no arguments and returns an agent signifier. It performs the following steps when called:

Let AR be the Agent Record of the surrounding agent.
Return AR.[[Signifier]].

9.7.2 AgentCanSuspend ( )

The abstract operation AgentCanSuspend takes no arguments and returns a Boolean. It performs the following steps when called:

Let AR be the Agent Record of the surrounding agent.
Return AR.[[CanBlock]].

Note

In some environments it may not be reasonable for a given agent to suspend. For example, in a web browser environment, it may be reasonable to disallow suspending a document's main event handling thread, while still allowing workers' event handling threads to suspend.

9.8 Agent Clusters

An agent cluster is a maximal set of agents that can communicate by operating on shared memory.

Note 1

Programs within different agents may share memory by unspecified means. At a minimum, the backing memory for SharedArrayBuffers can be shared among the agents in the cluster.

There may be agents that can communicate by message passing that cannot share memory; they are never in the same agent cluster.

Every agent belongs to exactly one agent cluster.

Note 2

The agents in a cluster need not all be alive at some particular point in time. If agent A creates another agent B, after which A terminates and B creates agent C, the three agents are in the same cluster if A could share some memory with B and B could share some memory with C.

All agents within a cluster must have the same value for the [[LittleEndian]] field in their respective Agent Records.

Note 3

If different agents within an agent cluster have different values of [[LittleEndian]] it becomes hard to use shared memory for multi-byte data.

All agents within a cluster must have the same values for the [[IsLockFree1]] field in their respective Agent Records; similarly for the [[IsLockFree2]] field.

All agents within a cluster must have different values for the [[Signifier]] field in their respective Agent Records.

An embedding may deactivate (stop forward progress) or activate (resume forward progress) an agent without the agent's knowledge or cooperation. If the embedding does so, it must not leave some agents in the cluster active while other agents in the cluster are deactivated indefinitely.

Note 4

The purpose of the preceding restriction is to avoid a situation where an agent deadlocks or starves because another agent has been deactivated. For example, if an HTML shared worker that has a lifetime independent of documents in any windows were allowed to share memory with the dedicated worker of such an independent document, and the document and its dedicated worker were to be deactivated while the dedicated worker holds a lock (say, the document is pushed into its window's history), and the shared worker then tries to acquire the lock, then the shared worker will be blocked until the dedicated worker is activated again, if ever. Meanwhile other workers trying to access the shared worker from other windows will starve.

The implication of the restriction is that it will not be possible to share memory between agents that don't belong to the same suspend/wake collective within the embedding.

An embedding may terminate an agent without any of the agent's cluster's other agents' prior knowledge or cooperation. If an agent is terminated not by programmatic action of its own or of another agent in the cluster but by forces external to the cluster, then the embedding must choose one of two strategies: Either terminate all the agents in the cluster, or provide reliable APIs that allow the agents in the cluster to coordinate so that at least one remaining member of the cluster will be able to detect the termination, with the termination data containing enough information to identify the agent that was terminated.

Note 5

Examples of that type of termination are: operating systems or users terminating agents that are running in separate processes; the embedding itself terminating an agent that is running in-process with the other agents when per-agent resource accounting indicates that the agent is runaway.

Each of the following specification values, and values transitively reachable from them, belong to exactly one agent cluster.

candidate execution Record
Shared Data Block
WaiterList Record

Prior to any evaluation of any ECMAScript code by any agent in a cluster, the [[CandidateExecution]] field of the Agent Record for all agents in the cluster is set to the initial candidate execution. The initial candidate execution is an empty candidate execution whose [[EventsRecords]] field is a List containing, for each agent, an Agent Events Record whose [[AgentSignifier]] field is that agent's agent signifier, and whose [[EventList]] and [[AgentSynchronizesWith]] fields are empty Lists.

Note 6

All agents in an agent cluster share the same candidate execution in its Agent Record's [[CandidateExecution]] field. The candidate execution is a specification mechanism used by the memory model.

Note 7

An agent cluster is a specification mechanism and need not correspond to any particular artefact of an ECMAScript implementation.

9.9 Forward Progress

For an agent to make forward progress is for it to perform an evaluation step according to this specification.

An agent becomes blocked when its running execution context waits synchronously and indefinitely for an external event. Only agents whose Agent Record's [[CanBlock]] field is true can become blocked in this sense. An unblocked agent is one that is not blocked.

Implementations must ensure that:

every unblocked agent with a dedicated executing thread eventually makes forward progress
in a set of agents that share an executing thread, one agent eventually makes forward progress
an agent does not cause another agent to become blocked except via explicit APIs that provide blocking.

Note

This, along with the liveness guarantee in the memory model, ensures that all seq-cst writes eventually become observable to all agents.

9.10 Processing Model of WeakRef and FinalizationRegistry Targets

9.10.1 Objectives

This specification does not make any guarantees that any object or symbol will be garbage collected. Objects or symbols which are not live may be released after long periods of time, or never at all. For this reason, this specification uses the term "may" when describing behaviour triggered by garbage collection.

The semantics of WeakRefs and FinalizationRegistrys is based on two operations which happen at particular points in time:

When WeakRef.prototype.deref is called, the referent (if undefined is not returned) is kept alive so that subsequent, synchronous accesses also return the same value. This list is reset when synchronous work is done using the ClearKeptObjects abstract operation.
When an object or symbol which is registered with a FinalizationRegistry becomes unreachable, a call of the FinalizationRegistry's cleanup callback may eventually be made, after synchronous ECMAScript execution completes. The FinalizationRegistry cleanup is performed with the CleanupFinalizationRegistry abstract operation.

Neither of these actions (ClearKeptObjects or CleanupFinalizationRegistry) may interrupt synchronous ECMAScript execution. Because hosts may assemble longer, synchronous ECMAScript execution runs, this specification defers the scheduling of ClearKeptObjects and CleanupFinalizationRegistry to the host environment.

Some ECMAScript implementations include garbage collector implementations which run in the background, including when ECMAScript is idle. Letting the host environment schedule CleanupFinalizationRegistry allows it to resume ECMAScript execution in order to run finalizer work, which may free up held values, reducing overall memory usage.

9.10.2 Liveness

For some set of objects and/or symbols S a hypothetical WeakRef-oblivious execution with respect to S is an execution whereby the abstract operation WeakRefDeref of a WeakRef whose referent is an element of S always returns undefined.

Note 1

WeakRef-obliviousness, together with liveness, capture two notions. One, that a WeakRef itself does not keep its referent alive. Two, that cycles in liveness does not imply that a value is live. To be concrete, if determining v's liveness depends on determining the liveness of a WeakRef referent, r, r's liveness cannot assume v's liveness, which would be circular reasoning.

Note 2

WeakRef- obliviousness is defined on sets of objects or symbols instead of individual values to account for cycles. If it were defined on individual values, then a WeakRef referent in a cycle will be considered live even though its identity is only observed via other WeakRef referents in the cycle.

Note 3

Colloquially, we say that an individual object or symbol is live if every set containing it is live.

At any point during evaluation, a set of objects and/or symbols S is considered live if either of the following conditions is met:

Any element in S is included in any agent's [[KeptAlive]] List.
There exists a valid future hypothetical WeakRef-oblivious execution with respect to S that observes the identity of any value in S.

Note 4

The second condition above intends to capture the intuition that a value is live if its identity is observable via non-WeakRef means. A value's identity may be observed by observing a strict equality comparison or observing the value being used as key in a Map.

Note 5

Presence of an object or a symbol in a field, an internal slot, or a property does not imply that the value is live. For example if the value in question is never passed back to the program, then it cannot be observed.

This is the case for keys in a WeakMap, members of a WeakSet, as well as the [[WeakRefTarget]] and [[UnregisterToken]] fields of a FinalizationRegistry Cell record.

The above definition implies that, if a key in a WeakMap is not live, then its corresponding value is not necessarily live either.

Note 6

Liveness is the lower bound for guaranteeing which WeakRefs engines must not empty. Liveness as defined here is undecidable. In practice, engines use conservative approximations such as reachability. There is expected to be significant implementation leeway.

9.10.3 Execution

At any time, if a set of objects and/or symbols S is not live, an ECMAScript implementation may perform the following steps atomically:

1. For each element value of S, do
1. a. For each WeakRef ref such that ref.[[WeakRefTarget]] is value, do
  1. Set ref.[[WeakRefTarget]] to empty.
2. b. For each FinalizationRegistry fg such that fg.[[Cells]] contains a Record cell such that cell.[[WeakRefTarget]] is value, do
  1. Set cell.[[WeakRefTarget]] to empty.
  2. Optionally, perform HostEnqueueFinalizationRegistryCleanupJob(fg).
3. c. For each WeakMap map such that map.[[WeakMapData]] contains a Record r such that r.[[Key]] is value, do
  1. Set r.[[Key]] to empty.
  2. Set r.[[Value]] to empty.
4. d. For each WeakSet set such that set.[[WeakSetData]] contains value, do
  1. Replace the element of set.[[WeakSetData]] whose value is value with an element whose value is empty.

Note 1

Together with the definition of liveness, this clause prescribes optimizations that an implementation may apply regarding WeakRefs.

It is possible to access an object without observing its identity. Optimizations such as dead variable elimination and scalar replacement on properties of non-escaping objects whose identity is not observed are allowed. These optimizations are thus allowed to observably empty WeakRefs that point to such objects.

On the other hand, if an object's identity is observable, and that object is in the [[WeakRefTarget]] internal slot of a WeakRef, optimizations such as rematerialization that observably empty the WeakRef are prohibited.

Because calling HostEnqueueFinalizationRegistryCleanupJob is optional, registered objects in a FinalizationRegistry do not necessarily hold that FinalizationRegistry live. Implementations may omit FinalizationRegistry callbacks for any reason, e.g., if the FinalizationRegistry itself becomes dead, or if the application is shutting down.

Note 2

Implementations are not obligated to empty WeakRefs for maximal sets of non-live objects or symbols.

If an implementation chooses a non-live set S in which to empty WeakRefs, this definition requires that it empties WeakRefs for all values in S simultaneously. In other words, it is not conformant for an implementation to empty a WeakRef pointing to a value v without emptying out other WeakRefs that, if not emptied, could result in an execution that observes the value of v.

9.10.4 Host Hooks

9.10.4.1 HostEnqueueFinalizationRegistryCleanupJob ( `finalizationRegistry` )

The host-defined abstract operation HostEnqueueFinalizationRegistryCleanupJob takes argument finalizationRegistry (a FinalizationRegistry) and returns unused.

Let cleanupJob be a new Job Abstract Closure with no parameters that captures finalizationRegistry and performs the following steps when called:

Let cleanupResult be Completion(CleanupFinalizationRegistry(finalizationRegistry)).
If cleanupResult is an abrupt completion, perform any host-defined steps for reporting the error.
Return unused.

An implementation of HostEnqueueFinalizationRegistryCleanupJob schedules cleanupJob to be performed at some future time, if possible. It must also conform to the requirements in 9.5.

9.11 ClearKeptObjects ( )

The abstract operation ClearKeptObjects takes no arguments and returns unused. ECMAScript implementations are expected to call ClearKeptObjects when a synchronous sequence of ECMAScript executions completes. It performs the following steps when called:

Let agentRecord be the surrounding agent's Agent Record.
Set agentRecord.[[KeptAlive]] to a new empty List.
Return unused.

9.12 AddToKeptObjects ( `value` )

The abstract operation AddToKeptObjects takes argument value (an Object or a Symbol) and returns unused. It performs the following steps when called:

Let agentRecord be the surrounding agent's Agent Record.
Append value to agentRecord.[[KeptAlive]].
Return unused.

Note

When the abstract operation AddToKeptObjects is called with a target object or symbol, it adds the target to a list that will point strongly at the target until ClearKeptObjects is called.

9.13 CleanupFinalizationRegistry ( `finalizationRegistry` )

The abstract operation CleanupFinalizationRegistry takes argument finalizationRegistry (a FinalizationRegistry) and returns either a normal completion containing unused or a throw completion. It performs the following steps when called:

Assert: finalizationRegistry has [[Cells]] and [[CleanupCallback]] internal slots.
Let callback be finalizationRegistry.[[CleanupCallback]].
3. While finalizationRegistry.[[Cells]] contains a Record cell such that cell.[[WeakRefTarget]] is empty, an implementation may perform the following steps:
1. Choose any such cell.
2. Remove cell from finalizationRegistry.[[Cells]].
3. Perform ? HostCallJobCallback(callback, undefined, « cell.[[HeldValue]] »).
Return unused.

9.14 CanBeHeldWeakly ( `v` )

The abstract operation CanBeHeldWeakly takes argument v (an ECMAScript language value) and returns a Boolean. It returns true if and only if v is suitable for use as a weak reference. Only values that are suitable for use as a weak reference may be a key of a WeakMap, an element of a WeakSet, the target of a WeakRef, or one of the targets of a FinalizationRegistry. It performs the following steps when called:

If v is an Object, return true.
If v is a Symbol and KeyForSymbol(v) is undefined, return true.
Return false.

Note

A language value without language identity can be manifested without prior reference and is unsuitable for use as a weak reference. A Symbol value produced by Symbol.for, unlike other Symbol values, does not have language identity and is unsuitable for use as a weak reference. Well-known symbols are likely to never be collected, but are nonetheless treated as suitable for use as a weak reference because they are limited in number and therefore manageable by a variety of implementation approaches. However, any value associated to a well-known symbol in a live WeakMap is unlikely to be collected and could “leak” memory resources in implementations.

10 Ordinary and Exotic Objects Behaviours

10.1 Ordinary Object Internal Methods and Internal Slots

All ordinary objects have an internal slot called [[Prototype]]. The value of this internal slot is either null or an object and is used for implementing inheritance. Assume a property named P is missing from an ordinary object O but exists on its [[Prototype]] object. If P refers to a data property on the [[Prototype]] object, O inherits it for get access, making it behave as if P was a property of O. If P refers to a writable data property on the [[Prototype]] object, set access of P on O creates a new data property named P on O. If P refers to a non-writable data property on the [[Prototype]] object, set access of P on O fails. If P refers to an accessor property on the [[Prototype]] object, the accessor is inherited by O for both get access and set access.

Every ordinary object has a Boolean-valued [[Extensible]] internal slot which is used to fulfill the extensibility-related internal method invariants specified in 6.1.7.3. Namely, once the value of an object's [[Extensible]] internal slot has been set to false, it is no longer possible to add properties to the object, to modify the value of the object's [[Prototype]] internal slot, or to subsequently change the value of [[Extensible]] to true.

In the following algorithm descriptions, assume O is an ordinary object, P is a property key value, V is any ECMAScript language value, and Desc is a Property Descriptor record.

Each ordinary object internal method delegates to a similarly-named abstract operation. If such an abstract operation depends on another internal method, then the internal method is invoked on O rather than calling the similarly-named abstract operation directly. These semantics ensure that exotic objects have their overridden internal methods invoked when ordinary object internal methods are applied to them.

10.1.1 `[[GetPrototypeOf]]` ( )

The [[GetPrototypeOf]] internal method of an ordinary object O takes no arguments and returns a normal completion containing either an Object or null. It performs the following steps when called:

Return OrdinaryGetPrototypeOf(O).

10.1.1.1 OrdinaryGetPrototypeOf ( `O` )

The abstract operation OrdinaryGetPrototypeOf takes argument O (an Object) and returns an Object or null. It performs the following steps when called:

Return O.[[Prototype]].

10.1.2 `[[SetPrototypeOf]]` ( `V` )

The [[SetPrototypeOf]] internal method of an ordinary object O takes argument V (an Object or null) and returns a normal completion containing a Boolean. It performs the following steps when called:

Return OrdinarySetPrototypeOf(O, V).

10.1.2.1 OrdinarySetPrototypeOf ( `O`, `V` )

The abstract operation OrdinarySetPrototypeOf takes arguments O (an Object) and V (an Object or null) and returns a Boolean. It performs the following steps when called:

Let current be O.[[Prototype]].
If SameValue(V, current) is true, return true.
Let extensible be O.[[Extensible]].
If extensible is false, return false.
Let p be V.
Let done be false.
7. Repeat, while done is false,
1. a. If p is null, then
  1. Set done to true.
2. b. Else if SameValue(p, O) is true, then
  1. Return false.
3. c. Else,
  1. If p.[[GetPrototypeOf]] is not the ordinary object internal method defined in 10.1.1, set done to true.
  2. Else, set p to p.[[Prototype]].
Set O.[[Prototype]] to V.
Return true.

Note

The loop in step 7 guarantees that there will be no circularities in any prototype chain that only includes objects that use the ordinary object definitions for [[GetPrototypeOf]] and [[SetPrototypeOf]].

10.1.3 `[[IsExtensible]]` ( )

The [[IsExtensible]] internal method of an ordinary object O takes no arguments and returns a normal completion containing a Boolean. It performs the following steps when called:

Return OrdinaryIsExtensible(O).

10.1.3.1 OrdinaryIsExtensible ( `O` )

The abstract operation OrdinaryIsExtensible takes argument O (an Object) and returns a Boolean. It performs the following steps when called:

Return O.[[Extensible]].

10.1.4 `[[PreventExtensions]]` ( )

The [[PreventExtensions]] internal method of an ordinary object O takes no arguments and returns a normal completion containing true. It performs the following steps when called:

Return OrdinaryPreventExtensions(O).

10.1.4.1 OrdinaryPreventExtensions ( `O` )

The abstract operation OrdinaryPreventExtensions takes argument O (an Object) and returns true. It performs the following steps when called:

Set O.[[Extensible]] to false.
Return true.

10.1.5 `[[GetOwnProperty]]` ( `P` )

The [[GetOwnProperty]] internal method of an ordinary object O takes argument P (a property key) and returns a normal completion containing either a Property Descriptor or undefined. It performs the following steps when called:

Return OrdinaryGetOwnProperty(O, P).

10.1.5.1 OrdinaryGetOwnProperty ( `O`, `P` )

The abstract operation OrdinaryGetOwnProperty takes arguments O (an Object) and P (a property key) and returns a Property Descriptor or undefined. It performs the following steps when called:

If O does not have an own property with key P, return undefined.
Let D be a newly created Property Descriptor with no fields.
Let X be O's own property whose key is P.
4. If X is a data property, then
1. Set D.[[Value]] to the value of X's [[Value]] attribute.
2. Set D.[[Writable]] to the value of X's [[Writable]] attribute.
5. Else,
1. Assert: X is an accessor property.
2. Set D.[[Get]] to the value of X's [[Get]] attribute.
3. Set D.[[Set]] to the value of X's [[Set]] attribute.
Set D.[[Enumerable]] to the value of X's [[Enumerable]] attribute.
Set D.[[Configurable]] to the value of X's [[Configurable]] attribute.
Return D.

10.1.6 `[[DefineOwnProperty]]` ( `P`, `Desc` )

The [[DefineOwnProperty]] internal method of an ordinary object O takes arguments P (a property key) and Desc (a Property Descriptor) and returns either a normal completion containing a Boolean or a throw completion. It performs the following steps when called:

Return ? OrdinaryDefineOwnProperty(O, P, Desc).

10.1.6.1 OrdinaryDefineOwnProperty ( `O`, `P`, `Desc` )

The abstract operation OrdinaryDefineOwnProperty takes arguments O (an Object), P (a property key), and Desc (a Property Descriptor) and returns either a normal completion containing a Boolean or a throw completion. It performs the following steps when called:

Let current be ? O.[[GetOwnProperty]](P).
Let extensible be ? IsExtensible(O).
Return ValidateAndApplyPropertyDescriptor(O, P, extensible, Desc, current).

10.1.6.2 IsCompatiblePropertyDescriptor ( `Extensible`, `Desc`, `Current` )

The abstract operation IsCompatiblePropertyDescriptor takes arguments Extensible (a Boolean), Desc (a Property Descriptor), and Current (a Property Descriptor or undefined) and returns a Boolean. It performs the following steps when called:

Return ValidateAndApplyPropertyDescriptor(undefined, "", Extensible, Desc, Current).

10.1.6.3 ValidateAndApplyPropertyDescriptor ( `O`, `P`, `extensible`, `Desc`, `current` )

The abstract operation ValidateAndApplyPropertyDescriptor takes arguments O (an Object or undefined), P (a property key), extensible (a Boolean), Desc (a Property Descriptor), and current (a Property Descriptor or undefined) and returns a Boolean. It returns true if and only if Desc can be applied as the property of an object with specified extensibility and current property current while upholding invariants. When such application is possible and O is not undefined, it is performed for the property named P (which is created if necessary). It performs the following steps when called:

Assert: IsPropertyKey(P) is true.
2. If current is undefined, then
1. If extensible is false, return false.
2. If O is undefined, return true.
3. c. If IsAccessorDescriptor(Desc) is true, then
  1. Create an own accessor property named P of object O whose [[Get]], [[Set]], [[Enumerable]], and [[Configurable]] attributes are set to the value of the corresponding field in Desc if Desc has that field, or to the attribute's default value otherwise.
4. d. Else,
  1. Create an own data property named P of object O whose [[Value]], [[Writable]], [[Enumerable]], and [[Configurable]] attributes are set to the value of the corresponding field in Desc if Desc has that field, or to the attribute's default value otherwise.
5. Return true.
Assert: current is a fully populated Property Descriptor.
If Desc does not have any fields, return true.
5. If current.[[Configurable]] is false, then
1. If Desc has a [[Configurable]] field and Desc.[[Configurable]] is true, return false.
2. If Desc has an [[Enumerable]] field and Desc.[[Enumerable]] is not current.[[Enumerable]], return false.
3. If IsGenericDescriptor(Desc) is false and IsAccessorDescriptor(Desc) is not IsAccessorDescriptor(current), return false.
4. d. If IsAccessorDescriptor(current) is true, then
  1. If Desc has a [[Get]] field and SameValue(Desc.[[Get]], current.[[Get]]) is false, return false.
  2. If Desc has a [[Set]] field and SameValue(Desc.[[Set]], current.[[Set]]) is false, return false.
5. e. Else if current.[[Writable]] is false, then
  1. If Desc has a [[Writable]] field and Desc.[[Writable]] is true, return false.
  2. If Desc has a [[Value]] field and SameValue(Desc.[[Value]], current.[[Value]]) is false, return false.
6. If O is not undefined, then
1. a. If IsDataDescriptor(current) is true and IsAccessorDescriptor(Desc) is true, then
  1. If Desc has a [[Configurable]] field, let configurable be Desc.[[Configurable]]; else let configurable be current.[[Configurable]].
  2. If Desc has a [[Enumerable]] field, let enumerable be Desc.[[Enumerable]]; else let enumerable be current.[[Enumerable]].
  3. Replace the property named P of object O with an accessor property whose [[Configurable]] and [[Enumerable]] attributes are set to configurable and enumerable, respectively, and whose [[Get]] and [[Set]] attributes are set to the value of the corresponding field in Desc if Desc has that field, or to the attribute's default value otherwise.
2. b. Else if IsAccessorDescriptor(current) is true and IsDataDescriptor(Desc) is true, then
  1. If Desc has a [[Configurable]] field, let configurable be Desc.[[Configurable]]; else let configurable be current.[[Configurable]].
  2. If Desc has a [[Enumerable]] field, let enumerable be Desc.[[Enumerable]]; else let enumerable be current.[[Enumerable]].
  3. Replace the property named P of object O with a data property whose [[Configurable]] and [[Enumerable]] attributes are set to configurable and enumerable, respectively, and whose [[Value]] and [[Writable]] attributes are set to the value of the corresponding field in Desc if Desc has that field, or to the attribute's default value otherwise.
3. c. Else,
  1. For each field of Desc, set the corresponding attribute of the property named P of object O to the value of the field.
Return true.

10.1.7 `[[HasProperty]]` ( `P` )

The [[HasProperty]] internal method of an ordinary object O takes argument P (a property key) and returns either a normal completion containing a Boolean or a throw completion. It performs the following steps when called:

Return ? OrdinaryHasProperty(O, P).

10.1.7.1 OrdinaryHasProperty ( `O`, `P` )

The abstract operation OrdinaryHasProperty takes arguments O (an Object) and P (a property key) and returns either a normal completion containing a Boolean or a throw completion. It performs the following steps when called:

Let hasOwn be ? O.[[GetOwnProperty]](P).
If hasOwn is not undefined, return true.
Let parent be ? O.[[GetPrototypeOf]]().
4. If parent is not null, then
1. Return ? parent.[[HasProperty]](P).
Return false.

10.1.8 `[[Get]]` ( `P`, `Receiver` )

The [[Get]] internal method of an ordinary object O takes arguments P (a property key) and Receiver (an ECMAScript language value) and returns either a normal completion containing an ECMAScript language value or a throw completion. It performs the following steps when called:

Return ? OrdinaryGet(O, P, Receiver).

10.1.8.1 OrdinaryGet ( `O`, `P`, `Receiver` )

The abstract operation OrdinaryGet takes arguments O (an Object), P (a property key), and Receiver (an ECMAScript language value) and returns either a normal completion containing an ECMAScript language value or a throw completion. It performs the following steps when called:

Let desc be ? O.[[GetOwnProperty]](P).
2. If desc is undefined, then
1. Let parent be ? O.[[GetPrototypeOf]]().
2. If parent is null, return undefined.
3. Return ? parent.[[Get]](P, Receiver).
If IsDataDescriptor(desc) is true, return desc.[[Value]].
Assert: IsAccessorDescriptor(desc) is true.
Let getter be desc.[[Get]].
If getter is undefined, return undefined.
Return ? Call(getter, Receiver).

10.1.9 `[[Set]]` ( `P`, `V`, `Receiver` )

The [[Set]] internal method of an ordinary object O takes arguments P (a property key), V (an ECMAScript language value), and Receiver (an ECMAScript language value) and returns either a normal completion containing a Boolean or a throw completion. It performs the following steps when called:

Return ? OrdinarySet(O, P, V, Receiver).

10.1.9.1 OrdinarySet ( `O`, `P`, `V`, `Receiver` )

The abstract operation OrdinarySet takes arguments O (an Object), P (a property key), V (an ECMAScript language value), and Receiver (an ECMAScript language value) and returns either a normal completion containing a Boolean or a throw completion. It performs the following steps when called:

Let ownDesc be ? O.[[GetOwnProperty]](P).
Return ? OrdinarySetWithOwnDescriptor(O, P, V, Receiver, ownDesc).

10.1.9.2 OrdinarySetWithOwnDescriptor ( `O`, `P`, `V`, `Receiver`, `ownDesc` )

The abstract operation OrdinarySetWithOwnDescriptor takes arguments O (an Object), P (a property key), V (an ECMAScript language value), Receiver (an ECMAScript language value), and ownDesc (a Property Descriptor or undefined) and returns either a normal completion containing a Boolean or a throw completion. It performs the following steps when called:

1. If ownDesc is undefined, then
1. Let parent be ? O.[[GetPrototypeOf]]().
2. b. If parent is not null, then
  1. Return ? parent.[[Set]](P, V, Receiver).
3. c. Else,
  1. Set ownDesc to the PropertyDescriptor { [[Value]]: undefined, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true }.
2. If IsDataDescriptor(ownDesc) is true, then
1. If ownDesc.[[Writable]] is false, return false.
2. If Receiver is not an Object, return false.
3. Let existingDescriptor be ? Receiver.[[GetOwnProperty]](P).
4. d. If existingDescriptor is not undefined, then
  1. If IsAccessorDescriptor(existingDescriptor) is true, return false.
  2. If existingDescriptor.[[Writable]] is false, return false.
  3. Let valueDesc be the PropertyDescriptor { [[Value]]: V }.
  4. Return ? Receiver.[[DefineOwnProperty]](P, valueDesc).
5. e. Else,
  1. Assert: Receiver does not currently have a property P.
  2. Return ? CreateDataProperty(Receiver, P, V).
Assert: IsAccessorDescriptor(ownDesc) is true.
Let setter be ownDesc.[[Set]].
If setter is undefined, return false.
Perform ? Call(setter, Receiver, « V »).
Return true.

10.1.10 `[[Delete]]` ( `P` )

The [[Delete]] internal method of an ordinary object O takes argument P (a property key) and returns either a normal completion containing a Boolean or a throw completion. It performs the following steps when called:

Return ? OrdinaryDelete(O, P).

10.1.10.1 OrdinaryDelete ( `O`, `P` )

The abstract operation OrdinaryDelete takes arguments O (an Object) and P (a property key) and returns either a normal completion containing a Boolean or a throw completion. It performs the following steps when called:

Let desc be ? O.[[GetOwnProperty]](P).
If desc is undefined, return true.
3. If desc.[[Configurable]] is true, then
1. Remove the own property with name P from O.
2. Return true.
Return false.

10.1.11 `[[OwnPropertyKeys]]` ( )

The [[OwnPropertyKeys]] internal method of an ordinary object O takes no arguments and returns a normal completion containing a List of property keys. It performs the following steps when called:

Return OrdinaryOwnPropertyKeys(O).

10.1.11.1 OrdinaryOwnPropertyKeys ( `O` )

The abstract operation OrdinaryOwnPropertyKeys takes argument O (an Object) and returns a List of property keys. It performs the following steps when called:

Let keys be a new empty List.
2. For each own property key P of O such that P is an array index, in ascending numeric index order, do
1. Append P to keys.
3. For each own property key P of O such that P is a String and P is not an array index, in ascending chronological order of property creation, do
1. Append P to keys.
4. For each own property key P of O such that P is a Symbol, in ascending chronological order of property creation, do
1. Append P to keys.
Return keys.

10.1.12 OrdinaryObjectCreate ( `proto` [ , `additionalInternalSlotsList` ] )

The abstract operation OrdinaryObjectCreate takes argument proto (an Object or null) and optional argument additionalInternalSlotsList (a List of names of internal slots) and returns an Object. It is used to specify the runtime creation of new ordinary objects. additionalInternalSlotsList contains the names of additional internal slots that must be defined as part of the object, beyond [[Prototype]] and [[Extensible]]. If additionalInternalSlotsList is not provided, a new empty List is used. It performs the following steps when called:

Let internalSlotsList be « [[Prototype]], [[Extensible]] ».
If additionalInternalSlotsList is present, set internalSlotsList to the list-concatenation of internalSlotsList and additionalInternalSlotsList.
Let O be MakeBasicObject(internalSlotsList).
Set O.[[Prototype]] to proto.
Return O.

Note

Although OrdinaryObjectCreate does little more than call MakeBasicObject, its use communicates the intention to create an ordinary object, and not an exotic one. Thus, within this specification, it is not called by any algorithm that subsequently modifies the internal methods of the object in ways that would make the result non-ordinary. Operations that create exotic objects invoke MakeBasicObject directly.

10.1.13 OrdinaryCreateFromConstructor ( `constructor`, `intrinsicDefaultProto` [ , `internalSlotsList` ] )

The abstract operation OrdinaryCreateFromConstructor takes arguments constructor (a constructor) and intrinsicDefaultProto (a String) and optional argument internalSlotsList (a List of names of internal slots) and returns either a normal completion containing an Object or a throw completion. It creates an ordinary object whose [[Prototype]] value is retrieved from a constructor's "prototype" property, if it exists. Otherwise the intrinsic named by intrinsicDefaultProto is used for [[Prototype]]. internalSlotsList contains the names of additional internal slots that must be defined as part of the object. If internalSlotsList is not provided, a new empty List is used. It performs the following steps when called:

Assert: intrinsicDefaultProto is this specification's name of an intrinsic object. The corresponding object must be an intrinsic that is intended to be used as the [[Prototype]] value of an object.
Let proto be ? GetPrototypeFromConstructor(constructor, intrinsicDefaultProto).
If internalSlotsList is present, let slotsList be internalSlotsList.
Else, let slotsList be a new empty List.
Return OrdinaryObjectCreate(proto, slotsList).

10.1.14 GetPrototypeFromConstructor ( `constructor`, `intrinsicDefaultProto` )

The abstract operation GetPrototypeFromConstructor takes arguments constructor (a function object) and intrinsicDefaultProto (a String) and returns either a normal completion containing an Object or a throw completion. It determines the [[Prototype]] value that should be used to create an object corresponding to a specific constructor. The value is retrieved from the constructor's "prototype" property, if it exists. Otherwise the intrinsic named by intrinsicDefaultProto is used for [[Prototype]]. It performs the following steps when called:

Assert: intrinsicDefaultProto is this specification's name of an intrinsic object. The corresponding object must be an intrinsic that is intended to be used as the [[Prototype]] value of an object.
Let proto be ? Get(constructor, "prototype").
3. If proto is not an Object, then
1. Let realm be ? GetFunctionRealm(constructor).
2. Set proto to realm's intrinsic object named intrinsicDefaultProto.
Return proto.

Note

If constructor does not supply a [[Prototype]] value, the default value that is used is obtained from the realm of the constructor function rather than from the running execution context.

10.1.15 RequireInternalSlot ( `O`, `internalSlot` )

The abstract operation RequireInternalSlot takes arguments O (an ECMAScript language value) and internalSlot (an internal slot name) and returns either a normal completion containing unused or a throw completion. It throws an exception unless O is an Object and has the given internal slot. It performs the following steps when called:

If O is not an Object, throw a TypeError exception.
If O does not have an internalSlot internal slot, throw a TypeError exception.
Return unused.

10.2 ECMAScript Function Objects

ECMAScript function objects encapsulate parameterized ECMAScript code closed over a lexical environment and support the dynamic evaluation of that code. An ECMAScript function object is an ordinary object and has the same internal slots and the same internal methods as other ordinary objects. The code of an ECMAScript function object may be either strict mode code (11.2.2) or non-strict code. An ECMAScript function object whose code is strict mode code is called a strict function. One whose code is not strict mode code is called a non-strict function.

In addition to [[Extensible]] and [[Prototype]], ECMAScript function objects also have the internal slots listed in Table 30.

Table 30: Internal Slots of ECMAScript Function Objects

Internal Slot	Type	Description
`[[Environment]]`	an Environment Record	The Environment Record that the function was closed over. Used as the outer environment when evaluating the code of the function.
`[[PrivateEnvironment]]`	a PrivateEnvironment Record or null	The PrivateEnvironment Record for Private Names that the function was closed over. null if this function is not syntactically contained within a class. Used as the outer PrivateEnvironment for inner classes when evaluating the code of the function.
`[[FormalParameters]]`	a Parse Node	The root parse node of the source text that defines the function's formal parameter list.
`[[ECMAScriptCode]]`	a Parse Node	The root parse node of the source text that defines the function's body.
`[[ConstructorKind]]`	base or derived	Whether or not the function is a derived class constructor.
`[[Realm]]`	a Realm Record	The realm in which the function was created and which provides any intrinsic objects that are accessed when evaluating the function.
`[[ScriptOrModule]]`	a Script Record or a Module Record	The script or module in which the function was created.
`[[ThisMode]]`	lexical, strict, or global	Defines how `this` references are interpreted within the formal parameters and code body of the function. lexical means that `this` refers to the this value of a lexically enclosing function. strict means that the this value is used exactly as provided by an invocation of the function. global means that a this value of undefined or null is interpreted as a reference to the global object, and any other this value is first passed to ToObject.
`[[Strict]]`	a Boolean	true if this is a strict function, false if this is a non-strict function.
`[[HomeObject]]`	an Object	If the function uses `super`, this is the object whose `[[GetPrototypeOf]]` provides the object where `super` property lookups begin.
`[[SourceText]]`	a sequence of Unicode code points	The source text that defines the function.
`[[Fields]]`	a List of ClassFieldDefinition Records	If the function is a class, this is a list of Records representing the non-static fields and corresponding initializers of the class.
`[[PrivateMethods]]`	a List of PrivateElements	If the function is a class, this is a list representing the non-static private methods and accessors of the class.
`[[ClassFieldInitializerName]]`	a String, a Symbol, a Private Name, or empty	If the function is created as the initializer of a class field, the name to use for NamedEvaluation of the field; empty otherwise.
`[[IsClassConstructor]]`	a Boolean	Indicates whether the function is a class constructor. (If true, invoking the function's `[[Call]]` will immediately throw a TypeError exception.)

All ECMAScript function objects have the [[Call]] internal method defined here. ECMAScript functions that are also constructors in addition have the [[Construct]] internal method.

10.2.1 `[[Call]]` ( `thisArgument`, `argumentsList` )

The [[Call]] internal method of an ECMAScript function object F takes arguments thisArgument (an ECMAScript language value) and argumentsList (a List of ECMAScript language values) and returns either a normal completion containing an ECMAScript language value or a throw completion. It performs the following steps when called:

Let callerContext be the running execution context.
Let calleeContext be PrepareForOrdinaryCall(F, undefined).
Assert: calleeContext is now the running execution context.
4. If F.[[IsClassConstructor]] is true, then
1. Let error be a newly created TypeError object.
2. NOTE: error is created in calleeContext with F's associated Realm Record.
3. Remove calleeContext from the execution context stack and restore callerContext as the running execution context.
4. Return ThrowCompletion(error).
Perform OrdinaryCallBindThis(F, calleeContext, thisArgument).
Let result be Completion(OrdinaryCallEvaluateBody(F, argumentsList)).
Remove calleeContext from the execution context stack and restore callerContext as the running execution context.
If result is a return completion, return result.[[Value]].
ReturnIfAbrupt(result).
Return undefined.

Note

When calleeContext is removed from the execution context stack in step 7 it must not be destroyed if it is suspended and retained for later resumption by an accessible Generator.

10.2.1.1 PrepareForOrdinaryCall ( `F`, `newTarget` )

The abstract operation PrepareForOrdinaryCall takes arguments F (an ECMAScript function object) and newTarget (an Object or undefined) and returns an execution context. It performs the following steps when called:

Let callerContext be the running execution context.
Let calleeContext be a new ECMAScript code execution context.
Set the Function of calleeContext to F.
Let calleeRealm be F.[[Realm]].
Set the Realm of calleeContext to calleeRealm.
Set the ScriptOrModule of calleeContext to F.[[ScriptOrModule]].
Let localEnv be NewFunctionEnvironment(F, newTarget).
Set the LexicalEnvironment of calleeContext to localEnv.
Set the VariableEnvironment of calleeContext to localEnv.
Set the PrivateEnvironment of calleeContext to F.[[PrivateEnvironment]].
If callerContext is not already suspended, suspend callerContext.
Push calleeContext onto the execution context stack; calleeContext is now the running execution context.
NOTE: Any exception objects produced after this point are associated with calleeRealm.
Return calleeContext.

10.2.1.2 OrdinaryCallBindThis ( `F`, `calleeContext`, `thisArgument` )

The abstract operation OrdinaryCallBindThis takes arguments F (an ECMAScript function object), calleeContext (an execution context), and thisArgument (an ECMAScript language value) and returns unused. It performs the following steps when called:

Let thisMode be F.[[ThisMode]].
If thisMode is lexical, return unused.
Let calleeRealm be F.[[Realm]].
Let localEnv be the LexicalEnvironment of calleeContext.
5. If thisMode is strict, then
1. Let thisValue be thisArgument.
6. Else,
1. a. If thisArgument is either undefined or null, then
  1. Let globalEnv be calleeRealm.[[GlobalEnv]].
  2. Assert: globalEnv is a Global Environment Record.
  3. Let thisValue be globalEnv.[[GlobalThisValue]].
2. b. Else,
  1. Let thisValue be ! ToObject(thisArgument).
  2. NOTE: ToObject produces wrapper objects using calleeRealm.
Assert: localEnv is a Function Environment Record.
Assert: The next step never returns an abrupt completion because localEnv.[[ThisBindingStatus]] is not initialized.
Perform ! localEnv.BindThisValue(thisValue).
Return unused.

10.2.1.3 Runtime Semantics: EvaluateBody

The syntax-directed operation EvaluateBody takes arguments functionObject (an ECMAScript function object) and argumentsList (a List of ECMAScript language values) and returns either a normal completion containing an ECMAScript language value or an abrupt completion. It is defined piecewise over the following productions:

FunctionBody

FunctionStatementList

Return ? EvaluateFunctionBody of FunctionBody with arguments functionObject and argumentsList.

ConciseBody

ExpressionBody

Return ? EvaluateConciseBody of ConciseBody with arguments functionObject and argumentsList.

GeneratorBody

FunctionBody

Return ? EvaluateGeneratorBody of GeneratorBody with arguments functionObject and argumentsList.

AsyncGeneratorBody

FunctionBody

Return ? EvaluateAsyncGeneratorBody of AsyncGeneratorBody with arguments functionObject and argumentsList.

AsyncFunctionBody

FunctionBody

Return ? EvaluateAsyncFunctionBody of AsyncFunctionBody with arguments functionObject and argumentsList.

AsyncConciseBody

ExpressionBody

Return ? EvaluateAsyncConciseBody of AsyncConciseBody with arguments functionObject and argumentsList.

Initializer

AssignmentExpression

Assert: argumentsList is empty.
Assert: functionObject.[[ClassFieldInitializerName]] is not empty.
3. If IsAnonymousFunctionDefinition(AssignmentExpression) is true, then
1. Let value be ? NamedEvaluation of Initializer with argument functionObject.[[ClassFieldInitializerName]].
4. Else,
1. Let rhs be ? Evaluation of AssignmentExpression.
2. Let value be ? GetValue(rhs).
Return Completion Record { [[Type]]: return, [[Value]]: value, [[Target]]: empty }.

Note

Even though field initializers constitute a function boundary, calling FunctionDeclarationInstantiation does not have any observable effect and so is omitted.

ClassStaticBlockBody

ClassStaticBlockStatementList

Assert: argumentsList is empty.
Return ? EvaluateClassStaticBlockBody of ClassStaticBlockBody with argument functionObject.

10.2.1.4 OrdinaryCallEvaluateBody ( `F`, `argumentsList` )

The abstract operation OrdinaryCallEvaluateBody takes arguments F (an ECMAScript function object) and argumentsList (a List of ECMAScript language values) and returns either a normal completion containing an ECMAScript language value or an abrupt completion. It performs the following steps when called:

Return ? EvaluateBody of F.[[ECMAScriptCode]] with arguments F and argumentsList.

10.2.2 `[[Construct]]` ( `argumentsList`, `newTarget` )

The [[Construct]] internal method of an ECMAScript function object F takes arguments argumentsList (a List of ECMAScript language values) and newTarget (a constructor) and returns either a normal completion containing an Object or a throw completion. It performs the following steps when called:

Let callerContext be the running execution context.
Let kind be F.[[ConstructorKind]].
3. If kind is base, then
1. Let thisArgument be ? OrdinaryCreateFromConstructor(newTarget, "%Object.prototype%").
Let calleeContext be PrepareForOrdinaryCall(F, newTarget).
Assert: calleeContext is now the running execution context.
6. If kind is base, then
1. Perform OrdinaryCallBindThis(F, calleeContext, thisArgument).
2. Let initializeResult be Completion(InitializeInstanceElements(thisArgument, F)).
3. c. If initializeResult is an abrupt completion, then
  1. Remove calleeContext from the execution context stack and restore callerContext as the running execution context.
  2. Return ? initializeResult.
Let constructorEnv be the LexicalEnvironment of calleeContext.
Let result be Completion(OrdinaryCallEvaluateBody(F, argumentsList)).
Remove calleeContext from the execution context stack and restore callerContext as the running execution context.
10. If result is a return completion, then
1. If result.[[Value]] is an Object, return result.[[Value]].
2. If kind is base, return thisArgument.
3. If result.[[Value]] is not undefined, throw a TypeError exception.
11. Else,
1. ReturnIfAbrupt(result).
Let thisBinding be ? constructorEnv.GetThisBinding().
Assert: thisBinding is an Object.
Return thisBinding.

10.2.3 OrdinaryFunctionCreate ( `functionPrototype`, `sourceText`, `ParameterList`, `Body`, `thisMode`, `env`, `privateEnv` )

The abstract operation OrdinaryFunctionCreate takes arguments functionPrototype (an Object), sourceText (a sequence of Unicode code points), ParameterList (a Parse Node), Body (a Parse Node), thisMode (lexical-this or non-lexical-this), env (an Environment Record), and privateEnv (a PrivateEnvironment Record or null) and returns an ECMAScript function object. It is used to specify the runtime creation of a new function with a default [[Call]] internal method and no [[Construct]] internal method (although one may be subsequently added by an operation such as MakeConstructor). sourceText is the source text of the syntactic definition of the function to be created. It performs the following steps when called:

Let internalSlotsList be the internal slots listed in Table 30.
Let F be OrdinaryObjectCreate(functionPrototype, internalSlotsList).
Set F.[[Call]] to the definition specified in 10.2.1.
Set F.[[SourceText]] to sourceText.
Set F.[[FormalParameters]] to ParameterList.
Set F.[[ECMAScriptCode]] to Body.
If the source text matched by Body is strict mode code, let Strict be true; else let Strict be false.
Set F.[[Strict]] to Strict.
If thisMode is lexical-this, set F.[[ThisMode]] to lexical.
Else if Strict is true, set F.[[ThisMode]] to strict.
Else, set F.[[ThisMode]] to global.
Set F.[[IsClassConstructor]] to false.
Set F.[[Environment]] to env.
Set F.[[PrivateEnvironment]] to privateEnv.
Set F.[[ScriptOrModule]] to GetActiveScriptOrModule().
Set F.[[Realm]] to the current Realm Record.
Set F.[[HomeObject]] to undefined.
Set F.[[Fields]] to a new empty List.
Set F.[[PrivateMethods]] to a new empty List.
Set F.[[ClassFieldInitializerName]] to empty.
Let len be the ExpectedArgumentCount of ParameterList.
Perform SetFunctionLength(F, len).
Return F.

10.2.4 AddRestrictedFunctionProperties ( `F`, `realm` )

The abstract operation AddRestrictedFunctionProperties takes arguments F (a function object) and realm (a Realm Record) and returns unused. It performs the following steps when called:

Assert: realm.[[Intrinsics]].[[%ThrowTypeError%]] exists and has been initialized.
Let thrower be realm.[[Intrinsics]].[[%ThrowTypeError%]].
Perform ! DefinePropertyOrThrow(F, "caller", PropertyDescriptor { [[Get]]: thrower, [[Set]]: thrower, [[Enumerable]]: false, [[Configurable]]: true }).
Perform ! DefinePropertyOrThrow(F, "arguments", PropertyDescriptor { [[Get]]: thrower, [[Set]]: thrower, [[Enumerable]]: false, [[Configurable]]: true }).
Return unused.

10.2.4.1 %ThrowTypeError% ( )

This function is the %ThrowTypeError% intrinsic object.

It is an anonymous built-in function object that is defined once for each realm.

It performs the following steps when called:

Throw a TypeError exception.

The value of the [[Extensible]] internal slot of this function is false.

The "length" property of this function has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

The "name" property of this function has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

10.2.5 MakeConstructor ( `F` [ , `writablePrototype` [ , `prototype` ] ] )

The abstract operation MakeConstructor takes argument F (an ECMAScript function object or a built-in function object) and optional arguments writablePrototype (a Boolean) and prototype (an Object) and returns unused. It converts F into a constructor. It performs the following steps when called:

1. If F is an ECMAScript function object, then
1. Assert: IsConstructor(F) is false.
2. Assert: F is an extensible object that does not have a "prototype" own property.
3. Set F.[[Construct]] to the definition specified in 10.2.2.
2. Else,
1. Set F.[[Construct]] to the definition specified in 10.3.2.
Set F.[[ConstructorKind]] to base.
If writablePrototype is not present, set writablePrototype to true.
5. If prototype is not present, then
1. Set prototype to OrdinaryObjectCreate(%Object.prototype%).
2. Perform ! DefinePropertyOrThrow(prototype, "constructor", PropertyDescriptor { [[Value]]: F, [[Writable]]: writablePrototype, [[Enumerable]]: false, [[Configurable]]: true }).
Perform ! DefinePropertyOrThrow(F, "prototype", PropertyDescriptor { [[Value]]: prototype, [[Writable]]: writablePrototype, [[Enumerable]]: false, [[Configurable]]: false }).
Return unused.

10.2.6 MakeClassConstructor ( `F` )

The abstract operation MakeClassConstructor takes argument F (an ECMAScript function object) and returns unused. It performs the following steps when called:

Assert: F.[[IsClassConstructor]] is false.
Set F.[[IsClassConstructor]] to true.
Return unused.

10.2.7 MakeMethod ( `F`, `homeObject` )

The abstract operation MakeMethod takes arguments F (an ECMAScript function object) and homeObject (an Object) and returns unused. It configures F as a method. It performs the following steps when called:

Set F.[[HomeObject]] to homeObject.
Return unused.

10.2.8 DefineMethodProperty ( `homeObject`, `key`, `closure`, `enumerable` )

The abstract operation DefineMethodProperty takes arguments homeObject (an Object), key (a property key or Private Name), closure (a function object), and enumerable (a Boolean) and returns either a normal completion containing either a PrivateElement or unused, or an abrupt completion. It performs the following steps when called:

Assert: homeObject is an ordinary, extensible object.
2. If key is a Private Name, then
1. Return PrivateElement { [[Key]]: key, [[Kind]]: method, [[Value]]: closure }.
3. Else,
1. Let desc be the PropertyDescriptor { [[Value]]: closure, [[Writable]]: true, [[Enumerable]]: enumerable, [[Configurable]]: true }.
2. Perform ? DefinePropertyOrThrow(homeObject, key, desc).
3. NOTE: DefinePropertyOrThrow only returns an abrupt completion when attempting to define a class static method whose key is "prototype".
4. Return unused.

10.2.9 SetFunctionName ( `F`, `name` [ , `prefix` ] )

The abstract operation SetFunctionName takes arguments F (a function object) and name (a property key or Private Name) and optional argument prefix (a String) and returns unused. It adds a "name" property to F. It performs the following steps when called:

Assert: F is an extensible object that does not have a "name" own property.
2. If name is a Symbol, then
1. Let description be name's [[Description]] value.
2. If description is undefined, set name to the empty String.
3. Else, set name to the string-concatenation of "[", description, and "]".
3. Else if name is a Private Name, then
1. Set name to name.[[Description]].
4. If F has an [[InitialName]] internal slot, then
1. Set F.[[InitialName]] to name.
5. If prefix is present, then
1. Set name to the string-concatenation of prefix, the code unit 0x0020 (SPACE), and name.
2. b. If F has an [[InitialName]] internal slot, then
  1. Optionally, set F.[[InitialName]] to name.
Perform ! DefinePropertyOrThrow(F, "name", PropertyDescriptor { [[Value]]: name, [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }).
Return unused.

10.2.10 SetFunctionLength ( `F`, `length` )

The abstract operation SetFunctionLength takes arguments F (a function object) and length (a non-negative integer or +∞) and returns unused. It adds a "length" property to F. It performs the following steps when called:

Assert: F is an extensible object that does not have a "length" own property.
Perform ! DefinePropertyOrThrow(F, "length", PropertyDescriptor { [[Value]]: 𝔽(length), [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }).
Return unused.

10.2.11 FunctionDeclarationInstantiation ( `func`, `argumentsList` )

The abstract operation FunctionDeclarationInstantiation takes arguments func (an ECMAScript function object) and argumentsList (a List of ECMAScript language values) and returns either a normal completion containing unused or an abrupt completion. func is the function object for which the execution context is being established.

Note 1

When an execution context is established for evaluating an ECMAScript function a new Function Environment Record is created and bindings for each formal parameter are instantiated in that Environment Record. Each declaration in the function body is also instantiated. If the function's formal parameters do not include any default value initializers then the body declarations are instantiated in the same Environment Record as the parameters. If default value parameter initializers exist, a second Environment Record is created for the body declarations. Formal parameters and functions are initialized as part of FunctionDeclarationInstantiation. All other bindings are initialized during evaluation of the function body.

It performs the following steps when called:

Let calleeContext be the running execution context.
Let code be func.[[ECMAScriptCode]].
Let strict be func.[[Strict]].
Let formals be func.[[FormalParameters]].
Let parameterNames be the BoundNames of formals.
If parameterNames has any duplicate entries, let hasDuplicates be true. Otherwise, let hasDuplicates be false.
Let simpleParameterList be IsSimpleParameterList of formals.
Let hasParameterExpressions be ContainsExpression of formals.
Let varNames be the VarDeclaredNames of code.
Let varDeclarations be the VarScopedDeclarations of code.
Let lexicalNames be the LexicallyDeclaredNames of code.
Let functionNames be a new empty List.
Let functionsToInitialize be a new empty List.
14. For each element d of varDeclarations, in reverse List order, do
1. a. If d is neither a VariableDeclaration nor a ForBinding nor a BindingIdentifier, then
  1. Assert: d is either a FunctionDeclaration, a GeneratorDeclaration, an AsyncFunctionDeclaration, or an AsyncGeneratorDeclaration.
  2. Let fn be the sole element of the BoundNames of d.
  3. iii. If functionNames does not contain fn, then
    1. Insert fn as the first element of functionNames.
    2. NOTE: If there are multiple function declarations for the same name, the last declaration is used.
    3. Insert d as the first element of functionsToInitialize.
Let argumentsObjectNeeded be true.
16. If func.[[ThisMode]] is lexical, then
1. NOTE: Arrow functions never have an arguments object.
2. Set argumentsObjectNeeded to false.
17. Else if parameterNames contains "arguments", then
1. Set argumentsObjectNeeded to false.
18. Else if hasParameterExpressions is false, then
1. a. If functionNames contains "arguments" or lexicalNames contains "arguments", then
  1. Set argumentsObjectNeeded to false.
19. If strict is true or hasParameterExpressions is false, then
1. NOTE: Only a single Environment Record is needed for the parameters, since calls to eval in strict mode code cannot create new bindings which are visible outside of the eval.
2. Let env be the LexicalEnvironment of calleeContext.
20. Else,
1. NOTE: A separate Environment Record is needed to ensure that bindings created by direct eval calls in the formal parameter list are outside the environment where parameters are declared.
2. Let calleeEnv be the LexicalEnvironment of calleeContext.
3. Let env be NewDeclarativeEnvironment(calleeEnv).
4. Assert: The VariableEnvironment of calleeContext is calleeEnv.
5. Set the LexicalEnvironment of calleeContext to env.
21. For each String paramName of parameterNames, do
1. Let alreadyDeclared be ! env.HasBinding(paramName).
2. NOTE: Early errors ensure that duplicate parameter names can only occur in non-strict functions that do not have parameter default values or rest parameters.
3. c. If alreadyDeclared is false, then
  1. Perform ! env.CreateMutableBinding(paramName, false).
  2. ii. If hasDuplicates is true, then
    1. Perform ! env.InitializeBinding(paramName, undefined).
22. If argumentsObjectNeeded is true, then
1. a. If strict is true or simpleParameterList is false, then
  1. Let ao be CreateUnmappedArgumentsObject(argumentsList).
2. b. Else,
  1. NOTE: A mapped argument object is only provided for non-strict functions that don't have a rest parameter, any parameter default value initializers, or any destructured parameters.
  2. Let ao be CreateMappedArgumentsObject(func, formals, argumentsList, env).
3. c. If strict is true, then
  1. Perform ! env.CreateImmutableBinding("arguments", false).
  2. NOTE: In strict mode code early errors prevent attempting to assign to this binding, so its mutability is not observable.
4. d. Else,
  1. Perform ! env.CreateMutableBinding("arguments", false).
5. Perform ! env.InitializeBinding("arguments", ao).
6. Let parameterBindings be the list-concatenation of parameterNames and « "arguments" ».
23. Else,
1. Let parameterBindings be parameterNames.
Let iteratorRecord be CreateListIteratorRecord(argumentsList).
25. If hasDuplicates is true, then
1. Perform ? IteratorBindingInitialization of formals with arguments iteratorRecord and undefined.
26. Else,
1. Perform ? IteratorBindingInitialization of formals with arguments iteratorRecord and env.
27. If hasParameterExpressions is false, then
1. NOTE: Only a single Environment Record is needed for the parameters and top-level vars.
2. Let instantiatedVarNames be a copy of the List parameterBindings.
3. c. For each element n of varNames, do
  1. i. If instantiatedVarNames does not contain n, then
    1. Append n to instantiatedVarNames.
    2. Perform ! env.CreateMutableBinding(n, false).
    3. Perform ! env.InitializeBinding(n, undefined).
4. Let varEnv be env.
28. Else,
1. NOTE: A separate Environment Record is needed to ensure that closures created by expressions in the formal parameter list do not have visibility of declarations in the function body.
2. Let varEnv be NewDeclarativeEnvironment(env).
3. Set the VariableEnvironment of calleeContext to varEnv.
4. Let instantiatedVarNames be a new empty List.
5. e. For each element n of varNames, do
  1. i. If instantiatedVarNames does not contain n, then
    1. Append n to instantiatedVarNames.
    2. Perform ! varEnv.CreateMutableBinding(n, false).
    3. 3. If parameterBindings does not contain n, or if functionNames contains n, then
      1. Let initialValue be undefined.
    4. 4. Else,
      1. Let initialValue be ! env.GetBindingValue(n, false).
    5. Perform ! varEnv.InitializeBinding(n, initialValue).
    6. NOTE: A var with the same name as a formal parameter initially has the same value as the corresponding initialized parameter.
NOTE: Annex B.3.2.1 adds additional steps at this point.
30. If strict is false, then
1. Let lexEnv be NewDeclarativeEnvironment(varEnv).
2. NOTE: Non-strict functions use a separate Environment Record for top-level lexical declarations so that a direct eval can determine whether any var scoped declarations introduced by the eval code conflict with pre-existing top-level lexically scoped declarations. This is not needed for strict functions because a strict direct eval always places all declarations into a new Environment Record.
31. Else,
1. Let lexEnv be varEnv.
Set the LexicalEnvironment of calleeContext to lexEnv.
Let lexDeclarations be the LexicallyScopedDeclarations of code.
34. For each element d of lexDeclarations, do
1. NOTE: A lexically declared name cannot be the same as a function/generator declaration, formal parameter, or a var name. Lexically declared names are only instantiated here but not initialized.
2. b. For each element dn of the BoundNames of d, do
  1. i. If IsConstantDeclaration of d is true, then
    1. Perform ! lexEnv.CreateImmutableBinding(dn, true).
  2. ii. Else,
    1. Perform ! lexEnv.CreateMutableBinding(dn, false).
Let privateEnv be the PrivateEnvironment of calleeContext.
36. For each Parse Node f of functionsToInitialize, do
1. Let fn be the sole element of the BoundNames of f.
2. Let fo be InstantiateFunctionObject of f with arguments lexEnv and privateEnv.
3. Perform ! varEnv.SetMutableBinding(fn, fo, false).
Return unused.

Note 2

B.3.2 provides an extension to the above algorithm that is necessary for backwards compatibility with web browser implementations of ECMAScript that predate ECMAScript 2015.

10.3 Built-in Function Objects

A built-in function object is an ordinary object; it must satisfy the requirements for ordinary objects set out in 10.1.

In addition to the internal slots required of every ordinary object (see 10.1), a built-in function object must also have the following internal slots:

[[Realm]], a Realm Record that represents the realm in which the function was created.
[[InitialName]], a String that is the initial name of the function. It is used by 20.2.3.5.

The initial value of a built-in function object's [[Prototype]] internal slot is %Function.prototype%, unless otherwise specified.

A built-in function object must have a [[Call]] internal method that conforms to the definition in 10.3.1.

A built-in function object has a [[Construct]] internal method if and only if it is described as a “constructor”, or some algorithm in this specification explicitly sets its [[Construct]] internal method. Such a [[Construct]] internal method must conform to the definition in 10.3.2.

An implementation may provide additional built-in function objects that are not defined in this specification.

10.3.1 `[[Call]]` ( `thisArgument`, `argumentsList` )

The [[Call]] internal method of a built-in function object F takes arguments thisArgument (an ECMAScript language value) and argumentsList (a List of ECMAScript language values) and returns either a normal completion containing an ECMAScript language value or a throw completion. It performs the following steps when called:

Return ? BuiltinCallOrConstruct(F, thisArgument, argumentsList, undefined).

10.3.2 `[[Construct]]` ( `argumentsList`, `newTarget` )

The [[Construct]] internal method of a built-in function object F (when the method is present) takes arguments argumentsList (a List of ECMAScript language values) and newTarget (a constructor) and returns either a normal completion containing an Object or a throw completion. It performs the following steps when called:

Return ? BuiltinCallOrConstruct(F, uninitialized, argumentsList, newTarget).

10.3.3 BuiltinCallOrConstruct ( `F`, `thisArgument`, `argumentsList`, `newTarget` )

The abstract operation BuiltinCallOrConstruct takes arguments F (a built-in function object), thisArgument (an ECMAScript language value or uninitialized), argumentsList (a List of ECMAScript language values), and newTarget (a constructor or undefined) and returns either a normal completion containing an ECMAScript language value or a throw completion. It performs the following steps when called:

Let callerContext be the running execution context.
If callerContext is not already suspended, suspend callerContext.
Let calleeContext be a new execution context.
Set the Function of calleeContext to F.
Let calleeRealm be F.[[Realm]].
Set the Realm of calleeContext to calleeRealm.
Set the ScriptOrModule of calleeContext to null.
Perform any necessary implementation-defined initialization of calleeContext.
Push calleeContext onto the execution context stack; calleeContext is now the running execution context.
Let result be the Completion Record that is the result of evaluating F in a manner that conforms to the specification of F. If thisArgument is uninitialized, the this value is uninitialized; otherwise, thisArgument provides the this value. argumentsList provides the named parameters. newTarget provides the NewTarget value.
NOTE: If F is defined in this document, “the specification of F” is the behaviour specified for it via algorithm steps or other means.
Remove calleeContext from the execution context stack and restore callerContext as the running execution context.
Return ? result.

Note

When calleeContext is removed from the execution context stack it must not be destroyed if it has been suspended and retained by an accessible Generator for later resumption.

10.3.4 CreateBuiltinFunction ( `behaviour`, `length`, `name`, `additionalInternalSlotsList` [ , `realm` [ , `prototype` [ , `prefix` ] ] ] )

The abstract operation CreateBuiltinFunction takes arguments behaviour (an Abstract Closure, a set of algorithm steps, or some other definition of a function's behaviour provided in this specification), length (a non-negative integer or +∞), name (a property key or a Private Name), and additionalInternalSlotsList (a List of names of internal slots) and optional arguments realm (a Realm Record), prototype (an Object or null), and prefix (a String) and returns a function object. additionalInternalSlotsList contains the names of additional internal slots that must be defined as part of the object. This operation creates a built-in function object. It performs the following steps when called:

If realm is not present, set realm to the current Realm Record.
If prototype is not present, set prototype to realm.[[Intrinsics]].[[%Function.prototype%]].
Let internalSlotsList be a List containing the names of all the internal slots that 10.3 requires for the built-in function object that is about to be created.
Append to internalSlotsList the elements of additionalInternalSlotsList.
Let func be a new built-in function object that, when called, performs the action described by behaviour using the provided arguments as the values of the corresponding parameters specified by behaviour. The new function object has internal slots whose names are the elements of internalSlotsList, and an [[InitialName]] internal slot.
Set func.[[Prototype]] to prototype.
Set func.[[Extensible]] to true.
Set func.[[Realm]] to realm.
Set func.[[InitialName]] to null.
Perform SetFunctionLength(func, length).
11. If prefix is not present, then
1. Perform SetFunctionName(func, name).
12. Else,
1. Perform SetFunctionName(func, name, prefix).
Return func.

Each built-in function defined in this specification is created by calling the CreateBuiltinFunction abstract operation.

10.4 Built-in Exotic Object Internal Methods and Slots

This specification defines several kinds of built-in exotic objects. These objects generally behave similar to ordinary objects except for a few specific situations. The following exotic objects use the ordinary object internal methods except where it is explicitly specified otherwise below:

10.4.1 Bound Function Exotic Objects

A bound function exotic object is an exotic object that wraps another function object. A bound function exotic object is callable (it has a [[Call]] internal method and may have a [[Construct]] internal method). Calling a bound function exotic object generally results in a call of its wrapped function.

An object is a bound function exotic object if its [[Call]] and (if applicable) [[Construct]] internal methods use the following implementations, and its other essential internal methods use the definitions found in 10.1. These methods are installed in BoundFunctionCreate.

Bound function exotic objects do not have the internal slots of ECMAScript function objects listed in Table 30. Instead they have the internal slots listed in Table 31, in addition to [[Prototype]] and [[Extensible]].

Table 31: Internal Slots of Bound Function Exotic Objects

Internal Slot	Type	Description
`[[BoundTargetFunction]]`	a callable Object	The wrapped function object.
`[[BoundThis]]`	an ECMAScript language value	The value that is always passed as the this value when calling the wrapped function.
`[[BoundArguments]]`	a List of ECMAScript language values	A list of values whose elements are used as the first arguments to any call to the wrapped function.

10.4.1.1 `[[Call]]` ( `thisArgument`, `argumentsList` )

The [[Call]] internal method of a bound function exotic object F takes arguments thisArgument (an ECMAScript language value) and argumentsList (a List of ECMAScript language values) and returns either a normal completion containing an ECMAScript language value or a throw completion. It performs the following steps when called:

Let target be F.[[BoundTargetFunction]].
Let boundThis be F.[[BoundThis]].
Let boundArgs be F.[[BoundArguments]].
Let args be the list-concatenation of boundArgs and argumentsList.
Return ? Call(target, boundThis, args).

10.4.1.2 `[[Construct]]` ( `argumentsList`, `newTarget` )

The [[Construct]] internal method of a bound function exotic object F takes arguments argumentsList (a List of ECMAScript language values) and newTarget (a constructor) and returns either a normal completion containing an Object or a throw completion. It performs the following steps when called:

Let target be F.[[BoundTargetFunction]].
Assert: IsConstructor(target) is true.
Let boundArgs be F.[[BoundArguments]].
Let args be the list-concatenation of boundArgs and argumentsList.
If SameValue(F, newTarget) is true, set newTarget to target.
Return ? Construct(target, args, newTarget).

10.4.1.3 BoundFunctionCreate ( `targetFunction`, `boundThis`, `boundArgs` )

The abstract operation BoundFunctionCreate takes arguments targetFunction (a function object), boundThis (an ECMAScript language value), and boundArgs (a List of ECMAScript language values) and returns either a normal completion containing a function object or a throw completion. It is used to specify the creation of new bound function exotic objects. It performs the following steps when called:

Let proto be ? targetFunction.[[GetPrototypeOf]]().
Let internalSlotsList be the list-concatenation of « [[Prototype]], [[Extensible]] » and the internal slots listed in Table 31.
Let obj be MakeBasicObject(internalSlotsList).
Set obj.[[Prototype]] to proto.
Set obj.[[Call]] as described in 10.4.1.1.
6. If IsConstructor(targetFunction) is true, then
1. Set obj.[[Construct]] as described in 10.4.1.2.
Set obj.[[BoundTargetFunction]] to targetFunction.
Set obj.[[BoundThis]] to boundThis.
Set obj.[[BoundArguments]] to boundArgs.
Return obj.

10.4.2 Array Exotic Objects

An Array is an exotic object that gives special treatment to array index property keys (see 6.1.7). A property whose property name is an array index is also called an element. Every Array has a non-configurable "length" property whose value is always a non-negative integral Number whose mathematical value is strictly less than 2³². The value of the "length" property is numerically greater than the name of every own property whose name is an array index; whenever an own property of an Array is created or changed, other properties are adjusted as necessary to maintain this invariant. Specifically, whenever an own property is added whose name is an array index, the value of the "length" property is changed, if necessary, to be one more than the numeric value of that array index; and whenever the value of the "length" property is changed, every own property whose name is an array index whose value is not smaller than the new length is deleted. This constraint applies only to own properties of an Array and is unaffected by "length" or array index properties that may be inherited from its prototypes.

An object is an Array exotic object (or simply, an Array) if its [[DefineOwnProperty]] internal method uses the following implementation, and its other essential internal methods use the definitions found in 10.1. These methods are installed in ArrayCreate.

10.4.2.1 `[[DefineOwnProperty]]` ( `P`, `Desc` )

The [[DefineOwnProperty]] internal method of an Array exotic object A takes arguments P (a property key) and Desc (a Property Descriptor) and returns either a normal completion containing a Boolean or a throw completion. It performs the following steps when called:

1. If P is "length", then
1. Return ? ArraySetLength(A, Desc).
2. Else if P is an array index, then
1. Let lengthDesc be OrdinaryGetOwnProperty(A, "length").
2. Assert: IsDataDescriptor(lengthDesc) is true.
3. Assert: lengthDesc.[[Configurable]] is false.
4. Let length be lengthDesc.[[Value]].
5. Assert: length is a non-negative integral Number.
6. Let index be ! ToUint32(P).
7. If index ≥ length and lengthDesc.[[Writable]] is false, return false.
8. Let succeeded be ! OrdinaryDefineOwnProperty(A, P, Desc).
9. If succeeded is false, return false.
10. j. If index ≥ length, then
  1. Set lengthDesc.[[Value]] to index + 1_𝔽.
  2. Set succeeded to ! OrdinaryDefineOwnProperty(A, "length", lengthDesc).
  3. Assert: succeeded is true.
11. Return true.
Return ? OrdinaryDefineOwnProperty(A, P, Desc).

10.4.2.2 ArrayCreate ( `length` [ , `proto` ] )

The abstract operation ArrayCreate takes argument length (a non-negative integer) and optional argument proto (an Object) and returns either a normal completion containing an Array exotic object or a throw completion. It is used to specify the creation of new Arrays. It performs the following steps when called:

If length > 2³² - 1, throw a RangeError exception.
If proto is not present, set proto to %Array.prototype%.
Let A be MakeBasicObject(« [[Prototype]], [[Extensible]] »).
Set A.[[Prototype]] to proto.
Set A.[[DefineOwnProperty]] as specified in 10.4.2.1.
Perform ! OrdinaryDefineOwnProperty(A, "length", PropertyDescriptor { [[Value]]: 𝔽(length), [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false }).
Return A.

10.4.2.3 ArraySpeciesCreate ( `originalArray`, `length` )

The abstract operation ArraySpeciesCreate takes arguments originalArray (an Object) and length (a non-negative integer) and returns either a normal completion containing an Object or a throw completion. It is used to specify the creation of a new Array or similar object using a constructor function that is derived from originalArray. It does not enforce that the constructor function returns an Array. It performs the following steps when called:

Let isArray be ? IsArray(originalArray).
If isArray is false, return ? ArrayCreate(length).
Let C be ? Get(originalArray, "constructor").
4. If IsConstructor(C) is true, then
1. Let thisRealm be the current Realm Record.
2. Let realmC be ? GetFunctionRealm(C).
3. c. If thisRealm and realmC are not the same Realm Record, then
  1. If SameValue(C, realmC.[[Intrinsics]].[[%Array%]]) is true, set C to undefined.
5. If C is an Object, then
1. Set C to ? Get(C, @@species).
2. If C is null, set C to undefined.
If C is undefined, return ? ArrayCreate(length).
If IsConstructor(C) is false, throw a TypeError exception.
Return ? Construct(C, « 𝔽(length) »).

Note

If originalArray was created using the standard built-in Array constructor for a realm that is not the realm of the running execution context, then a new Array is created using the realm of the running execution context. This maintains compatibility with Web browsers that have historically had that behaviour for the Array.prototype methods that now are defined using ArraySpeciesCreate.

10.4.2.4 ArraySetLength ( `A`, `Desc` )

The abstract operation ArraySetLength takes arguments A (an Array) and Desc (a Property Descriptor) and returns either a normal completion containing a Boolean or a throw completion. It performs the following steps when called:

1. If Desc does not have a [[Value]] field, then
1. Return ! OrdinaryDefineOwnProperty(A, "length", Desc).
Let newLenDesc be a copy of Desc.
Let newLen be ? ToUint32(Desc.[[Value]]).
Let numberLen be ? ToNumber(Desc.[[Value]]).
If SameValueZero(newLen, numberLen) is false, throw a RangeError exception.
Set newLenDesc.[[Value]] to newLen.
Let oldLenDesc be OrdinaryGetOwnProperty(A, "length").
Assert: IsDataDescriptor(oldLenDesc) is true.
Assert: oldLenDesc.[[Configurable]] is false.
Let oldLen be oldLenDesc.[[Value]].
11. If newLen ≥ oldLen, then
1. Return ! OrdinaryDefineOwnProperty(A, "length", newLenDesc).
If oldLenDesc.[[Writable]] is false, return false.
13. If newLenDesc does not have a [[Writable]] field or newLenDesc.[[Writable]] is true, then
1. Let newWritable be true.
14. Else,
1. NOTE: Setting the [[Writable]] attribute to false is deferred in case any elements cannot be deleted.
2. Let newWritable be false.
3. Set newLenDesc.[[Writable]] to true.
Let succeeded be ! OrdinaryDefineOwnProperty(A, "length", newLenDesc).
If succeeded is false, return false.
17. For each own property key P of A such that P is an array index and ! ToUint32(P) ≥ newLen, in descending numeric index order, do
1. Let deleteSucceeded be ! A.[[Delete]](P).
2. b. If deleteSucceeded is false, then
  1. Set newLenDesc.[[Value]] to ! ToUint32(P) + 1_𝔽.
  2. If newWritable is false, set newLenDesc.[[Writable]] to false.
  3. Perform ! OrdinaryDefineOwnProperty(A, "length", newLenDesc).
  4. Return false.
18. If newWritable is false, then
1. Set succeeded to ! OrdinaryDefineOwnProperty(A, "length", PropertyDescriptor { [[Writable]]: false }).
2. Assert: succeeded is true.
Return true.

Note

In steps 3 and 4, if Desc.[[Value]] is an object then its valueOf method is called twice. This is legacy behaviour that was specified with this effect starting with the 2^nd Edition of this specification.

10.4.3 String Exotic Objects

A String object is an exotic object that encapsulates a String value and exposes virtual integer-indexed data properties corresponding to the individual code unit elements of the String value. String exotic objects always have a data property named "length" whose value is the length of the encapsulated String value. Both the code unit data properties and the "length" property are non-writable and non-configurable.

An object is a String exotic object (or simply, a String object) if its [[GetOwnProperty]], [[DefineOwnProperty]], and [[OwnPropertyKeys]] internal methods use the following implementations, and its other essential internal methods use the definitions found in 10.1. These methods are installed in StringCreate.

String exotic objects have the same internal slots as ordinary objects. They also have a [[StringData]] internal slot.

10.4.3.1 `[[GetOwnProperty]]` ( `P` )

The [[GetOwnProperty]] internal method of a String exotic object S takes argument P (a property key) and returns a normal completion containing either a Property Descriptor or undefined. It performs the following steps when called:

Let desc be OrdinaryGetOwnProperty(S, P).
If desc is not undefined, return desc.
Return StringGetOwnProperty(S, P).

10.4.3.2 `[[DefineOwnProperty]]` ( `P`, `Desc` )

The [[DefineOwnProperty]] internal method of a String exotic object S takes arguments P (a property key) and Desc (a Property Descriptor) and returns a normal completion containing a Boolean. It performs the following steps when called:

Let stringDesc be StringGetOwnProperty(S, P).
2. If stringDesc is not undefined, then
1. Let extensible be S.[[Extensible]].
2. Return IsCompatiblePropertyDescriptor(extensible, Desc, stringDesc).
Return ! OrdinaryDefineOwnProperty(S, P, Desc).

10.4.3.3 `[[OwnPropertyKeys]]` ( )

The [[OwnPropertyKeys]] internal method of a String exotic object O takes no arguments and returns a normal completion containing a List of property keys. It performs the following steps when called:

Let keys be a new empty List.
Let str be O.[[StringData]].
Assert: str is a String.
Let len be the length of str.
5. For each integer i such that 0 ≤ i < len, in ascending order, do
1. Append ! ToString(𝔽(i)) to keys.
6. For each own property key P of O such that P is an array index and ! ToIntegerOrInfinity(P) ≥ len, in ascending numeric index order, do
1. Append P to keys.
7. For each own property key P of O such that P is a String and P is not an array index, in ascending chronological order of property creation, do
1. Append P to keys.
8. For each own property key P of O such that P is a Symbol, in ascending chronological order of property creation, do
1. Append P to keys.
Return keys.

10.4.3.4 StringCreate ( `value`, `prototype` )

The abstract operation StringCreate takes arguments value (a String) and prototype (an Object) and returns a String exotic object. It is used to specify the creation of new String exotic objects. It performs the following steps when called:

Let S be MakeBasicObject(« [[Prototype]], [[Extensible]], [[StringData]] »).
Set S.[[Prototype]] to prototype.
Set S.[[StringData]] to value.
Set S.[[GetOwnProperty]] as specified in 10.4.3.1.
Set S.[[DefineOwnProperty]] as specified in 10.4.3.2.
Set S.[[OwnPropertyKeys]] as specified in 10.4.3.3.
Let length be the length of value.
Perform ! DefinePropertyOrThrow(S, "length", PropertyDescriptor { [[Value]]: 𝔽(length), [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }).
Return S.

10.4.3.5 StringGetOwnProperty ( `S`, `P` )

The abstract operation StringGetOwnProperty takes arguments S (an Object that has a [[StringData]] internal slot) and P (a property key) and returns a Property Descriptor or undefined. It performs the following steps when called:

If P is not a String, return undefined.
Let index be CanonicalNumericIndexString(P).
If index is undefined, return undefined.
If IsIntegralNumber(index) is false, return undefined.
If index is -0_𝔽, return undefined.
Let str be S.[[StringData]].
Assert: str is a String.
Let len be the length of str.
If ℝ(index) < 0 or len ≤ ℝ(index), return undefined.
Let resultStr be the substring of str from ℝ(index) to ℝ(index) + 1.
Return the PropertyDescriptor { [[Value]]: resultStr, [[Writable]]: false, [[Enumerable]]: true, [[Configurable]]: false }.

10.4.4 Arguments Exotic Objects

Most ECMAScript functions make an arguments object available to their code. Depending upon the characteristics of the function definition, its arguments object is either an ordinary object or an arguments exotic object. An arguments exotic object is an exotic object whose array index properties map to the formal parameters bindings of an invocation of its associated ECMAScript function.

An object is an arguments exotic object if its internal methods use the following implementations, with the ones not specified here using those found in 10.1. These methods are installed in CreateMappedArgumentsObject.

Note 1

While CreateUnmappedArgumentsObject is grouped into this clause, it creates an ordinary object, not an arguments exotic object.

Arguments exotic objects have the same internal slots as ordinary objects. They also have a [[ParameterMap]] internal slot. Ordinary arguments objects also have a [[ParameterMap]] internal slot whose value is always undefined. For ordinary argument objects the [[ParameterMap]] internal slot is only used by Object.prototype.toString (20.1.3.6) to identify them as such.

Note 2

The integer-indexed data properties of an arguments exotic object whose numeric name values are less than the number of formal parameters of the corresponding function object initially share their values with the corresponding argument bindings in the function's execution context. This means that changing the property changes the corresponding value of the argument binding and vice-versa. This correspondence is broken if such a property is deleted and then redefined or if the property is changed into an accessor property. If the arguments object is an ordinary object, the values of its properties are simply a copy of the arguments passed to the function and there is no dynamic linkage between the property values and the formal parameter values.

Note 3

The ParameterMap object and its property values are used as a device for specifying the arguments object correspondence to argument bindings. The ParameterMap object and the objects that are the values of its properties are not directly observable from ECMAScript code. An ECMAScript implementation does not need to actually create or use such objects to implement the specified semantics.

Note 4

Ordinary arguments objects define a non-configurable accessor property named "callee" which throws a TypeError exception on access. The "callee" property has a more specific meaning for arguments exotic objects, which are created only for some class of non-strict functions. The definition of this property in the ordinary variant exists to ensure that it is not defined in any other manner by conforming ECMAScript implementations.

Note 5

ECMAScript implementations of arguments exotic objects have historically contained an accessor property named "caller". Prior to ECMAScript 2017, this specification included the definition of a throwing "caller" property on ordinary arguments objects. Since implementations do not contain this extension any longer, ECMAScript 2017 dropped the requirement for a throwing "caller" accessor.

10.4.4.1 `[[GetOwnProperty]]` ( `P` )

The [[GetOwnProperty]] internal method of an arguments exotic object args takes argument P (a property key) and returns a normal completion containing either a Property Descriptor or undefined. It performs the following steps when called:

Let desc be OrdinaryGetOwnProperty(args, P).
If desc is undefined, return undefined.
Let map be args.[[ParameterMap]].
Let isMapped be ! HasOwnProperty(map, P).
5. If isMapped is true, then
1. Set desc.[[Value]] to ! Get(map, P).
Return desc.

10.4.4.2 `[[DefineOwnProperty]]` ( `P`, `Desc` )

The [[DefineOwnProperty]] internal method of an arguments exotic object args takes arguments P (a property key) and Desc (a Property Descriptor) and returns a normal completion containing a Boolean. It performs the following steps when called:

Let map be args.[[ParameterMap]].
Let isMapped be ! HasOwnProperty(map, P).
Let newArgDesc be Desc.
4. If isMapped is true and IsDataDescriptor(Desc) is true, then
1. a. If Desc does not have a [[Value]] field, Desc has a [[Writable]] field, and Desc.[[Writable]] is false, then
  1. Set newArgDesc to a copy of Desc.
  2. Set newArgDesc.[[Value]] to ! Get(map, P).
Let allowed be ! OrdinaryDefineOwnProperty(args, P, newArgDesc).
If allowed is false, return false.
7. If isMapped is true, then
1. a. If IsAccessorDescriptor(Desc) is true, then
  1. Perform ! map.[[Delete]](P).
2. b. Else,
  1. i. If Desc has a [[Value]] field, then
    1. Assert: The following Set will succeed, since formal parameters mapped by arguments objects are always writable.
    2. Perform ! Set(map, P, Desc.[[Value]], false).
  2. ii. If Desc has a [[Writable]] field and Desc.[[Writable]] is false, then
    1. Perform ! map.[[Delete]](P).
Return true.

10.4.4.3 `[[Get]]` ( `P`, `Receiver` )

The [[Get]] internal method of an arguments exotic object args takes arguments P (a property key) and Receiver (an ECMAScript language value) and returns either a normal completion containing an ECMAScript language value or a throw completion. It performs the following steps when called:

Let map be args.[[ParameterMap]].
Let isMapped be ! HasOwnProperty(map, P).
3. If isMapped is false, then
1. Return ? OrdinaryGet(args, P, Receiver).
4. Else,
1. Assert: map contains a formal parameter mapping for P.
2. Return ! Get(map, P).

10.4.4.4 `[[Set]]` ( `P`, `V`, `Receiver` )

The [[Set]] internal method of an arguments exotic object args takes arguments P (a property key), V (an ECMAScript language value), and Receiver (an ECMAScript language value) and returns either a normal completion containing a Boolean or a throw completion. It performs the following steps when called:

1. If SameValue(args, Receiver) is false, then
1. Let isMapped be false.
2. Else,
1. Let map be args.[[ParameterMap]].
2. Let isMapped be ! HasOwnProperty(map, P).
3. If isMapped is true, then
1. Assert: The following Set will succeed, since formal parameters mapped by arguments objects are always writable.
2. Perform ! Set(map, P, V, false).
Return ? OrdinarySet(args, P, V, Receiver).

10.4.4.5 `[[Delete]]` ( `P` )

The [[Delete]] internal method of an arguments exotic object args takes argument P (a property key) and returns either a normal completion containing a Boolean or a throw completion. It performs the following steps when called:

Let map be args.[[ParameterMap]].
Let isMapped be ! HasOwnProperty(map, P).
Let result be ? OrdinaryDelete(args, P).
4. If result is true and isMapped is true, then
1. Perform ! map.[[Delete]](P).
Return result.

10.4.4.6 CreateUnmappedArgumentsObject ( `argumentsList` )

The abstract operation CreateUnmappedArgumentsObject takes argument argumentsList (a List of ECMAScript language values) and returns an ordinary object. It performs the following steps when called:

Let len be the number of elements in argumentsList.
Let obj be OrdinaryObjectCreate(%Object.prototype%, « [[ParameterMap]] »).
Set obj.[[ParameterMap]] to undefined.
Perform ! DefinePropertyOrThrow(obj, "length", PropertyDescriptor { [[Value]]: 𝔽(len), [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: true }).
Let index be 0.
6. Repeat, while index < len,
1. Let val be argumentsList[index].
2. Perform ! CreateDataPropertyOrThrow(obj, ! ToString(𝔽(index)), val).
3. Set index to index + 1.
Perform ! DefinePropertyOrThrow(obj, @@iterator, PropertyDescriptor { [[Value]]: %Array.prototype.values%, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: true }).
Perform ! DefinePropertyOrThrow(obj, "callee", PropertyDescriptor { [[Get]]: %ThrowTypeError%, [[Set]]: %ThrowTypeError%, [[Enumerable]]: false, [[Configurable]]: false }).
Return obj.

10.4.4.7 CreateMappedArgumentsObject ( `func`, `formals`, `argumentsList`, `env` )

The abstract operation CreateMappedArgumentsObject takes arguments func (an Object), formals (a Parse Node), argumentsList (a List of ECMAScript language values), and env (an Environment Record) and returns an arguments exotic object. It performs the following steps when called:

Assert: formals does not contain a rest parameter, any binding patterns, or any initializers. It may contain duplicate identifiers.
Let len be the number of elements in argumentsList.
Let obj be MakeBasicObject(« [[Prototype]], [[Extensible]], [[ParameterMap]] »).
Set obj.[[GetOwnProperty]] as specified in 10.4.4.1.
Set obj.[[DefineOwnProperty]] as specified in 10.4.4.2.
Set obj.[[Get]] as specified in 10.4.4.3.
Set obj.[[Set]] as specified in 10.4.4.4.
Set obj.[[Delete]] as specified in 10.4.4.5.
Set obj.[[Prototype]] to %Object.prototype%.
Let map be OrdinaryObjectCreate(null).
Set obj.[[ParameterMap]] to map.
Let parameterNames be the BoundNames of formals.
Let numberOfParameters be the number of elements in parameterNames.
Let index be 0.
15. Repeat, while index < len,
1. Let val be argumentsList[index].
2. Perform ! CreateDataPropertyOrThrow(obj, ! ToString(𝔽(index)), val).
3. Set index to index + 1.
Perform ! DefinePropertyOrThrow(obj, "length", PropertyDescriptor { [[Value]]: 𝔽(len), [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: true }).
Let mappedNames be a new empty List.
Set index to numberOfParameters - 1.
19. Repeat, while index ≥ 0,
1. Let name be parameterNames[index].
2. b. If mappedNames does not contain name, then
  1. Append name to mappedNames.
  2. ii. If index < len, then
    1. Let g be MakeArgGetter(name, env).
    2. Let p be MakeArgSetter(name, env).
    3. Perform ! map.[[DefineOwnProperty]](! ToString(𝔽(index)), PropertyDescriptor { [[Set]]: p, [[Get]]: g, [[Enumerable]]: false, [[Configurable]]: true }).
3. Set index to index - 1.
Perform ! DefinePropertyOrThrow(obj, @@iterator, PropertyDescriptor { [[Value]]: %Array.prototype.values%, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: true }).
Perform ! DefinePropertyOrThrow(obj, "callee", PropertyDescriptor { [[Value]]: func, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: true }).
Return obj.

10.4.4.7.1 MakeArgGetter ( `name`, `env` )

The abstract operation MakeArgGetter takes arguments name (a String) and env (an Environment Record) and returns a function object. It creates a built-in function object that when executed returns the value bound for name in env. It performs the following steps when called:

1. Let getterClosure be a new Abstract Closure with no parameters that captures name and env and performs the following steps when called:
1. Return env.GetBindingValue(name, false).
Let getter be CreateBuiltinFunction(getterClosure, 0, "", « »).
NOTE: getter is never directly accessible to ECMAScript code.
Return getter.

10.4.4.7.2 MakeArgSetter ( `name`, `env` )

The abstract operation MakeArgSetter takes arguments name (a String) and env (an Environment Record) and returns a function object. It creates a built-in function object that when executed sets the value bound for name in env. It performs the following steps when called:

1. Let setterClosure be a new Abstract Closure with parameters (value) that captures name and env and performs the following steps when called:
1. Return ! env.SetMutableBinding(name, value, false).
Let setter be CreateBuiltinFunction(setterClosure, 1, "", « »).
NOTE: setter is never directly accessible to ECMAScript code.
Return setter.

10.4.5 TypedArray Exotic Objects

A TypedArray is an exotic object that performs special handling of integer index property keys.

TypedArrays have the same internal slots as ordinary objects and additionally [[ViewedArrayBuffer]], [[ArrayLength]], [[ByteOffset]], [[ContentType]], and [[TypedArrayName]] internal slots.

An object is a TypedArray if its [[GetOwnProperty]], [[HasProperty]], [[DefineOwnProperty]], [[Get]], [[Set]], [[Delete]], and [[OwnPropertyKeys]] internal methods use the definitions in this section, and its other essential internal methods use the definitions found in 10.1. These methods are installed by TypedArrayCreate.

10.4.5.1 `[[GetOwnProperty]]` ( `P` )

The [[GetOwnProperty]] internal method of a TypedArray O takes argument P (a property key) and returns a normal completion containing either a Property Descriptor or undefined. It performs the following steps when called:

1. If P is a String, then
1. Let numericIndex be CanonicalNumericIndexString(P).
2. b. If numericIndex is not undefined, then
  1. Let value be TypedArrayGetElement(O, numericIndex).
  2. If value is undefined, return undefined.
  3. Return the PropertyDescriptor { [[Value]]: value, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true }.
Return OrdinaryGetOwnProperty(O, P).

10.4.5.2 `[[HasProperty]]` ( `P` )

The [[HasProperty]] internal method of a TypedArray O takes argument P (a property key) and returns either a normal completion containing a Boolean or a throw completion. It performs the following steps when called:

1. If P is a String, then
1. Let numericIndex be CanonicalNumericIndexString(P).
2. If numericIndex is not undefined, return IsValidIntegerIndex(O, numericIndex).
Return ? OrdinaryHasProperty(O, P).

10.4.5.3 `[[DefineOwnProperty]]` ( `P`, `Desc` )

The [[DefineOwnProperty]] internal method of a TypedArray O takes arguments P (a property key) and Desc (a Property Descriptor) and returns either a normal completion containing a Boolean or a throw completion. It performs the following steps when called:

1. If P is a String, then
1. Let numericIndex be CanonicalNumericIndexString(P).
2. b. If numericIndex is not undefined, then
  1. If IsValidIntegerIndex(O, numericIndex) is false, return false.
  2. If Desc has a [[Configurable]] field and Desc.[[Configurable]] is false, return false.
  3. If Desc has an [[Enumerable]] field and Desc.[[Enumerable]] is false, return false.
  4. If IsAccessorDescriptor(Desc) is true, return false.
  5. If Desc has a [[Writable]] field and Desc.[[Writable]] is false, return false.
  6. If Desc has a [[Value]] field, perform ? TypedArraySetElement(O, numericIndex, Desc.[[Value]]).
  7. Return true.
Return ! OrdinaryDefineOwnProperty(O, P, Desc).

10.4.5.4 `[[Get]]` ( `P`, `Receiver` )

The [[Get]] internal method of a TypedArray O takes arguments P (a property key) and Receiver (an ECMAScript language value) and returns either a normal completion containing an ECMAScript language value or a throw completion. It performs the following steps when called:

1. If P is a String, then
1. Let numericIndex be CanonicalNumericIndexString(P).
2. b. If numericIndex is not undefined, then
  1. Return TypedArrayGetElement(O, numericIndex).
Return ? OrdinaryGet(O, P, Receiver).

10.4.5.5 `[[Set]]` ( `P`, `V`, `Receiver` )

The [[Set]] internal method of a TypedArray O takes arguments P (a property key), V (an ECMAScript language value), and Receiver (an ECMAScript language value) and returns either a normal completion containing a Boolean or a throw completion. It performs the following steps when called:

1. If P is a String, then
1. Let numericIndex be CanonicalNumericIndexString(P).
2. b. If numericIndex is not undefined, then
  1. i. If SameValue(O, Receiver) is true, then
    1. Perform ? TypedArraySetElement(O, numericIndex, V).
    2. Return true.
  2. If IsValidIntegerIndex(O, numericIndex) is false, return true.
Return ? OrdinarySet(O, P, V, Receiver).

10.4.5.6 `[[Delete]]` ( `P` )

The [[Delete]] internal method of a TypedArray O takes argument P (a property key) and returns a normal completion containing a Boolean. It performs the following steps when called:

1. If P is a String, then
1. Let numericIndex be CanonicalNumericIndexString(P).
2. b. If numericIndex is not undefined, then
  1. If IsValidIntegerIndex(O, numericIndex) is false, return true; else return false.
Return ! OrdinaryDelete(O, P).

10.4.5.7 `[[OwnPropertyKeys]]` ( )

The [[OwnPropertyKeys]] internal method of a TypedArray O takes no arguments and returns a normal completion containing a List of property keys. It performs the following steps when called:

Let taRecord be MakeTypedArrayWithBufferWitnessRecord(O, seq-cst).
Let keys be a new empty List.
3. If IsTypedArrayOutOfBounds(taRecord) is false, then
1. Let length be TypedArrayLength(taRecord).
2. b. For each integer i such that 0 ≤ i < length, in ascending order, do
  1. Append ! ToString(𝔽(i)) to keys.
4. For each own property key P of O such that P is a String and P is not an integer index, in ascending chronological order of property creation, do
1. Append P to keys.
5. For each own property key P of O such that P is a Symbol, in ascending chronological order of property creation, do
1. Append P to keys.
Return keys.

10.4.5.8 TypedArray With Buffer Witness Records

An TypedArray With Buffer Witness Record is a Record value used to encapsulate a TypedArray along with a cached byte length of the viewed buffer. It is used to help ensure there is a single shared memory read event of the byte length data block when the viewed buffer is a growable SharedArrayBuffer.

TypedArray With Buffer Witness Records have the fields listed in Table 32.

Table 32: TypedArray With Buffer Witness Record Fields

Field Name	Value	Meaning
`[[Object]]`	a TypedArray	The TypedArray whose buffer's byte length is loaded.
`[[CachedBufferByteLength]]`	a non-negative integer or detached	The byte length of the object's `[[ViewedArrayBuffer]]` when the Record was created.

10.4.5.9 MakeTypedArrayWithBufferWitnessRecord ( `obj`, `order` )

The abstract operation MakeTypedArrayWithBufferWitnessRecord takes arguments obj (a TypedArray) and order (seq-cst or unordered) and returns a TypedArray With Buffer Witness Record. It performs the following steps when called:

Let buffer be obj.[[ViewedArrayBuffer]].
2. If IsDetachedBuffer(buffer) is true, then
1. Let byteLength be detached.
3. Else,
1. Let byteLength be ArrayBufferByteLength(buffer, order).
Return the TypedArray With Buffer Witness Record { [[Object]]: obj, [[CachedBufferByteLength]]: byteLength }.

10.4.5.10 TypedArrayCreate ( `prototype` )

The abstract operation TypedArrayCreate takes argument prototype (an Object) and returns a TypedArray. It is used to specify the creation of new TypedArrays. It performs the following steps when called:

Let internalSlotsList be « [[Prototype]], [[Extensible]], [[ViewedArrayBuffer]], [[TypedArrayName]], [[ContentType]], [[ByteLength]], [[ByteOffset]], [[ArrayLength]] ».
Let A be MakeBasicObject(internalSlotsList).
Set A.[[GetOwnProperty]] as specified in 10.4.5.1.
Set A.[[HasProperty]] as specified in 10.4.5.2.
Set A.[[DefineOwnProperty]] as specified in 10.4.5.3.
Set A.[[Get]] as specified in 10.4.5.4.
Set A.[[Set]] as specified in 10.4.5.5.
Set A.[[Delete]] as specified in 10.4.5.6.
Set A.[[OwnPropertyKeys]] as specified in 10.4.5.7.
Set A.[[Prototype]] to prototype.
Return A.

10.4.5.11 TypedArrayByteLength ( `taRecord` )

The abstract operation TypedArrayByteLength takes argument taRecord (a TypedArray With Buffer Witness Record) and returns a non-negative integer. It performs the following steps when called:

If IsTypedArrayOutOfBounds(taRecord) is true, return 0.
Let length be TypedArrayLength(taRecord).
If length = 0, return 0.
Let O be taRecord.[[Object]].
If O.[[ByteLength]] is not auto, return O.[[ByteLength]].
Let elementSize be TypedArrayElementSize(O).
Return length × elementSize.

10.4.5.12 TypedArrayLength ( `taRecord` )

The abstract operation TypedArrayLength takes argument taRecord (a TypedArray With Buffer Witness Record) and returns a non-negative integer. It performs the following steps when called:

Assert: IsTypedArrayOutOfBounds(taRecord) is false.
Let O be taRecord.[[Object]].
If O.[[ArrayLength]] is not auto, return O.[[ArrayLength]].
Assert: IsFixedLengthArrayBuffer(O.[[ViewedArrayBuffer]]) is false.
Let byteOffset be O.[[ByteOffset]].
Let elementSize be TypedArrayElementSize(O).
Let byteLength be taRecord.[[CachedBufferByteLength]].
Assert: byteLength is not detached.
Return floor((byteLength - byteOffset) / elementSize).

10.4.5.13 IsTypedArrayOutOfBounds ( `taRecord` )

The abstract operation IsTypedArrayOutOfBounds takes argument taRecord (a TypedArray With Buffer Witness Record) and returns a Boolean. It checks if any of the object's numeric properties reference a value at an index not contained within the underlying buffer's bounds. It performs the following steps when called:

Let O be taRecord.[[Object]].
Let bufferByteLength be taRecord.[[CachedBufferByteLength]].
Assert: IsDetachedBuffer(O.[[ViewedArrayBuffer]]) is true if and only if bufferByteLength is detached.
If bufferByteLength is detached, return true.
Let byteOffsetStart be O.[[ByteOffset]].
6. If O.[[ArrayLength]] is auto, then
1. Let byteOffsetEnd be bufferByteLength.
7. Else,
1. Let elementSize be TypedArrayElementSize(O).
2. Let byteOffsetEnd be byteOffsetStart + O.[[ArrayLength]] × elementSize.
If byteOffsetStart > bufferByteLength or byteOffsetEnd > bufferByteLength, return true.
NOTE: 0-length TypedArrays are not considered out-of-bounds.
Return false.

10.4.5.14 IsValidIntegerIndex ( `O`, `index` )

The abstract operation IsValidIntegerIndex takes arguments O (a TypedArray) and index (a Number) and returns a Boolean. It performs the following steps when called:

If IsDetachedBuffer(O.[[ViewedArrayBuffer]]) is true, return false.
If IsIntegralNumber(index) is false, return false.
If index is -0_𝔽, return false.
Let taRecord be MakeTypedArrayWithBufferWitnessRecord(O, unordered).
NOTE: Bounds checking is not a synchronizing operation when O's backing buffer is a growable SharedArrayBuffer.
If IsTypedArrayOutOfBounds(taRecord) is true, return false.
Let length be TypedArrayLength(taRecord).
If ℝ(index) < 0 or ℝ(index) ≥ length, return false.
Return true.

10.4.5.15 TypedArrayGetElement ( `O`, `index` )

The abstract operation TypedArrayGetElement takes arguments O (a TypedArray) and index (a Number) and returns a Number, a BigInt, or undefined. It performs the following steps when called:

If IsValidIntegerIndex(O, index) is false, return undefined.
Let offset be O.[[ByteOffset]].
Let elementSize be TypedArrayElementSize(O).
Let byteIndexInBuffer be (ℝ(index) × elementSize) + offset.
Let elementType be TypedArrayElementType(O).
Return GetValueFromBuffer(O.[[ViewedArrayBuffer]], byteIndexInBuffer, elementType, true, unordered).

10.4.5.16 TypedArraySetElement ( `O`, `index`, `value` )

The abstract operation TypedArraySetElement takes arguments O (a TypedArray), index (a Number), and value (an ECMAScript language value) and returns either a normal completion containing unused or a throw completion. It performs the following steps when called:

If O.[[ContentType]] is bigint, let numValue be ? ToBigInt(value).
Otherwise, let numValue be ? ToNumber(value).
3. If IsValidIntegerIndex(O, index) is true, then
1. Let offset be O.[[ByteOffset]].
2. Let elementSize be TypedArrayElementSize(O).
3. Let byteIndexInBuffer be (ℝ(index) × elementSize) + offset.
4. Let elementType be TypedArrayElementType(O).
5. Perform SetValueInBuffer(O.[[ViewedArrayBuffer]], byteIndexInBuffer, elementType, numValue, true, unordered).
Return unused.

Note

This operation always appears to succeed, but it has no effect when attempting to write past the end of a TypedArray or to a TypedArray which is backed by a detached ArrayBuffer.

10.4.5.17 IsArrayBufferViewOutOfBounds ( `O` )

The abstract operation IsArrayBufferViewOutOfBounds takes argument O (a TypedArray or a DataView) and returns a Boolean. It checks if either any of a TypedArray's numeric properties or a DataView object's methods can reference a value at an index not contained within the underlying data block's bounds. This abstract operation exists as a convenience for upstream specifications. It performs the following steps when called:

1. If O has a [[DataView]] internal slot, then
1. Let viewRecord be MakeDataViewWithBufferWitnessRecord(O, seq-cst).
2. Return IsViewOutOfBounds(viewRecord).
Let taRecord be MakeTypedArrayWithBufferWitnessRecord(O, seq-cst).
Return IsTypedArrayOutOfBounds(taRecord).

10.4.6 Module Namespace Exotic Objects

A module namespace exotic object is an exotic object that exposes the bindings exported from an ECMAScript Module (See 16.2.3). There is a one-to-one correspondence between the String-keyed own properties of a module namespace exotic object and the binding names exported by the Module. The exported bindings include any bindings that are indirectly exported using export * export items. Each String-valued own property key is the StringValue of the corresponding exported binding name. These are the only String-keyed properties of a module namespace exotic object. Each such property has the attributes { [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: false }. Module namespace exotic objects are not extensible.

An object is a module namespace exotic object if its [[GetPrototypeOf]], [[SetPrototypeOf]], [[IsExtensible]], [[PreventExtensions]], [[GetOwnProperty]], [[DefineOwnProperty]], [[HasProperty]], [[Get]], [[Set]], [[Delete]], and [[OwnPropertyKeys]] internal methods use the definitions in this section, and its other essential internal methods use the definitions found in 10.1. These methods are installed by ModuleNamespaceCreate.

Module namespace exotic objects have the internal slots defined in Table 33.

Table 33: Internal Slots of Module Namespace Exotic Objects

Internal Slot	Type	Description
`[[Module]]`	a Module Record	The Module Record whose exports this namespace exposes.
`[[Exports]]`	a List of Strings	A List whose elements are the String values of the exported names exposed as own properties of this object. The list is ordered as if an Array of those String values had been sorted using %Array.prototype.sort% using undefined as `comparefn`.

10.4.6.1 `[[GetPrototypeOf]]` ( )

The [[GetPrototypeOf]] internal method of a module namespace exotic object takes no arguments and returns a normal completion containing null. It performs the following steps when called:

Return null.

10.4.6.2 `[[SetPrototypeOf]]` ( `V` )

The [[SetPrototypeOf]] internal method of a module namespace exotic object O takes argument V (an Object or null) and returns a normal completion containing a Boolean. It performs the following steps when called:

Return ! SetImmutablePrototype(O, V).

10.4.6.3 `[[IsExtensible]]` ( )

The [[IsExtensible]] internal method of a module namespace exotic object takes no arguments and returns a normal completion containing false. It performs the following steps when called:

Return false.

10.4.6.4 `[[PreventExtensions]]` ( )

The [[PreventExtensions]] internal method of a module namespace exotic object takes no arguments and returns a normal completion containing true. It performs the following steps when called:

Return true.

10.4.6.5 `[[GetOwnProperty]]` ( `P` )

The [[GetOwnProperty]] internal method of a module namespace exotic object O takes argument P (a property key) and returns either a normal completion containing either a Property Descriptor or undefined, or a throw completion. It performs the following steps when called:

If P is a Symbol, return OrdinaryGetOwnProperty(O, P).
Let exports be O.[[Exports]].
If exports does not contain P, return undefined.
Let value be ? O.[[Get]](P, O).
Return PropertyDescriptor { [[Value]]: value, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: false }.

10.4.6.6 `[[DefineOwnProperty]]` ( `P`, `Desc` )

The [[DefineOwnProperty]] internal method of a module namespace exotic object O takes arguments P (a property key) and Desc (a Property Descriptor) and returns either a normal completion containing a Boolean or a throw completion. It performs the following steps when called:

If P is a Symbol, return ! OrdinaryDefineOwnProperty(O, P, Desc).
Let current be ? O.[[GetOwnProperty]](P).
If current is undefined, return false.
If Desc has a [[Configurable]] field and Desc.[[Configurable]] is true, return false.
If Desc has an [[Enumerable]] field and Desc.[[Enumerable]] is false, return false.
If IsAccessorDescriptor(Desc) is true, return false.
If Desc has a [[Writable]] field and Desc.[[Writable]] is false, return false.
If Desc has a [[Value]] field, return SameValue(Desc.[[Value]], current.[[Value]]).
Return true.

10.4.6.7 `[[HasProperty]]` ( `P` )

The [[HasProperty]] internal method of a module namespace exotic object O takes argument P (a property key) and returns a normal completion containing a Boolean. It performs the following steps when called:

If P is a Symbol, return ! OrdinaryHasProperty(O, P).
Let exports be O.[[Exports]].
If exports contains P, return true.
Return false.

10.4.6.8 `[[Get]]` ( `P`, `Receiver` )

The [[Get]] internal method of a module namespace exotic object O takes arguments P (a property key) and Receiver (an ECMAScript language value) and returns either a normal completion containing an ECMAScript language value or a throw completion. It performs the following steps when called:

1. If P is a Symbol, then
1. Return ! OrdinaryGet(O, P, Receiver).
Let exports be O.[[Exports]].
If exports does not contain P, return undefined.
Let m be O.[[Module]].
Let binding be m.ResolveExport(P).
Assert: binding is a ResolvedBinding Record.
Let targetModule be binding.[[Module]].
Assert: targetModule is not undefined.
9. If binding.[[BindingName]] is namespace, then
1. Return GetModuleNamespace(targetModule).
Let targetEnv be targetModule.[[Environment]].
If targetEnv is empty, throw a ReferenceError exception.
Return ? targetEnv.GetBindingValue(binding.[[BindingName]], true).

Note

ResolveExport is side-effect free. Each time this operation is called with a specific exportName, resolveSet pair as arguments it must return the same result. An implementation might choose to pre-compute or cache the ResolveExport results for the [[Exports]] of each module namespace exotic object.

10.4.6.9 `[[Set]]` ( `P`, `V`, `Receiver` )

The [[Set]] internal method of a module namespace exotic object takes arguments P (a property key), V (an ECMAScript language value), and Receiver (an ECMAScript language value) and returns a normal completion containing false. It performs the following steps when called:

Return false.

10.4.6.10 `[[Delete]]` ( `P` )

The [[Delete]] internal method of a module namespace exotic object O takes argument P (a property key) and returns a normal completion containing a Boolean. It performs the following steps when called:

1. If P is a Symbol, then
1. Return ! OrdinaryDelete(O, P).
Let exports be O.[[Exports]].
If exports contains P, return false.
Return true.

10.4.6.11 `[[OwnPropertyKeys]]` ( )

The [[OwnPropertyKeys]] internal method of a module namespace exotic object O takes no arguments and returns a normal completion containing a List of property keys. It performs the following steps when called:

Let exports be O.[[Exports]].
Let symbolKeys be OrdinaryOwnPropertyKeys(O).
Return the list-concatenation of exports and symbolKeys.

10.4.6.12 ModuleNamespaceCreate ( `module`, `exports` )

The abstract operation ModuleNamespaceCreate takes arguments module (a Module Record) and exports (a List of Strings) and returns a module namespace exotic object. It is used to specify the creation of new module namespace exotic objects. It performs the following steps when called:

Assert: module.[[Namespace]] is empty.
Let internalSlotsList be the internal slots listed in Table 33.
Let M be MakeBasicObject(internalSlotsList).
Set M's essential internal methods to the definitions specified in 10.4.6.
Set M.[[Module]] to module.
Let sortedExports be a List whose elements are the elements of exports ordered as if an Array of the same values had been sorted using %Array.prototype.sort% using undefined as comparefn.
Set M.[[Exports]] to sortedExports.
Create own properties of M corresponding to the definitions in 28.3.
Set module.[[Namespace]] to M.
Return M.

10.4.7 Immutable Prototype Exotic Objects

An immutable prototype exotic object is an exotic object that has a [[Prototype]] internal slot that will not change once it is initialized.

An object is an immutable prototype exotic object if its [[SetPrototypeOf]] internal method uses the following implementation. (Its other essential internal methods may use any implementation, depending on the specific immutable prototype exotic object in question.)

Note

Unlike other exotic objects, there is not a dedicated creation abstract operation provided for immutable prototype exotic objects. This is because they are only used by %Object.prototype% and by host environments, and in host environments, the relevant objects are potentially exotic in other ways and thus need their own dedicated creation operation.

10.4.7.1 `[[SetPrototypeOf]]` ( `V` )

The [[SetPrototypeOf]] internal method of an immutable prototype exotic object O takes argument V (an Object or null) and returns either a normal completion containing a Boolean or a throw completion. It performs the following steps when called:

Return ? SetImmutablePrototype(O, V).

10.4.7.2 SetImmutablePrototype ( `O`, `V` )

The abstract operation SetImmutablePrototype takes arguments O (an Object) and V (an Object or null) and returns either a normal completion containing a Boolean or a throw completion. It performs the following steps when called:

Let current be ? O.[[GetPrototypeOf]]().
If SameValue(V, current) is true, return true.
Return false.

10.5 Proxy Object Internal Methods and Internal Slots

A Proxy object is an exotic object whose essential internal methods are partially implemented using ECMAScript code. Every Proxy object has an internal slot called [[ProxyHandler]]. The value of [[ProxyHandler]] is an object, called the proxy's handler object, or null. Methods (see Table 34) of a handler object may be used to augment the implementation for one or more of the Proxy object's internal methods. Every Proxy object also has an internal slot called [[ProxyTarget]] whose value is either an object or the null value. This object is called the proxy's target object.

An object is a Proxy exotic object if its essential internal methods (including [[Call]] and [[Construct]], if applicable) use the definitions in this section. These internal methods are installed in ProxyCreate.

Table 34: Proxy Handler Methods

Internal Method	Handler Method
`[[GetPrototypeOf]]`	`getPrototypeOf`
`[[SetPrototypeOf]]`	`setPrototypeOf`
`[[IsExtensible]]`	`isExtensible`
`[[PreventExtensions]]`	`preventExtensions`
`[[GetOwnProperty]]`	`getOwnPropertyDescriptor`
`[[DefineOwnProperty]]`	`defineProperty`
`[[HasProperty]]`	`has`
`[[Get]]`	`get`
`[[Set]]`	`set`
`[[Delete]]`	`deleteProperty`
`[[OwnPropertyKeys]]`	`ownKeys`
`[[Call]]`	`apply`
`[[Construct]]`	`construct`

When a handler method is called to provide the implementation of a Proxy object internal method, the handler method is passed the proxy's target object as a parameter. A proxy's handler object does not necessarily have a method corresponding to every essential internal method. Invoking an internal method on the proxy results in the invocation of the corresponding internal method on the proxy's target object if the handler object does not have a method corresponding to the internal trap.

The [[ProxyHandler]] and [[ProxyTarget]] internal slots of a Proxy object are always initialized when the object is created and typically may not be modified. Some Proxy objects are created in a manner that permits them to be subsequently revoked. When a proxy is revoked, its [[ProxyHandler]] and [[ProxyTarget]] internal slots are set to null causing subsequent invocations of internal methods on that Proxy object to throw a TypeError exception.

Because Proxy objects permit the implementation of internal methods to be provided by arbitrary ECMAScript code, it is possible to define a Proxy object whose handler methods violates the invariants defined in 6.1.7.3. Some of the internal method invariants defined in 6.1.7.3 are essential integrity invariants. These invariants are explicitly enforced by the Proxy object internal methods specified in this section. An ECMAScript implementation must be robust in the presence of all possible invariant violations.

In the following algorithm descriptions, assume O is an ECMAScript Proxy object, P is a property key value, V is any ECMAScript language value and Desc is a Property Descriptor record.

10.5.1 `[[GetPrototypeOf]]` ( )

The [[GetPrototypeOf]] internal method of a Proxy exotic object O takes no arguments and returns either a normal completion containing either an Object or null, or a throw completion. It performs the following steps when called:

Perform ? ValidateNonRevokedProxy(O).
Let target be O.[[ProxyTarget]].
Let handler be O.[[ProxyHandler]].
Assert: handler is an Object.
Let trap be ? GetMethod(handler, "getPrototypeOf").
6. If trap is undefined, then
1. Return ? target.[[GetPrototypeOf]]().
Let handlerProto be ? Call(trap, handler, « target »).
If handlerProto is not an Object and handlerProto is not null, throw a TypeError exception.
Let extensibleTarget be ? IsExtensible(target).
If extensibleTarget is true, return handlerProto.
Let targetProto be ? target.[[GetPrototypeOf]]().
If SameValue(handlerProto, targetProto) is false, throw a TypeError exception.
Return handlerProto.

Note

[[GetPrototypeOf]] for Proxy objects enforces the following invariants:

The result of [[GetPrototypeOf]] must be either an Object or null.
If the target object is not extensible, [[GetPrototypeOf]] applied to the Proxy object must return the same value as [[GetPrototypeOf]] applied to the Proxy object's target object.

10.5.2 `[[SetPrototypeOf]]` ( `V` )

The [[SetPrototypeOf]] internal method of a Proxy exotic object O takes argument V (an Object or null) and returns either a normal completion containing a Boolean or a throw completion. It performs the following steps when called:

Perform ? ValidateNonRevokedProxy(O).
Let target be O.[[ProxyTarget]].
Let handler be O.[[ProxyHandler]].
Assert: handler is an Object.
Let trap be ? GetMethod(handler, "setPrototypeOf").
6. If trap is undefined, then
1. Return ? target.[[SetPrototypeOf]](V).
Let booleanTrapResult be ToBoolean(? Call(trap, handler, « target, V »)).
If booleanTrapResult is false, return false.
Let extensibleTarget be ? IsExtensible(target).
If extensibleTarget is true, return true.
Let targetProto be ? target.[[GetPrototypeOf]]().
If SameValue(V, targetProto) is false, throw a TypeError exception.
Return true.

Note

[[SetPrototypeOf]] for Proxy objects enforces the following invariants:

The result of [[SetPrototypeOf]] is a Boolean value.
If the target object is not extensible, the argument value must be the same as the result of [[GetPrototypeOf]] applied to target object.

10.5.3 `[[IsExtensible]]` ( )

The [[IsExtensible]] internal method of a Proxy exotic object O takes no arguments and returns either a normal completion containing a Boolean or a throw completion. It performs the following steps when called:

Perform ? ValidateNonRevokedProxy(O).
Let target be O.[[ProxyTarget]].
Let handler be O.[[ProxyHandler]].
Assert: handler is an Object.
Let trap be ? GetMethod(handler, "isExtensible").
6. If trap is undefined, then
1. Return ? IsExtensible(target).
Let booleanTrapResult be ToBoolean(? Call(trap, handler, « target »)).
Let targetResult be ? IsExtensible(target).
If booleanTrapResult is not targetResult, throw a TypeError exception.
Return booleanTrapResult.

Note

[[IsExtensible]] for Proxy objects enforces the following invariants:

The result of [[IsExtensible]] is a Boolean value.
[[IsExtensible]] applied to the Proxy object must return the same value as [[IsExtensible]] applied to the Proxy object's target object with the same argument.

10.5.4 `[[PreventExtensions]]` ( )

The [[PreventExtensions]] internal method of a Proxy exotic object O takes no arguments and returns either a normal completion containing a Boolean or a throw completion. It performs the following steps when called:

Perform ? ValidateNonRevokedProxy(O).
Let target be O.[[ProxyTarget]].
Let handler be O.[[ProxyHandler]].
Assert: handler is an Object.
Let trap be ? GetMethod(handler, "preventExtensions").
6. If trap is undefined, then
1. Return ? target.[[PreventExtensions]]().
Let booleanTrapResult be ToBoolean(? Call(trap, handler, « target »)).
8. If booleanTrapResult is true, then
1. Let extensibleTarget be ? IsExtensible(target).
2. If extensibleTarget is true, throw a TypeError exception.
Return booleanTrapResult.

Note

[[PreventExtensions]] for Proxy objects enforces the following invariants:

The result of [[PreventExtensions]] is a Boolean value.
[[PreventExtensions]] applied to the Proxy object only returns true if [[IsExtensible]] applied to the Proxy object's target object is false.

10.5.5 `[[GetOwnProperty]]` ( `P` )

The [[GetOwnProperty]] internal method of a Proxy exotic object O takes argument P (a property key) and returns either a normal completion containing either a Property Descriptor or undefined, or a throw completion. It performs the following steps when called:

Perform ? ValidateNonRevokedProxy(O).
Let target be O.[[ProxyTarget]].
Let handler be O.[[ProxyHandler]].
Assert: handler is an Object.
Let trap be ? GetMethod(handler, "getOwnPropertyDescriptor").
6. If trap is undefined, then
1. Return ? target.[[GetOwnProperty]](P).
Let trapResultObj be ? Call(trap, handler, « target, P »).
If trapResultObj is not an Object and trapResultObj is not undefined, throw a TypeError exception.
Let targetDesc be ? target.[[GetOwnProperty]](P).
10. If trapResultObj is undefined, then
1. If targetDesc is undefined, return undefined.
2. If targetDesc.[[Configurable]] is false, throw a TypeError exception.
3. Let extensibleTarget be ? IsExtensible(target).
4. If extensibleTarget is false, throw a TypeError exception.
5. Return undefined.
Let extensibleTarget be ? IsExtensible(target).
Let resultDesc be ? ToPropertyDescriptor(trapResultObj).
Perform CompletePropertyDescriptor(resultDesc).
Let valid be IsCompatiblePropertyDescriptor(extensibleTarget, resultDesc, targetDesc).
If valid is false, throw a TypeError exception.
16. If resultDesc.[[Configurable]] is false, then
1. a. If targetDesc is undefined or targetDesc.[[Configurable]] is true, then
  1. Throw a TypeError exception.
2. b. If resultDesc has a [[Writable]] field and resultDesc.[[Writable]] is false, then
  1. Assert: targetDesc has a [[Writable]] field.
  2. If targetDesc.[[Writable]] is true, throw a TypeError exception.
Return resultDesc.

Note

[[GetOwnProperty]] for Proxy objects enforces the following invariants:

The result of [[GetOwnProperty]] must be either an Object or undefined.
A property cannot be reported as non-existent, if it exists as a non-configurable own property of the target object.
A property cannot be reported as non-existent, if it exists as an own property of a non-extensible target object.
A property cannot be reported as existent, if it does not exist as an own property of the target object and the target object is not extensible.
A property cannot be reported as non-configurable, unless it exists as a non-configurable own property of the target object.
A property cannot be reported as both non-configurable and non-writable, unless it exists as a non-configurable, non-writable own property of the target object.

10.5.6 `[[DefineOwnProperty]]` ( `P`, `Desc` )

The [[DefineOwnProperty]] internal method of a Proxy exotic object O takes arguments P (a property key) and Desc (a Property Descriptor) and returns either a normal completion containing a Boolean or a throw completion. It performs the following steps when called:

Perform ? ValidateNonRevokedProxy(O).
Let target be O.[[ProxyTarget]].
Let handler be O.[[ProxyHandler]].
Assert: handler is an Object.
Let trap be ? GetMethod(handler, "defineProperty").
6. If trap is undefined, then
1. Return ? target.[[DefineOwnProperty]](P, Desc).
Let descObj be FromPropertyDescriptor(Desc).
Let booleanTrapResult be ToBoolean(? Call(trap, handler, « target, P, descObj »)).
If booleanTrapResult is false, return false.
Let targetDesc be ? target.[[GetOwnProperty]](P).
Let extensibleTarget be ? IsExtensible(target).
12. If Desc has a [[Configurable]] field and Desc.[[Configurable]] is false, then
1. Let settingConfigFalse be true.
13. Else,
1. Let settingConfigFalse be false.
14. If targetDesc is undefined, then
1. If extensibleTarget is false, throw a TypeError exception.
2. If settingConfigFalse is true, throw a TypeError exception.
15. Else,
1. If IsCompatiblePropertyDescriptor(extensibleTarget, Desc, targetDesc) is false, throw a TypeError exception.
2. If settingConfigFalse is true and targetDesc.[[Configurable]] is true, throw a TypeError exception.
3. c. If IsDataDescriptor(targetDesc) is true, targetDesc.[[Configurable]] is false, and targetDesc.[[Writable]] is true, then
  1. If Desc has a [[Writable]] field and Desc.[[Writable]] is false, throw a TypeError exception.
Return true.

Note

[[DefineOwnProperty]] for Proxy objects enforces the following invariants:

The result of [[DefineOwnProperty]] is a Boolean value.
A property cannot be added, if the target object is not extensible.
A property cannot be non-configurable, unless there exists a corresponding non-configurable own property of the target object.
A non-configurable property cannot be non-writable, unless there exists a corresponding non-configurable, non-writable own property of the target object.
If a property has a corresponding target object property then applying the Property Descriptor of the property to the target object using [[DefineOwnProperty]] will not throw an exception.

10.5.7 `[[HasProperty]]` ( `P` )

The [[HasProperty]] internal method of a Proxy exotic object O takes argument P (a property key) and returns either a normal completion containing a Boolean or a throw completion. It performs the following steps when called:

Perform ? ValidateNonRevokedProxy(O).
Let target be O.[[ProxyTarget]].
Let handler be O.[[ProxyHandler]].
Assert: handler is an Object.
Let trap be ? GetMethod(handler, "has").
6. If trap is undefined, then
1. Return ? target.[[HasProperty]](P).
Let booleanTrapResult be ToBoolean(? Call(trap, handler, « target, P »)).
8. If booleanTrapResult is false, then
1. Let targetDesc be ? target.[[GetOwnProperty]](P).
2. b. If targetDesc is not undefined, then
  1. If targetDesc.[[Configurable]] is false, throw a TypeError exception.
  2. Let extensibleTarget be ? IsExtensible(target).
  3. If extensibleTarget is false, throw a TypeError exception.
Return booleanTrapResult.

Note

[[HasProperty]] for Proxy objects enforces the following invariants:

The result of [[HasProperty]] is a Boolean value.
A property cannot be reported as non-existent, if it exists as a non-configurable own property of the target object.
A property cannot be reported as non-existent, if it exists as an own property of the target object and the target object is not extensible.

10.5.8 `[[Get]]` ( `P`, `Receiver` )

The [[Get]] internal method of a Proxy exotic object O takes arguments P (a property key) and Receiver (an ECMAScript language value) and returns either a normal completion containing an ECMAScript language value or a throw completion. It performs the following steps when called:

Perform ? ValidateNonRevokedProxy(O).
Let target be O.[[ProxyTarget]].
Let handler be O.[[ProxyHandler]].
Assert: handler is an Object.
Let trap be ? GetMethod(handler, "get").
6. If trap is undefined, then
1. Return ? target.[[Get]](P, Receiver).
Let trapResult be ? Call(trap, handler, « target, P, Receiver »).
Let targetDesc be ? target.[[GetOwnProperty]](P).
9. If targetDesc is not undefined and targetDesc.[[Configurable]] is false, then
1. a. If IsDataDescriptor(targetDesc) is true and targetDesc.[[Writable]] is false, then
  1. If SameValue(trapResult, targetDesc.[[Value]]) is false, throw a TypeError exception.
2. b. If IsAccessorDescriptor(targetDesc) is true and targetDesc.[[Get]] is undefined, then
  1. If trapResult is not undefined, throw a TypeError exception.
Return trapResult.

Note

[[Get]] for Proxy objects enforces the following invariants:

The value reported for a property must be the same as the value of the corresponding target object property if the target object property is a non-writable, non-configurable own data property.
The value reported for a property must be undefined if the corresponding target object property is a non-configurable own accessor property that has undefined as its [[Get]] attribute.

10.5.9 `[[Set]]` ( `P`, `V`, `Receiver` )

The [[Set]] internal method of a Proxy exotic object O takes arguments P (a property key), V (an ECMAScript language value), and Receiver (an ECMAScript language value) and returns either a normal completion containing a Boolean or a throw completion. It performs the following steps when called:

Perform ? ValidateNonRevokedProxy(O).
Let target be O.[[ProxyTarget]].
Let handler be O.[[ProxyHandler]].
Assert: handler is an Object.
Let trap be ? GetMethod(handler, "set").
6. If trap is undefined, then
1. Return ? target.[[Set]](P, V, Receiver).
Let booleanTrapResult be ToBoolean(? Call(trap, handler, « target, P, V, Receiver »)).
If booleanTrapResult is false, return false.
Let targetDesc be ? target.[[GetOwnProperty]](P).
10. If targetDesc is not undefined and targetDesc.[[Configurable]] is false, then
1. a. If IsDataDescriptor(targetDesc) is true and targetDesc.[[Writable]] is false, then
  1. If SameValue(V, targetDesc.[[Value]]) is false, throw a TypeError exception.
2. b. If IsAccessorDescriptor(targetDesc) is true, then
  1. If targetDesc.[[Set]] is undefined, throw a TypeError exception.
Return true.

Note

[[Set]] for Proxy objects enforces the following invariants:

The result of [[Set]] is a Boolean value.
Cannot change the value of a property to be different from the value of the corresponding target object property if the corresponding target object property is a non-writable, non-configurable own data property.
Cannot set the value of a property if the corresponding target object property is a non-configurable own accessor property that has undefined as its [[Set]] attribute.

10.5.10 `[[Delete]]` ( `P` )

The [[Delete]] internal method of a Proxy exotic object O takes argument P (a property key) and returns either a normal completion containing a Boolean or a throw completion. It performs the following steps when called:

Perform ? ValidateNonRevokedProxy(O).
Let target be O.[[ProxyTarget]].
Let handler be O.[[ProxyHandler]].
Assert: handler is an Object.
Let trap be ? GetMethod(handler, "deleteProperty").
6. If trap is undefined, then
1. Return ? target.[[Delete]](P).
Let booleanTrapResult be ToBoolean(? Call(trap, handler, « target, P »)).
If booleanTrapResult is false, return false.
Let targetDesc be ? target.[[GetOwnProperty]](P).
If targetDesc is undefined, return true.
If targetDesc.[[Configurable]] is false, throw a TypeError exception.
Let extensibleTarget be ? IsExtensible(target).
If extensibleTarget is false, throw a TypeError exception.
Return true.

Note

[[Delete]] for Proxy objects enforces the following invariants:

The result of [[Delete]] is a Boolean value.
A property cannot be reported as deleted, if it exists as a non-configurable own property of the target object.
A property cannot be reported as deleted, if it exists as an own property of the target object and the target object is non-extensible.

10.5.11 `[[OwnPropertyKeys]]` ( )

The [[OwnPropertyKeys]] internal method of a Proxy exotic object O takes no arguments and returns either a normal completion containing a List of property keys or a throw completion. It performs the following steps when called:

Perform ? ValidateNonRevokedProxy(O).
Let target be O.[[ProxyTarget]].
Let handler be O.[[ProxyHandler]].
Assert: handler is an Object.
Let trap be ? GetMethod(handler, "ownKeys").
6. If trap is undefined, then
1. Return ? target.[[OwnPropertyKeys]]().
Let trapResultArray be ? Call(trap, handler, « target »).
Let trapResult be ? CreateListFromArrayLike(trapResultArray, « String, Symbol »).
If trapResult contains any duplicate entries, throw a TypeError exception.
Let extensibleTarget be ? IsExtensible(target).
Let targetKeys be ? target.[[OwnPropertyKeys]]().
Assert: targetKeys is a List of property keys.
Assert: targetKeys contains no duplicate entries.
Let targetConfigurableKeys be a new empty List.
Let targetNonconfigurableKeys be a new empty List.
16. For each element key of targetKeys, do
1. Let desc be ? target.[[GetOwnProperty]](key).
2. b. If desc is not undefined and desc.[[Configurable]] is false, then
  1. Append key to targetNonconfigurableKeys.
3. c. Else,
  1. Append key to targetConfigurableKeys.
17. If extensibleTarget is true and targetNonconfigurableKeys is empty, then
1. Return trapResult.
Let uncheckedResultKeys be a List whose elements are the elements of trapResult.
19. For each element key of targetNonconfigurableKeys, do
1. If uncheckedResultKeys does not contain key, throw a TypeError exception.
2. Remove key from uncheckedResultKeys.
If extensibleTarget is true, return trapResult.
21. For each element key of targetConfigurableKeys, do
1. If uncheckedResultKeys does not contain key, throw a TypeError exception.
2. Remove key from uncheckedResultKeys.
If uncheckedResultKeys is not empty, throw a TypeError exception.
Return trapResult.

Note

[[OwnPropertyKeys]] for Proxy objects enforces the following invariants:

The result of [[OwnPropertyKeys]] is a List.
The returned List contains no duplicate entries.
The Type of each result List element is either String or Symbol.
The result List must contain the keys of all non-configurable own properties of the target object.
If the target object is not extensible, then the result List must contain all the keys of the own properties of the target object and no other values.

10.5.12 `[[Call]]` ( `thisArgument`, `argumentsList` )

The [[Call]] internal method of a Proxy exotic object O takes arguments thisArgument (an ECMAScript language value) and argumentsList (a List of ECMAScript language values) and returns either a normal completion containing an ECMAScript language value or a throw completion. It performs the following steps when called:

Perform ? ValidateNonRevokedProxy(O).
Let target be O.[[ProxyTarget]].
Let handler be O.[[ProxyHandler]].
Assert: handler is an Object.
Let trap be ? GetMethod(handler, "apply").
6. If trap is undefined, then
1. Return ? Call(target, thisArgument, argumentsList).
Let argArray be CreateArrayFromList(argumentsList).
Return ? Call(trap, handler, « target, thisArgument, argArray »).

Note

A Proxy exotic object only has a [[Call]] internal method if the initial value of its [[ProxyTarget]] internal slot is an object that has a [[Call]] internal method.

10.5.13 `[[Construct]]` ( `argumentsList`, `newTarget` )

The [[Construct]] internal method of a Proxy exotic object O takes arguments argumentsList (a List of ECMAScript language values) and newTarget (a constructor) and returns either a normal completion containing an Object or a throw completion. It performs the following steps when called:

Perform ? ValidateNonRevokedProxy(O).
Let target be O.[[ProxyTarget]].
Assert: IsConstructor(target) is true.
Let handler be O.[[ProxyHandler]].
Assert: handler is an Object.
Let trap be ? GetMethod(handler, "construct").
7. If trap is undefined, then
1. Return ? Construct(target, argumentsList, newTarget).
Let argArray be CreateArrayFromList(argumentsList).
Let newObj be ? Call(trap, handler, « target, argArray, newTarget »).
If newObj is not an Object, throw a TypeError exception.
Return newObj.

Note 1

A Proxy exotic object only has a [[Construct]] internal method if the initial value of its [[ProxyTarget]] internal slot is an object that has a [[Construct]] internal method.

Note 2

[[Construct]] for Proxy objects enforces the following invariants:

The result of [[Construct]] must be an Object.

10.5.14 ValidateNonRevokedProxy ( `proxy` )

The abstract operation ValidateNonRevokedProxy takes argument proxy (a Proxy exotic object) and returns either a normal completion containing unused or a throw completion. It throws a TypeError exception if proxy has been revoked. It performs the following steps when called:

If proxy.[[ProxyTarget]] is null, throw a TypeError exception.
Assert: proxy.[[ProxyHandler]] is not null.
Return unused.

10.5.15 ProxyCreate ( `target`, `handler` )

The abstract operation ProxyCreate takes arguments target (an ECMAScript language value) and handler (an ECMAScript language value) and returns either a normal completion containing a Proxy exotic object or a throw completion. It is used to specify the creation of new Proxy objects. It performs the following steps when called:

If target is not an Object, throw a TypeError exception.
If handler is not an Object, throw a TypeError exception.
Let P be MakeBasicObject(« [[ProxyHandler]], [[ProxyTarget]] »).
Set P's essential internal methods, except for [[Call]] and [[Construct]], to the definitions specified in 10.5.
5. If IsCallable(target) is true, then
1. Set P.[[Call]] as specified in 10.5.12.
2. b. If IsConstructor(target) is true, then
  1. Set P.[[Construct]] as specified in 10.5.13.
Set P.[[ProxyTarget]] to target.
Set P.[[ProxyHandler]] to handler.
Return P.

11 ECMAScript Language: Source Text

11.1 Source Text

Syntax

SourceCharacter

any Unicode code point

ECMAScript source text is a sequence of Unicode code points. All Unicode code point values from U+0000 to U+10FFFF, including surrogate code points, may occur in ECMAScript source text where permitted by the ECMAScript grammars. The actual encodings used to store and interchange ECMAScript source text is not relevant to this specification. Regardless of the external source text encoding, a conforming ECMAScript implementation processes the source text as if it was an equivalent sequence of SourceCharacter values, each SourceCharacter being a Unicode code point. Conforming ECMAScript implementations are not required to perform any normalization of source text, or behave as though they were performing normalization of source text.

The components of a combining character sequence are treated as individual Unicode code points even though a user might think of the whole sequence as a single character.

Note

In string literals, regular expression literals, template literals and identifiers, any Unicode code point may also be expressed using Unicode escape sequences that explicitly express a code point's numeric value. Within a comment, such an escape sequence is effectively ignored as part of the comment.

ECMAScript differs from the Java programming language in the behaviour of Unicode escape sequences. In a Java program, if the Unicode escape sequence \u000A, for example, occurs within a single-line comment, it is interpreted as a line terminator (Unicode code point U+000A is LINE FEED (LF)) and therefore the next code point is not part of the comment. Similarly, if the Unicode escape sequence \u000A occurs within a string literal in a Java program, it is likewise interpreted as a line terminator, which is not allowed within a string literal—one must write \n instead of \u000A to cause a LINE FEED (LF) to be part of the String value of a string literal. In an ECMAScript program, a Unicode escape sequence occurring within a comment is never interpreted and therefore cannot contribute to termination of the comment. Similarly, a Unicode escape sequence occurring within a string literal in an ECMAScript program always contributes to the literal and is never interpreted as a line terminator or as a code point that might terminate the string literal.

11.1.1 Static Semantics: UTF16EncodeCodePoint ( `cp` )

The abstract operation UTF16EncodeCodePoint takes argument cp (a Unicode code point) and returns a String. It performs the following steps when called:

Assert: 0 ≤ cp ≤ 0x10FFFF.
If cp ≤ 0xFFFF, return the String value consisting of the code unit whose numeric value is cp.
Let cu1 be the code unit whose numeric value is floor((cp - 0x10000) / 0x400) + 0xD800.
Let cu2 be the code unit whose numeric value is ((cp - 0x10000) modulo 0x400) + 0xDC00.
Return the string-concatenation of cu1 and cu2.

11.1.2 Static Semantics: CodePointsToString ( `text` )

The abstract operation CodePointsToString takes argument text (a sequence of Unicode code points) and returns a String. It converts text into a String value, as described in 6.1.4. It performs the following steps when called:

Let result be the empty String.
2. For each code point cp of text, do
1. Set result to the string-concatenation of result and UTF16EncodeCodePoint(cp).
Return result.

11.1.3 Static Semantics: UTF16SurrogatePairToCodePoint ( `lead`, `trail` )

The abstract operation UTF16SurrogatePairToCodePoint takes arguments lead (a code unit) and trail (a code unit) and returns a code point. Two code units that form a UTF-16 surrogate pair are converted to a code point. It performs the following steps when called:

Assert: lead is a leading surrogate and trail is a trailing surrogate.
Let cp be (lead - 0xD800) × 0x400 + (trail - 0xDC00) + 0x10000.
Return the code point cp.

11.1.4 Static Semantics: CodePointAt ( `string`, `position` )

The abstract operation CodePointAt takes arguments string (a String) and position (a non-negative integer) and returns a Record with fields [[CodePoint]] (a code point), [[CodeUnitCount]] (a positive integer), and [[IsUnpairedSurrogate]] (a Boolean). It interprets string as a sequence of UTF-16 encoded code points, as described in 6.1.4, and reads from it a single code point starting with the code unit at index position. It performs the following steps when called:

Let size be the length of string.
Assert: position ≥ 0 and position < size.
Let first be the code unit at index position within string.
Let cp be the code point whose numeric value is the numeric value of first.
5. If first is neither a leading surrogate nor a trailing surrogate, then
1. Return the Record { [[CodePoint]]: cp, [[CodeUnitCount]]: 1, [[IsUnpairedSurrogate]]: false }.
6. If first is a trailing surrogate or position + 1 = size, then
1. Return the Record { [[CodePoint]]: cp, [[CodeUnitCount]]: 1, [[IsUnpairedSurrogate]]: true }.
Let second be the code unit at index position + 1 within string.
8. If second is not a trailing surrogate, then
1. Return the Record { [[CodePoint]]: cp, [[CodeUnitCount]]: 1, [[IsUnpairedSurrogate]]: true }.
Set cp to UTF16SurrogatePairToCodePoint(first, second).
Return the Record { [[CodePoint]]: cp, [[CodeUnitCount]]: 2, [[IsUnpairedSurrogate]]: false }.

11.1.5 Static Semantics: StringToCodePoints ( `string` )

The abstract operation StringToCodePoints takes argument string (a String) and returns a List of code points. It returns the sequence of Unicode code points that results from interpreting string as UTF-16 encoded Unicode text as described in 6.1.4. It performs the following steps when called:

Let codePoints be a new empty List.
Let size be the length of string.
Let position be 0.
4. Repeat, while position < size,
1. Let cp be CodePointAt(string, position).
2. Append cp.[[CodePoint]] to codePoints.
3. Set position to position + cp.[[CodeUnitCount]].
Return codePoints.

11.1.6 Static Semantics: ParseText ( `sourceText`, `goalSymbol` )

The abstract operation ParseText takes arguments sourceText (a sequence of Unicode code points) and goalSymbol (a nonterminal in one of the ECMAScript grammars) and returns a Parse Node or a non-empty List of SyntaxError objects. It performs the following steps when called:

Attempt to parse sourceText using goalSymbol as the goal symbol, and analyse the parse result for any early error conditions. Parsing and early error detection may be interleaved in an implementation-defined manner.
If the parse succeeded and no early errors were found, return the Parse Node (an instance of goalSymbol) at the root of the parse tree resulting from the parse.
Otherwise, return a List of one or more SyntaxError objects representing the parsing errors and/or early errors. If more than one parsing error or early error is present, the number and ordering of error objects in the list is implementation-defined, but at least one must be present.

Note 1

Consider a text that has an early error at a particular point, and also a syntax error at a later point. An implementation that does a parse pass followed by an early errors pass might report the syntax error and not proceed to the early errors pass. An implementation that interleaves the two activities might report the early error and not proceed to find the syntax error. A third implementation might report both errors. All of these behaviours are conformant.

Note 2

11.2 Types of Source Code

There are four types of ECMAScript code:

Global code is source text that is treated as an ECMAScript Script. The global code of a particular Script does not include any source text that is parsed as part of a FunctionDeclaration, FunctionExpression, GeneratorDeclaration, GeneratorExpression, AsyncFunctionDeclaration, AsyncFunctionExpression, AsyncGeneratorDeclaration, AsyncGeneratorExpression, MethodDefinition, ArrowFunction, AsyncArrowFunction, ClassDeclaration, or ClassExpression.
Eval code is the source text supplied to the built-in eval function. More precisely, if the parameter to the built-in eval function is a String, it is treated as an ECMAScript Script. The eval code for a particular invocation of eval is the global code portion of that Script.
Function code is source text that is parsed to supply the value of the [[ECMAScriptCode]] and [[FormalParameters]] internal slots (see 10.2) of an ECMAScript function object. The function code of a particular ECMAScript function does not include any source text that is parsed as the function code of a nested FunctionDeclaration, FunctionExpression, GeneratorDeclaration, GeneratorExpression, AsyncFunctionDeclaration, AsyncFunctionExpression, AsyncGeneratorDeclaration, AsyncGeneratorExpression, MethodDefinition, ArrowFunction, AsyncArrowFunction, ClassDeclaration, or ClassExpression.

In addition, if the source text referred to above is parsed as:
- the FormalParameters and FunctionBody of a FunctionDeclaration or FunctionExpression,
- the FormalParameters and GeneratorBody of a GeneratorDeclaration or GeneratorExpression,
- the FormalParameters and AsyncFunctionBody of an AsyncFunctionDeclaration or AsyncFunctionExpression, or
- the FormalParameters and AsyncGeneratorBody of an AsyncGeneratorDeclaration or AsyncGeneratorExpression,
then the source text matched by the BindingIdentifier (if any) of that declaration or expression is also included in the function code of the corresponding function.
Module code is source text that is code that is provided as a ModuleBody. It is the code that is directly evaluated when a module is initialized. The module code of a particular module does not include any source text that is parsed as part of a nested FunctionDeclaration, FunctionExpression, GeneratorDeclaration, GeneratorExpression, AsyncFunctionDeclaration, AsyncFunctionExpression, AsyncGeneratorDeclaration, AsyncGeneratorExpression, MethodDefinition, ArrowFunction, AsyncArrowFunction, ClassDeclaration, or ClassExpression.

Note 1

Function code is generally provided as the bodies of Function Definitions (15.2), Arrow Function Definitions (15.3), Method Definitions (15.4), Generator Function Definitions (15.5), Async Function Definitions (15.8), Async Generator Function Definitions (15.6), and Async Arrow Functions (15.9). Function code is also derived from the arguments to the Function constructor (20.2.1.1), the GeneratorFunction constructor (27.3.1.1), and the AsyncFunction constructor (27.7.1.1).

Note 2

The practical effect of including the BindingIdentifier in function code is that the Early Errors for strict mode code are applied to a BindingIdentifier that is the name of a function whose body contains a "use strict" directive, even if the surrounding code is not strict mode code.

11.2.1 Directive Prologues and the Use Strict Directive

A Directive Prologue is the longest sequence of ExpressionStatements occurring as the initial StatementListItems or ModuleItems of a FunctionBody, a ScriptBody, or a ModuleBody and where each ExpressionStatement in the sequence consists entirely of a StringLiteral token followed by a semicolon. The semicolon may appear explicitly or may be inserted by automatic semicolon insertion (12.10). A Directive Prologue may be an empty sequence.

A Use Strict Directive is an ExpressionStatement in a Directive Prologue whose StringLiteral is either of the exact code point sequences "use strict" or 'use strict'. A Use Strict Directive may not contain an EscapeSequence or LineContinuation.

A Directive Prologue may contain more than one Use Strict Directive. However, an implementation may issue a warning if this occurs.

Note

The ExpressionStatements of a Directive Prologue are evaluated normally during evaluation of the containing production. Implementations may define implementation specific meanings for ExpressionStatements which are not a Use Strict Directive and which occur in a Directive Prologue. If an appropriate notification mechanism exists, an implementation should issue a warning if it encounters in a Directive Prologue an ExpressionStatement that is not a Use Strict Directive and which does not have a meaning defined by the implementation.

11.2.2 Strict Mode Code

An ECMAScript syntactic unit may be processed using either unrestricted or strict mode syntax and semantics (4.3.2). Code is interpreted as strict mode code in the following situations:

Global code is strict mode code if it begins with a Directive Prologue that contains a Use Strict Directive.
Module code is always strict mode code.
All parts of a ClassDeclaration or a ClassExpression are strict mode code.
Eval code is strict mode code if it begins with a Directive Prologue that contains a Use Strict Directive or if the call to eval is a direct eval that is contained in strict mode code.
Function code is strict mode code if the associated FunctionDeclaration, FunctionExpression, GeneratorDeclaration, GeneratorExpression, AsyncFunctionDeclaration, AsyncFunctionExpression, AsyncGeneratorDeclaration, AsyncGeneratorExpression, MethodDefinition, ArrowFunction, or AsyncArrowFunction is contained in strict mode code or if the code that produces the value of the function's [[ECMAScriptCode]] internal slot begins with a Directive Prologue that contains a Use Strict Directive.
Function code that is supplied as the arguments to the built-in Function, Generator, AsyncFunction, and AsyncGenerator constructors is strict mode code if the last argument is a String that when processed is a FunctionBody that begins with a Directive Prologue that contains a Use Strict Directive.

ECMAScript code that is not strict mode code is called non-strict code.

11.2.3 Non-ECMAScript Functions

An ECMAScript implementation may support the evaluation of function exotic objects whose evaluative behaviour is expressed in some host-defined form of executable code other than ECMAScript source text. Whether a function object is defined within ECMAScript code or is a built-in function is not observable from the perspective of ECMAScript code that calls or is called by such a function object.

12 ECMAScript Language: Lexical Grammar

The source text of an ECMAScript Script or Module is first converted into a sequence of input elements, which are tokens, line terminators, comments, or white space. The source text is scanned from left to right, repeatedly taking the longest possible sequence of code points as the next input element.

There are several situations where the identification of lexical input elements is sensitive to the syntactic grammar context that is consuming the input elements. This requires multiple goal symbols for the lexical grammar. The InputElementHashbangOrRegExp goal is used at the start of a Script or Module. The InputElementRegExpOrTemplateTail goal is used in syntactic grammar contexts where a RegularExpressionLiteral, a TemplateMiddle, or a TemplateTail is permitted. The InputElementRegExp goal symbol is used in all syntactic grammar contexts where a RegularExpressionLiteral is permitted but neither a TemplateMiddle, nor a TemplateTail is permitted. The InputElementTemplateTail goal is used in all syntactic grammar contexts where a TemplateMiddle or a TemplateTail is permitted but a RegularExpressionLiteral is not permitted. In all other contexts, InputElementDiv is used as the lexical goal symbol.

Note

The use of multiple lexical goals ensures that there are no lexical ambiguities that would affect automatic semicolon insertion. For example, there are no syntactic grammar contexts where both a leading division or division-assignment, and a leading RegularExpressionLiteral are permitted. This is not affected by semicolon insertion (see 12.10); in examples such as the following:

a = b
/hi/g.exec(c).map(d);

where the first non-whitespace, non-comment code point after a LineTerminator is U+002F (SOLIDUS) and the syntactic context allows division or division-assignment, no semicolon is inserted at the LineTerminator. That is, the above example is interpreted in the same way as:

a = b / hi / g.exec(c).map(d);

Syntax

RegularExpressionLiteral

InputElementRegExpOrTemplateTail

RegularExpressionLiteral

TemplateSubstitutionTail

InputElementTemplateTail

TemplateSubstitutionTail

InputElementHashbangOrRegExp

RegularExpressionLiteral

12.1 Unicode Format-Control Characters

The Unicode format-control characters (i.e., the characters in category “Cf” in the Unicode Character Database such as LEFT-TO-RIGHT MARK or RIGHT-TO-LEFT MARK) are control codes used to control the formatting of a range of text in the absence of higher-level protocols for this (such as mark-up languages).

It is useful to allow format-control characters in source text to facilitate editing and display. All format control characters may be used within comments, and within string literals, template literals, and regular expression literals.

U+200C (ZERO WIDTH NON-JOINER) and U+200D (ZERO WIDTH JOINER) are format-control characters that are used to make necessary distinctions when forming words or phrases in certain languages. In ECMAScript source text these code points may also be used in an IdentifierName after the first character.

U+FEFF (ZERO WIDTH NO-BREAK SPACE) is a format-control character used primarily at the start of a text to mark it as Unicode and to allow detection of the text's encoding and byte order. <ZWNBSP> characters intended for this purpose can sometimes also appear after the start of a text, for example as a result of concatenating files. In ECMAScript source text <ZWNBSP> code points are treated as white space characters (see 12.2).

The special treatment of certain format-control characters outside of comments, string literals, and regular expression literals is summarized in Table 35.

Table 35: Format-Control Code Point Usage

Code Point	Name	Abbreviation	Usage
`U+200C`	ZERO WIDTH NON-JOINER	<ZWNJ>	IdentifierPart
`U+200D`	ZERO WIDTH JOINER	<ZWJ>	IdentifierPart
`U+FEFF`	ZERO WIDTH NO-BREAK SPACE	<ZWNBSP>	WhiteSpace

12.2 White Space

White space code points are used to improve source text readability and to separate tokens (indivisible lexical units) from each other, but are otherwise insignificant. White space code points may occur between any two tokens and at the start or end of input. White space code points may occur within a StringLiteral, a RegularExpressionLiteral, a Template, or a TemplateSubstitutionTail where they are considered significant code points forming part of a literal value. They may also occur within a Comment, but cannot appear within any other kind of token.

The ECMAScript white space code points are listed in Table 36.

Table 36: White Space Code Points

Code Points	Name	Abbreviation
`U+0009`	CHARACTER TABULATION	<TAB>
`U+000B`	LINE TABULATION	<VT>
`U+000C`	FORM FEED (FF)	<FF>
`U+FEFF`	ZERO WIDTH NO-BREAK SPACE	<ZWNBSP>
any code point in general category “Space_Separator”		<USP>

Note 1

U+0020 (SPACE) and U+00A0 (NO-BREAK SPACE) code points are part of <USP>.

Note 2

Other than for the code points listed in Table 36, ECMAScript WhiteSpace intentionally excludes all code points that have the Unicode “White_Space” property but which are not classified in general category “Space_Separator” (“Zs”).

Syntax

WhiteSpace

<TAB>

<VT>

<FF>

<USP>

12.3 Line Terminators

Like white space code points, line terminator code points are used to improve source text readability and to separate tokens (indivisible lexical units) from each other. However, unlike white space code points, line terminators have some influence over the behaviour of the syntactic grammar. In general, line terminators may occur between any two tokens, but there are a few places where they are forbidden by the syntactic grammar. Line terminators also affect the process of automatic semicolon insertion (12.10). A line terminator cannot occur within any token except a StringLiteral, Template, or TemplateSubstitutionTail. <LF> and <CR> line terminators cannot occur within a StringLiteral token except as part of a LineContinuation.

A line terminator can occur within a MultiLineComment but cannot occur within a SingleLineComment.

Line terminators are included in the set of white space code points that are matched by the \s class in regular expressions.

The ECMAScript line terminator code points are listed in Table 37.

Table 37: Line Terminator Code Points

Code Point	Unicode Name	Abbreviation
`U+000A`	LINE FEED (LF)	<LF>
`U+000D`	CARRIAGE RETURN (CR)	<CR>
`U+2028`	LINE SEPARATOR	<LS>
`U+2029`	PARAGRAPH SEPARATOR	<PS>

Only the Unicode code points in Table 37 are treated as line terminators. Other new line or line breaking Unicode code points are not treated as line terminators but are treated as white space if they meet the requirements listed in Table 36. The sequence <CR><LF> is commonly used as a line terminator. It should be considered a single SourceCharacter for the purpose of reporting line numbers.

Syntax

LineTerminator

<LF>

<CR>

<LS>

<PS>

LineTerminatorSequence

<LF>

<CR>

[lookahead ≠ <LF>]

<LS>

<PS>

<CR>

<LF>

12.4 Comments

Comments can be either single or multi-line. Multi-line comments cannot nest.

Because a single-line comment can contain any Unicode code point except a LineTerminator code point, and because of the general rule that a token is always as long as possible, a single-line comment always consists of all code points from the // marker to the end of the line. However, the LineTerminator at the end of the line is not considered to be part of the single-line comment; it is recognized separately by the lexical grammar and becomes part of the stream of input elements for the syntactic grammar. This point is very important, because it implies that the presence or absence of single-line comments does not affect the process of automatic semicolon insertion (see 12.10).

Comments behave like white space and are discarded except that, if a MultiLineComment contains a line terminator code point, then the entire comment is considered to be a LineTerminator for purposes of parsing by the syntactic grammar.

Syntax

MultiLineCommentChars

opt

MultiLineCommentChars

MultiLineNotAsteriskChar

MultiLineCommentChars

opt

PostAsteriskCommentChars

opt

PostAsteriskCommentChars

MultiLineNotForwardSlashOrAsteriskChar

MultiLineCommentChars

opt

PostAsteriskCommentChars

opt

MultiLineNotAsteriskChar

SourceCharacter

but not *

MultiLineNotForwardSlashOrAsteriskChar

SourceCharacter

but not one of / or *

SingleLineComment

SingleLineCommentChars

opt

SingleLineCommentChars

SingleLineCommentChar

SingleLineCommentChars

opt

SingleLineCommentChar

SourceCharacter

but not LineTerminator

A number of productions in this section are given alternative definitions in section B.1.1

12.5 Hashbang Comments

Hashbang Comments are location-sensitive and like other types of comments are discarded from the stream of input elements for the syntactic grammar.

Syntax

HashbangComment

SingleLineCommentChars

opt

12.6 Tokens

Syntax

Note

The DivPunctuator, RegularExpressionLiteral, RightBracePunctuator, and TemplateSubstitutionTail productions derive additional tokens that are not included in the CommonToken production.

12.7 Names and Keywords

IdentifierName and ReservedWord are tokens that are interpreted according to the Default Identifier Syntax given in Unicode Standard Annex #31, Identifier and Pattern Syntax, with some small modifications. ReservedWord is an enumerated subset of IdentifierName. The syntactic grammar defines Identifier as an IdentifierName that is not a ReservedWord. The Unicode identifier grammar is based on character properties specified by the Unicode Standard. The Unicode code points in the specified categories in the latest version of the Unicode Standard must be treated as in those categories by all conforming ECMAScript implementations. ECMAScript implementations may recognize identifier code points defined in later editions of the Unicode Standard.

Note 1

This standard specifies specific code point additions: U+0024 (DOLLAR SIGN) and U+005F (LOW LINE) are permitted anywhere in an IdentifierName, and the code points U+200C (ZERO WIDTH NON-JOINER) and U+200D (ZERO WIDTH JOINER) are permitted anywhere after the first code point of an IdentifierName.

Syntax

UnicodeEscapeSequence

IdentifierPart

IdentifierPartChar

UnicodeEscapeSequence

<ZWNJ>

<ZWJ>

AsciiLetter

one of

UnicodeIDStart

any Unicode code point with the Unicode property “ID_Start”

UnicodeIDContinue

any Unicode code point with the Unicode property “ID_Continue”

The definitions of the nonterminal UnicodeEscapeSequence is given in 12.9.4.

Note 2

The nonterminal IdentifierPart derives _ via UnicodeIDContinue.

Note 3

The sets of code points with Unicode properties “ID_Start” and “ID_Continue” include, respectively, the code points with Unicode properties “Other_ID_Start” and “Other_ID_Continue”.

12.7.1 Identifier Names

Unicode escape sequences are permitted in an IdentifierName, where they contribute a single Unicode code point equal to the IdentifierCodePoint of the UnicodeEscapeSequence. The \ preceding the UnicodeEscapeSequence does not contribute any code points. A UnicodeEscapeSequence cannot be used to contribute a code point to an IdentifierName that would otherwise be invalid. In other words, if a \ UnicodeEscapeSequence sequence were replaced by the SourceCharacter it contributes, the result must still be a valid IdentifierName that has the exact same sequence of SourceCharacter elements as the original IdentifierName. All interpretations of IdentifierName within this specification are based upon their actual code points regardless of whether or not an escape sequence was used to contribute any particular code point.

Two IdentifierNames that are canonically equivalent according to the Unicode Standard are not equal unless, after replacement of each UnicodeEscapeSequence, they are represented by the exact same sequence of code points.

12.7.1.1 Static Semantics: Early Errors

IdentifierStart

UnicodeEscapeSequence

It is a Syntax Error if IdentifierCodePoint of UnicodeEscapeSequence is not some Unicode code point matched by the IdentifierStartChar lexical grammar production.

IdentifierPart

UnicodeEscapeSequence

It is a Syntax Error if IdentifierCodePoint of UnicodeEscapeSequence is not some Unicode code point matched by the IdentifierPartChar lexical grammar production.

12.7.1.2 Static Semantics: IdentifierCodePoints

The syntax-directed operation IdentifierCodePoints takes no arguments and returns a List of code points. It is defined piecewise over the following productions:

IdentifierName

IdentifierStart

Let cp be IdentifierCodePoint of IdentifierStart.
Return « cp ».

IdentifierName

IdentifierPart

Let cps be IdentifierCodePoints of the derived IdentifierName.
Let cp be IdentifierCodePoint of IdentifierPart.
Return the list-concatenation of cps and « cp ».

12.7.1.3 Static Semantics: IdentifierCodePoint

The syntax-directed operation IdentifierCodePoint takes no arguments and returns a code point. It is defined piecewise over the following productions:

IdentifierStart

IdentifierStartChar

Return the code point matched by IdentifierStartChar.

IdentifierPart

IdentifierPartChar

Return the code point matched by IdentifierPartChar.

UnicodeEscapeSequence

Hex4Digits

Return the code point whose numeric value is the MV of Hex4Digits.

UnicodeEscapeSequence

CodePoint

}

Return the code point whose numeric value is the MV of CodePoint.

12.7.2 Keywords and Reserved Words

A keyword is a token that matches IdentifierName, but also has a syntactic use; that is, it appears literally, in a fixed width font, in some syntactic production. The keywords of ECMAScript include if, while, async, await, and many others.

A reserved word is an IdentifierName that cannot be used as an identifier. Many keywords are reserved words, but some are not, and some are reserved only in certain contexts. if and while are reserved words. await is reserved only inside async functions and modules. async is not reserved; it can be used as a variable name or statement label without restriction.

This specification uses a combination of grammatical productions and early error rules to specify which names are valid identifiers and which are reserved words. All tokens in the ReservedWord list below, except for await and yield, are unconditionally reserved. Exceptions for await and yield are specified in 13.1, using parameterized syntactic productions. Lastly, several early error rules restrict the set of valid identifiers. See 13.1.1, 14.3.1.1, 14.7.5.1, and 15.7.1. In summary, there are five categories of identifier names:

Those that are always allowed as identifiers, and are not keywords, such as Math, window, toString, and _;
Those that are never allowed as identifiers, namely the ReservedWords listed below except await and yield;
Those that are contextually allowed as identifiers, namely await and yield;
Those that are contextually disallowed as identifiers, in strict mode code: let, static, implements, interface, package, private, protected, and public;
Those that are always allowed as identifiers, but also appear as keywords within certain syntactic productions, at places where Identifier is not allowed: as, async, from, get, meta, of, set, and target.

The term conditional keyword, or contextual keyword, is sometimes used to refer to the keywords that fall in the last three categories, and thus can be used as identifiers in some contexts and as keywords in others.

Syntax

ReservedWord

one of

await

break

case

catch

class

const

continue

debugger

default

delete

else

enum

export

extends

false

finally

for

function

import

instanceof

new

null

return

super

switch

this

throw

true

try

typeof

var

void

while

with

yield

Note 1

Per 5.1.5, keywords in the grammar match literal sequences of specific SourceCharacter elements. A code point in a keyword cannot be expressed by a \ UnicodeEscapeSequence.

An IdentifierName can contain \ UnicodeEscapeSequences, but it is not possible to declare a variable named "else" by spelling it els\u{65}. The early error rules in 13.1.1 rule out identifiers with the same StringValue as a reserved word.

Note 2

enum is not currently used as a keyword in this specification. It is a future reserved word, set aside for use as a keyword in future language extensions.

Similarly, implements, interface, package, private, protected, and public are future reserved words in strict mode code.

Note 3

The names arguments and eval are not keywords, but they are subject to some restrictions in strict mode code. See 13.1.1, 8.6.4, 15.2.1, 15.5.1, 15.6.1, and 15.8.1.

12.8 Punctuators

Syntax

Punctuator

OptionalChainingPunctuator

OtherPunctuator

OptionalChainingPunctuator

[lookahead ∉ DecimalDigit]

OtherPunctuator

one of

{

(

)

[

]

...

;

===

!==

>>>

**=

<<=

>>=

>>>=

&&=

||=

??=

DivPunctuator

RightBracePunctuator

}

12.9 Literals

12.9.1 Null Literals

Syntax

NullLiteral

null

12.9.2 Boolean Literals

Syntax

BooleanLiteral

true

false

12.9.3 Numeric Literals

Syntax

NumericLiteralSeparator

NumericLiteral

DecimalLiteral

DecimalBigIntegerLiteral

NonDecimalIntegerLiteral

[+Sep]

NonDecimalIntegerLiteral

[+Sep]

BigIntLiteralSuffix

LegacyOctalIntegerLiteral

DecimalBigIntegerLiteral

BigIntLiteralSuffix

NonZeroDigit

DecimalDigits

[+Sep]

opt

BigIntLiteralSuffix

NonZeroDigit

NumericLiteralSeparator

DecimalDigits

[+Sep]

BigIntLiteralSuffix

NonDecimalIntegerLiteral

[Sep]

[?Sep]

[?Sep]

[?Sep]

DecimalIntegerLiteral

DecimalDigits

[+Sep]

opt

ExponentPart

[+Sep]

opt

DecimalDigits

[+Sep]

ExponentPart

[+Sep]

opt

DecimalIntegerLiteral

ExponentPart

[+Sep]

opt

DecimalIntegerLiteral

NonZeroDigit

NumericLiteralSeparator

opt

DecimalDigits

[+Sep]

NonOctalDecimalIntegerLiteral

[Sep]

[?Sep]

[+Sep]

[+Sep]

NumericLiteralSeparator

one of

one of

[Sep]

[?Sep]

one of

[Sep]

[?Sep]

[?Sep]

[?Sep]

[Sep]

[?Sep]

[?Sep]

[Sep]

[?Sep]

[+Sep]

[+Sep]

NumericLiteralSeparator

one of

[Sep]

[?Sep]

[?Sep]

[Sep]

[?Sep]

[+Sep]

[+Sep]

NumericLiteralSeparator

OctalDigit

LegacyOctalIntegerLiteral

OctalDigit

LegacyOctalIntegerLiteral

OctalDigit

NonOctalDecimalIntegerLiteral

NonOctalDigit

LegacyOctalLikeDecimalIntegerLiteral

NonOctalDigit

NonOctalDecimalIntegerLiteral

DecimalDigit

LegacyOctalLikeDecimalIntegerLiteral

OctalDigit

LegacyOctalLikeDecimalIntegerLiteral

one of

one of

[Sep]

[?Sep]

[?Sep]

[Sep]

[?Sep]

[+Sep]

[+Sep]

NumericLiteralSeparator

HexDigit

one of

The SourceCharacter immediately following a NumericLiteral must not be an IdentifierStart or DecimalDigit.

Note

For example: 3in is an error and not the two input elements 3 and in.

12.9.3.1 Static Semantics: Early Errors

NumericLiteral

LegacyOctalIntegerLiteral

DecimalIntegerLiteral

NonOctalDecimalIntegerLiteral

It is a Syntax Error if the source text matched by this production is strict mode code.

Note

In non-strict code, this syntax is Legacy.

12.9.3.2 Static Semantics: MV

A numeric literal stands for a value of the Number type or the BigInt type.

The MV of DecimalLiteral :: DecimalIntegerLiteral . DecimalDigits is the MV of DecimalIntegerLiteral plus (the MV of DecimalDigits × 10^-n), where n is the number of code points in DecimalDigits, excluding all occurrences of NumericLiteralSeparator.
The MV of DecimalLiteral :: DecimalIntegerLiteral . ExponentPart is the MV of DecimalIntegerLiteral × 10^e, where e is the MV of ExponentPart.
The MV of DecimalLiteral :: DecimalIntegerLiteral . DecimalDigits ExponentPart is (the MV of DecimalIntegerLiteral plus (the MV of DecimalDigits × 10^-n)) × 10^e, where n is the number of code points in DecimalDigits, excluding all occurrences of NumericLiteralSeparator and e is the MV of ExponentPart.
The MV of DecimalLiteral :: . DecimalDigits is the MV of DecimalDigits × 10^-n, where n is the number of code points in DecimalDigits, excluding all occurrences of NumericLiteralSeparator.
The MV of DecimalLiteral :: . DecimalDigits ExponentPart is the MV of DecimalDigits × 10^{e - n}, where n is the number of code points in DecimalDigits, excluding all occurrences of NumericLiteralSeparator, and e is the MV of ExponentPart.
The MV of DecimalLiteral :: DecimalIntegerLiteral ExponentPart is the MV of DecimalIntegerLiteral × 10^e, where e is the MV of ExponentPart.
The MV of DecimalIntegerLiteral :: 0 is 0.
The MV of DecimalIntegerLiteral :: NonZeroDigit NumericLiteralSeparatoropt DecimalDigits is (the MV of NonZeroDigit × 10ⁿ) plus the MV of DecimalDigits, where n is the number of code points in DecimalDigits, excluding all occurrences of NumericLiteralSeparator.
The MV of DecimalDigits :: DecimalDigits DecimalDigit is (the MV of DecimalDigits × 10) plus the MV of DecimalDigit.
The MV of DecimalDigits :: DecimalDigits NumericLiteralSeparator DecimalDigit is (the MV of DecimalDigits × 10) plus the MV of DecimalDigit.
The MV of ExponentPart :: ExponentIndicator SignedInteger is the MV of SignedInteger.
The MV of SignedInteger :: - DecimalDigits is the negative of the MV of DecimalDigits.
The MV of DecimalDigit :: 0 or of HexDigit :: 0 or of OctalDigit :: 0 or of LegacyOctalEscapeSequence :: 0 or of BinaryDigit :: 0 is 0.
The MV of DecimalDigit :: 1 or of NonZeroDigit :: 1 or of HexDigit :: 1 or of OctalDigit :: 1 or of BinaryDigit :: 1 is 1.
The MV of DecimalDigit :: 2 or of NonZeroDigit :: 2 or of HexDigit :: 2 or of OctalDigit :: 2 is 2.
The MV of DecimalDigit :: 3 or of NonZeroDigit :: 3 or of HexDigit :: 3 or of OctalDigit :: 3 is 3.
The MV of DecimalDigit :: 4 or of NonZeroDigit :: 4 or of HexDigit :: 4 or of OctalDigit :: 4 is 4.
The MV of DecimalDigit :: 5 or of NonZeroDigit :: 5 or of HexDigit :: 5 or of OctalDigit :: 5 is 5.
The MV of DecimalDigit :: 6 or of NonZeroDigit :: 6 or of HexDigit :: 6 or of OctalDigit :: 6 is 6.
The MV of DecimalDigit :: 7 or of NonZeroDigit :: 7 or of HexDigit :: 7 or of OctalDigit :: 7 is 7.
The MV of DecimalDigit :: 8 or of NonZeroDigit :: 8 or of NonOctalDigit :: 8 or of HexDigit :: 8 is 8.
The MV of DecimalDigit :: 9 or of NonZeroDigit :: 9 or of NonOctalDigit :: 9 or of HexDigit :: 9 is 9.
The MV of HexDigit :: a or of HexDigit :: A is 10.
The MV of HexDigit :: b or of HexDigit :: B is 11.
The MV of HexDigit :: c or of HexDigit :: C is 12.
The MV of HexDigit :: d or of HexDigit :: D is 13.
The MV of HexDigit :: e or of HexDigit :: E is 14.
The MV of HexDigit :: f or of HexDigit :: F is 15.
The MV of BinaryDigits :: BinaryDigits BinaryDigit is (the MV of BinaryDigits × 2) plus the MV of BinaryDigit.
The MV of BinaryDigits :: BinaryDigits NumericLiteralSeparator BinaryDigit is (the MV of BinaryDigits × 2) plus the MV of BinaryDigit.
The MV of OctalDigits :: OctalDigits OctalDigit is (the MV of OctalDigits × 8) plus the MV of OctalDigit.
The MV of OctalDigits :: OctalDigits NumericLiteralSeparator OctalDigit is (the MV of OctalDigits × 8) plus the MV of OctalDigit.
The MV of LegacyOctalIntegerLiteral :: LegacyOctalIntegerLiteral OctalDigit is (the MV of LegacyOctalIntegerLiteral times 8) plus the MV of OctalDigit.
The MV of NonOctalDecimalIntegerLiteral :: LegacyOctalLikeDecimalIntegerLiteral NonOctalDigit is (the MV of LegacyOctalLikeDecimalIntegerLiteral times 10) plus the MV of NonOctalDigit.
The MV of NonOctalDecimalIntegerLiteral :: NonOctalDecimalIntegerLiteral DecimalDigit is (the MV of NonOctalDecimalIntegerLiteral times 10) plus the MV of DecimalDigit.
The MV of LegacyOctalLikeDecimalIntegerLiteral :: LegacyOctalLikeDecimalIntegerLiteral OctalDigit is (the MV of LegacyOctalLikeDecimalIntegerLiteral times 10) plus the MV of OctalDigit.
The MV of HexDigits :: HexDigits HexDigit is (the MV of HexDigits × 16) plus the MV of HexDigit.
The MV of HexDigits :: HexDigits NumericLiteralSeparator HexDigit is (the MV of HexDigits × 16) plus the MV of HexDigit.

12.9.3.3 Static Semantics: NumericValue

The syntax-directed operation NumericValue takes no arguments and returns a Number or a BigInt. It is defined piecewise over the following productions:

NumericLiteral

DecimalLiteral

Return RoundMVResult(MV of DecimalLiteral).

NumericLiteral

NonDecimalIntegerLiteral

Return 𝔽(MV of NonDecimalIntegerLiteral).

NumericLiteral

LegacyOctalIntegerLiteral

Return 𝔽(MV of LegacyOctalIntegerLiteral).

NumericLiteral

NonDecimalIntegerLiteral

BigIntLiteralSuffix

Return the BigInt value for the MV of NonDecimalIntegerLiteral.

DecimalBigIntegerLiteral

BigIntLiteralSuffix

Return 0_ℤ.

DecimalBigIntegerLiteral

NonZeroDigit

BigIntLiteralSuffix

Return the BigInt value for the MV of NonZeroDigit.

DecimalBigIntegerLiteral

NumericLiteralSeparator

DecimalDigits

BigIntLiteralSuffix

Let n be the number of code points in DecimalDigits, excluding all occurrences of NumericLiteralSeparator.
Let mv be (the MV of NonZeroDigit × 10ⁿ) plus the MV of DecimalDigits.
Return ℤ(mv).

12.9.4 String Literals

Note 1

A string literal is 0 or more Unicode code points enclosed in single or double quotes. Unicode code points may also be represented by an escape sequence. All code points may appear literally in a string literal except for the closing quote code points, U+005C (REVERSE SOLIDUS), U+000D (CARRIAGE RETURN), and U+000A (LINE FEED). Any code points may appear in the form of an escape sequence. String literals evaluate to ECMAScript String values. When generating these String values Unicode code points are UTF-16 encoded as defined in 11.1.1. Code points belonging to the Basic Multilingual Plane are encoded as a single code unit element of the string. All other code points are encoded as two code unit elements of the string.

Syntax

StringLiteral

DoubleStringCharacters

opt

SingleStringCharacters

opt

DoubleStringCharacters

DoubleStringCharacter

DoubleStringCharacters

opt

SingleStringCharacters

SingleStringCharacter

SingleStringCharacters

opt

DoubleStringCharacter

SourceCharacter

but not one of " or \ or LineTerminator

<LS>

<PS>

EscapeSequence

LineContinuation

SingleStringCharacter

SourceCharacter

but not one of ' or \ or LineTerminator

<LS>

<PS>

EscapeSequence

LineContinuation

LineTerminatorSequence

EscapeSequence

CharacterEscapeSequence

[lookahead ∉ DecimalDigit]

LegacyOctalEscapeSequence

NonOctalDecimalEscapeSequence

HexEscapeSequence

UnicodeEscapeSequence

CharacterEscapeSequence

SingleEscapeCharacter

NonEscapeCharacter

SingleEscapeCharacter

one of

NonEscapeCharacter

SourceCharacter

but not one of EscapeCharacter or LineTerminator

EscapeCharacter

SingleEscapeCharacter

DecimalDigit

LegacyOctalEscapeSequence

[lookahead ∈ { 8, 9 }]

NonZeroOctalDigit

[lookahead ∉ OctalDigit]

ZeroToThree

OctalDigit

[lookahead ∉ OctalDigit]

but not 0

one of

one of

NonOctalDecimalEscapeSequence

one of

HexEscapeSequence

HexDigit

UnicodeEscapeSequence

}

The definition of the nonterminal HexDigit is given in 12.9.3. SourceCharacter is defined in 11.1.

Note 2

<LF> and <CR> cannot appear in a string literal, except as part of a LineContinuation to produce the empty code points sequence. The proper way to include either in the String value of a string literal is to use an escape sequence such as \n or \u000A.

12.9.4.1 Static Semantics: Early Errors

EscapeSequence

LegacyOctalEscapeSequence

NonOctalDecimalEscapeSequence

It is a Syntax Error if the source text matched by this production is strict mode code.

Note 1

In non-strict code, this syntax is Legacy.

Note 2

It is possible for string literals to precede a Use Strict Directive that places the enclosing code in strict mode, and implementations must take care to enforce the above rules for such literals. For example, the following source text contains a Syntax Error:

function invalid() { "\7"; "use strict"; }

12.9.4.2 Static Semantics: SV

The syntax-directed operation SV takes no arguments and returns a String.

A string literal stands for a value of the String type. SV produces String values for string literals through recursive application on the various parts of the string literal. As part of this process, some Unicode code points within the string literal are interpreted as having a mathematical value, as described below or in 12.9.3.

The SV of StringLiteral :: " " is the empty String.
The SV of StringLiteral :: ' ' is the empty String.
The SV of DoubleStringCharacters :: DoubleStringCharacter DoubleStringCharacters is the string-concatenation of the SV of DoubleStringCharacter and the SV of DoubleStringCharacters.
The SV of SingleStringCharacters :: SingleStringCharacter SingleStringCharacters is the string-concatenation of the SV of SingleStringCharacter and the SV of SingleStringCharacters.
The SV of DoubleStringCharacter :: SourceCharacter but not one of " or \ or LineTerminator is the result of performing UTF16EncodeCodePoint on the code point matched by SourceCharacter.
The SV of DoubleStringCharacter :: <LS> is the String value consisting of the code unit 0x2028 (LINE SEPARATOR).
The SV of DoubleStringCharacter :: <PS> is the String value consisting of the code unit 0x2029 (PARAGRAPH SEPARATOR).
The SV of DoubleStringCharacter :: LineContinuation is the empty String.
The SV of SingleStringCharacter :: SourceCharacter but not one of ' or \ or LineTerminator is the result of performing UTF16EncodeCodePoint on the code point matched by SourceCharacter.
The SV of SingleStringCharacter :: <LS> is the String value consisting of the code unit 0x2028 (LINE SEPARATOR).
The SV of SingleStringCharacter :: <PS> is the String value consisting of the code unit 0x2029 (PARAGRAPH SEPARATOR).
The SV of SingleStringCharacter :: LineContinuation is the empty String.
The SV of EscapeSequence :: 0 is the String value consisting of the code unit 0x0000 (NULL).
The SV of CharacterEscapeSequence :: SingleEscapeCharacter is the String value consisting of the code unit whose numeric value is determined by the SingleEscapeCharacter according to Table 38.

Table 38: String Single Character Escape Sequences

Escape Sequence	Code Unit Value	Unicode Character Name	Symbol
`\b`	`0x0008`	BACKSPACE	<BS>
`\t`	`0x0009`	CHARACTER TABULATION	<HT>
`\n`	`0x000A`	LINE FEED (LF)	<LF>
`\v`	`0x000B`	LINE TABULATION	<VT>
`\f`	`0x000C`	FORM FEED (FF)	<FF>
`\r`	`0x000D`	CARRIAGE RETURN (CR)	<CR>
`\"`	`0x0022`	QUOTATION MARK	`"`
`\'`	`0x0027`	APOSTROPHE	`'`
`\\`	`0x005C`	REVERSE SOLIDUS	`\`

The SV of NonEscapeCharacter :: SourceCharacter but not one of EscapeCharacter or LineTerminator is the result of performing UTF16EncodeCodePoint on the code point matched by SourceCharacter.
The SV of EscapeSequence :: LegacyOctalEscapeSequence is the String value consisting of the code unit whose numeric value is the MV of LegacyOctalEscapeSequence.
The SV of NonOctalDecimalEscapeSequence :: 8 is the String value consisting of the code unit 0x0038 (DIGIT EIGHT).
The SV of NonOctalDecimalEscapeSequence :: 9 is the String value consisting of the code unit 0x0039 (DIGIT NINE).
The SV of HexEscapeSequence :: x HexDigit HexDigit is the String value consisting of the code unit whose numeric value is the MV of HexEscapeSequence.
The SV of Hex4Digits :: HexDigit HexDigit HexDigit HexDigit is the String value consisting of the code unit whose numeric value is the MV of Hex4Digits.
The SV of UnicodeEscapeSequence :: u{ CodePoint } is the result of performing UTF16EncodeCodePoint on the MV of CodePoint.
The SV of TemplateEscapeSequence :: 0 is the String value consisting of the code unit 0x0000 (NULL).

12.9.4.3 Static Semantics: MV

The MV of LegacyOctalEscapeSequence :: ZeroToThree OctalDigit is (8 times the MV of ZeroToThree) plus the MV of OctalDigit.
The MV of LegacyOctalEscapeSequence :: FourToSeven OctalDigit is (8 times the MV of FourToSeven) plus the MV of OctalDigit.
The MV of LegacyOctalEscapeSequence :: ZeroToThree OctalDigit OctalDigit is (64 (that is, 8²) times the MV of ZeroToThree) plus (8 times the MV of the first OctalDigit) plus the MV of the second OctalDigit.
The MV of ZeroToThree :: 0 is 0.
The MV of ZeroToThree :: 1 is 1.
The MV of ZeroToThree :: 2 is 2.
The MV of ZeroToThree :: 3 is 3.
The MV of FourToSeven :: 4 is 4.
The MV of FourToSeven :: 5 is 5.
The MV of FourToSeven :: 6 is 6.
The MV of FourToSeven :: 7 is 7.
The MV of HexEscapeSequence :: x HexDigit HexDigit is (16 times the MV of the first HexDigit) plus the MV of the second HexDigit.
The MV of Hex4Digits :: HexDigit HexDigit HexDigit HexDigit is (0x1000 × the MV of the first HexDigit) plus (0x100 × the MV of the second HexDigit) plus (0x10 × the MV of the third HexDigit) plus the MV of the fourth HexDigit.

12.9.5 Regular Expression Literals

Note 1

A regular expression literal is an input element that is converted to a RegExp object (see 22.2) each time the literal is evaluated. Two regular expression literals in a program evaluate to regular expression objects that never compare as === to each other even if the two literals' contents are identical. A RegExp object may also be created at runtime by new RegExp or calling the RegExp constructor as a function (see 22.2.4).

The productions below describe the syntax for a regular expression literal and are used by the input element scanner to find the end of the regular expression literal. The source text comprising the RegularExpressionBody and the RegularExpressionFlags are subsequently parsed again using the more stringent ECMAScript Regular Expression grammar (22.2.1).

An implementation may extend the ECMAScript Regular Expression grammar defined in 22.2.1, but it must not extend the RegularExpressionBody and RegularExpressionFlags productions defined below or the productions used by these productions.

Syntax

RegularExpressionLiteral

RegularExpressionBody

RegularExpressionFlags

RegularExpressionBody

RegularExpressionFirstChar

RegularExpressionChars

[empty]

RegularExpressionChars

RegularExpressionChar

RegularExpressionFirstChar

RegularExpressionNonTerminator

but not one of * or \ or / or [

RegularExpressionBackslashSequence

RegularExpressionClass

RegularExpressionChar

RegularExpressionNonTerminator

but not one of \ or / or [

RegularExpressionBackslashSequence

RegularExpressionClass

RegularExpressionBackslashSequence

RegularExpressionNonTerminator

SourceCharacter

but not LineTerminator

RegularExpressionClass

[

RegularExpressionClassChars

]

RegularExpressionClassChars

[empty]

RegularExpressionClassChars

RegularExpressionClassChar

RegularExpressionNonTerminator

but not one of ] or \

RegularExpressionBackslashSequence

RegularExpressionFlags

[empty]

RegularExpressionFlags

IdentifierPartChar

Note 2

Regular expression literals may not be empty; instead of representing an empty regular expression literal, the code unit sequence // starts a single-line comment. To specify an empty regular expression, use: /(?:)/.

12.9.5.1 Static Semantics: BodyText

The syntax-directed operation BodyText takes no arguments and returns source text. It is defined piecewise over the following productions:

RegularExpressionLiteral

RegularExpressionBody

RegularExpressionFlags

Return the source text that was recognized as RegularExpressionBody.

12.9.5.2 Static Semantics: FlagText

The syntax-directed operation FlagText takes no arguments and returns source text. It is defined piecewise over the following productions:

RegularExpressionLiteral

RegularExpressionBody

RegularExpressionFlags

Return the source text that was recognized as RegularExpressionFlags.

12.9.6 Template Literal Lexical Components

Syntax

Template

NoSubstitutionTemplate

TemplateHead

NoSubstitutionTemplate

TemplateCharacters

opt

TemplateHead

TemplateCharacters

opt

TemplateSubstitutionTail

}

opt

}

opt

opt

[lookahead ≠ {]

TemplateEscapeSequence

NotEscapeSequence

LineContinuation

LineTerminatorSequence

SourceCharacter

but not one of ` or \ or $ or LineTerminator

TemplateEscapeSequence

CharacterEscapeSequence

[lookahead ∉ DecimalDigit]

HexEscapeSequence

UnicodeEscapeSequence

NotEscapeSequence

DecimalDigit

but not 0

[lookahead ∉ HexDigit]

HexDigit

[lookahead ∉ HexDigit]

[lookahead ≠ {]

HexDigit

[lookahead ∉ HexDigit]

HexDigit

[lookahead ∉ HexDigit]

HexDigit

[lookahead ∉ HexDigit]

{

[lookahead ∉ HexDigit]

{

NotCodePoint

[lookahead ∉ HexDigit]

{

CodePoint

[lookahead ∉ HexDigit]

[lookahead ≠ }]

NotCodePoint

HexDigits

[~Sep]

but only if MV of HexDigits > 0x10FFFF

CodePoint

HexDigits

[~Sep]

but only if MV of HexDigits ≤ 0x10FFFF

Note

TemplateSubstitutionTail is used by the InputElementTemplateTail alternative lexical goal.

12.9.6.1 Static Semantics: TV

The syntax-directed operation TV takes no arguments and returns a String or undefined. A template literal component is interpreted by TV as a value of the String type. TV is used to construct the indexed components of a template object (colloquially, the template values). In TV, escape sequences are replaced by the UTF-16 code unit(s) of the Unicode code point represented by the escape sequence.

The TV of NoSubstitutionTemplate :: ` ` is the empty String.
The TV of TemplateHead :: ` ${ is the empty String.
The TV of TemplateMiddle :: } ${ is the empty String.
The TV of TemplateTail :: } ` is the empty String.
The TV of TemplateCharacters :: TemplateCharacter TemplateCharacters is undefined if the TV of TemplateCharacter is undefined or the TV of TemplateCharacters is undefined. Otherwise, it is the string-concatenation of the TV of TemplateCharacter and the TV of TemplateCharacters.
The TV of TemplateCharacter :: SourceCharacter but not one of ` or \ or $ or LineTerminator is the result of performing UTF16EncodeCodePoint on the code point matched by SourceCharacter.
The TV of TemplateCharacter :: $ is the String value consisting of the code unit 0x0024 (DOLLAR SIGN).
The TV of TemplateCharacter :: \ TemplateEscapeSequence is the SV of TemplateEscapeSequence.
The TV of TemplateCharacter :: \ NotEscapeSequence is undefined.
The TV of TemplateCharacter :: LineTerminatorSequence is the TRV of LineTerminatorSequence.
The TV of LineContinuation :: \ LineTerminatorSequence is the empty String.

12.9.6.2 Static Semantics: TRV

The syntax-directed operation TRV takes no arguments and returns a String. A template literal component is interpreted by TRV as a value of the String type. TRV is used to construct the raw components of a template object (colloquially, the template raw values). TRV is similar to TV with the difference being that in TRV, escape sequences are interpreted as they appear in the literal.

The TRV of NoSubstitutionTemplate :: ` ` is the empty String.
The TRV of TemplateHead :: ` ${ is the empty String.
The TRV of TemplateMiddle :: } ${ is the empty String.
The TRV of TemplateTail :: } ` is the empty String.
The TRV of TemplateCharacters :: TemplateCharacter TemplateCharacters is the string-concatenation of the TRV of TemplateCharacter and the TRV of TemplateCharacters.
The TRV of TemplateCharacter :: SourceCharacter but not one of ` or \ or $ or LineTerminator is the result of performing UTF16EncodeCodePoint on the code point matched by SourceCharacter.
The TRV of TemplateCharacter :: $ is the String value consisting of the code unit 0x0024 (DOLLAR SIGN).
The TRV of TemplateCharacter :: \ TemplateEscapeSequence is the string-concatenation of the code unit 0x005C (REVERSE SOLIDUS) and the TRV of TemplateEscapeSequence.
The TRV of TemplateCharacter :: \ NotEscapeSequence is the string-concatenation of the code unit 0x005C (REVERSE SOLIDUS) and the TRV of NotEscapeSequence.
The TRV of TemplateEscapeSequence :: 0 is the String value consisting of the code unit 0x0030 (DIGIT ZERO).
The TRV of NotEscapeSequence :: 0 DecimalDigit is the string-concatenation of the code unit 0x0030 (DIGIT ZERO) and the TRV of DecimalDigit.
The TRV of NotEscapeSequence :: x [lookahead ∉ HexDigit] is the String value consisting of the code unit 0x0078 (LATIN SMALL LETTER X).
The TRV of NotEscapeSequence :: x HexDigit [lookahead ∉ HexDigit] is the string-concatenation of the code unit 0x0078 (LATIN SMALL LETTER X) and the TRV of HexDigit.
The TRV of NotEscapeSequence :: u [lookahead ∉ HexDigit] [lookahead ≠ {] is the String value consisting of the code unit 0x0075 (LATIN SMALL LETTER U).
The TRV of NotEscapeSequence :: u HexDigit [lookahead ∉ HexDigit] is the string-concatenation of the code unit 0x0075 (LATIN SMALL LETTER U) and the TRV of HexDigit.
The TRV of NotEscapeSequence :: u HexDigit HexDigit [lookahead ∉ HexDigit] is the string-concatenation of the code unit 0x0075 (LATIN SMALL LETTER U), the TRV of the first HexDigit, and the TRV of the second HexDigit.
The TRV of NotEscapeSequence :: u HexDigit HexDigit HexDigit [lookahead ∉ HexDigit] is the string-concatenation of the code unit 0x0075 (LATIN SMALL LETTER U), the TRV of the first HexDigit, the TRV of the second HexDigit, and the TRV of the third HexDigit.
The TRV of NotEscapeSequence :: u { [lookahead ∉ HexDigit] is the string-concatenation of the code unit 0x0075 (LATIN SMALL LETTER U) and the code unit 0x007B (LEFT CURLY BRACKET).
The TRV of NotEscapeSequence :: u { NotCodePoint [lookahead ∉ HexDigit] is the string-concatenation of the code unit 0x0075 (LATIN SMALL LETTER U), the code unit 0x007B (LEFT CURLY BRACKET), and the TRV of NotCodePoint.
The TRV of NotEscapeSequence :: u { CodePoint [lookahead ∉ HexDigit] [lookahead ≠ }] is the string-concatenation of the code unit 0x0075 (LATIN SMALL LETTER U), the code unit 0x007B (LEFT CURLY BRACKET), and the TRV of CodePoint.
The TRV of DecimalDigit :: one of 0 1 2 3 4 5 6 7 8 9 is the result of performing UTF16EncodeCodePoint on the single code point matched by this production.
The TRV of CharacterEscapeSequence :: NonEscapeCharacter is the SV of NonEscapeCharacter.
The TRV of SingleEscapeCharacter :: one of ' " \ b f n r t v is the result of performing UTF16EncodeCodePoint on the single code point matched by this production.
The TRV of HexEscapeSequence :: x HexDigit HexDigit is the string-concatenation of the code unit 0x0078 (LATIN SMALL LETTER X), the TRV of the first HexDigit, and the TRV of the second HexDigit.
The TRV of UnicodeEscapeSequence :: u Hex4Digits is the string-concatenation of the code unit 0x0075 (LATIN SMALL LETTER U) and the TRV of Hex4Digits.
The TRV of UnicodeEscapeSequence :: u{ CodePoint } is the string-concatenation of the code unit 0x0075 (LATIN SMALL LETTER U), the code unit 0x007B (LEFT CURLY BRACKET), the TRV of CodePoint, and the code unit 0x007D (RIGHT CURLY BRACKET).
The TRV of Hex4Digits :: HexDigit HexDigit HexDigit HexDigit is the string-concatenation of the TRV of the first HexDigit, the TRV of the second HexDigit, the TRV of the third HexDigit, and the TRV of the fourth HexDigit.
The TRV of HexDigits :: HexDigits HexDigit is the string-concatenation of the TRV of HexDigits and the TRV of HexDigit.
The TRV of HexDigit :: one of 0 1 2 3 4 5 6 7 8 9 a b c d e f A B C D E F is the result of performing UTF16EncodeCodePoint on the single code point matched by this production.
The TRV of LineContinuation :: \ LineTerminatorSequence is the string-concatenation of the code unit 0x005C (REVERSE SOLIDUS) and the TRV of LineTerminatorSequence.
The TRV of LineTerminatorSequence :: <LF> is the String value consisting of the code unit 0x000A (LINE FEED).
The TRV of LineTerminatorSequence :: <CR> is the String value consisting of the code unit 0x000A (LINE FEED).
The TRV of LineTerminatorSequence :: <LS> is the String value consisting of the code unit 0x2028 (LINE SEPARATOR).
The TRV of LineTerminatorSequence :: <PS> is the String value consisting of the code unit 0x2029 (PARAGRAPH SEPARATOR).
The TRV of LineTerminatorSequence :: <CR> <LF> is the String value consisting of the code unit 0x000A (LINE FEED).

Note

TV excludes the code units of LineContinuation while TRV includes them. <CR><LF> and <CR> LineTerminatorSequences are normalized to <LF> for both TV and TRV. An explicit TemplateEscapeSequence is needed to include a <CR> or <CR><LF> sequence.

12.10 Automatic Semicolon Insertion

Most ECMAScript statements and declarations must be terminated with a semicolon. Such semicolons may always appear explicitly in the source text. For convenience, however, such semicolons may be omitted from the source text in certain situations. These situations are described by saying that semicolons are automatically inserted into the source code token stream in those situations.

12.10.1 Rules of Automatic Semicolon Insertion

In the following rules, “token” means the actual recognized lexical token determined using the current lexical goal symbol as described in clause 12.

There are three basic rules of semicolon insertion:

When, as the source text is parsed from left to right, a token (called the offending token) is encountered that is not allowed by any production of the grammar, then a semicolon is automatically inserted before the offending token if one or more of the following conditions is true:
- The offending token is separated from the previous token by at least one LineTerminator.
- The offending token is }.
- The previous token is ) and the inserted semicolon would then be parsed as the terminating semicolon of a do-while statement (14.7.2).
When, as the source text is parsed from left to right, the end of the input stream of tokens is encountered and the parser is unable to parse the input token stream as a single instance of the goal nonterminal, then a semicolon is automatically inserted at the end of the input stream.
When, as the source text is parsed from left to right, a token is encountered that is allowed by some production of the grammar, but the production is a restricted production and the token would be the first token for a terminal or nonterminal immediately following the annotation “[no LineTerminator here]” within the restricted production (and therefore such a token is called a restricted token), and the restricted token is separated from the previous token by at least one LineTerminator, then a semicolon is automatically inserted before the restricted token.

However, there is an additional overriding condition on the preceding rules: a semicolon is never inserted automatically if the semicolon would then be parsed as an empty statement or if that semicolon would become one of the two semicolons in the header of a for statement (see 14.7.4).

Note

The following are the only restricted productions in the grammar:

UpdateExpression

[Yield, Await]

LeftHandSideExpression

[?Yield, ?Await]

[no LineTerminator here]

LeftHandSideExpression

[?Yield, ?Await]

[no LineTerminator here]

ContinueStatement

[Yield, Await]

continue

;

continue

[no LineTerminator here]

LabelIdentifier

[?Yield, ?Await]

;

BreakStatement

[Yield, Await]

break

;

break

[no LineTerminator here]

LabelIdentifier

[?Yield, ?Await]

;

ReturnStatement

[Yield, Await]

return

;

return

[no LineTerminator here]

Expression

[+In, ?Yield, ?Await]

;

ThrowStatement

[Yield, Await]

throw

[no LineTerminator here]

Expression

[+In, ?Yield, ?Await]

;

YieldExpression

[In, Await]

yield

[no LineTerminator here]

AssignmentExpression

[?In, +Yield, ?Await]

yield

[no LineTerminator here]

AssignmentExpression

[?In, +Yield, ?Await]

ArrowFunction

[In, Yield, Await]

ArrowParameters

[?Yield, ?Await]

[no LineTerminator here]

ConciseBody

[?In]

AsyncFunctionDeclaration

[Yield, Await, Default]

async

[no LineTerminator here]

function

BindingIdentifier

[?Yield, ?Await]

(

FormalParameters

[~Yield, +Await]

)

{

AsyncFunctionBody

}

[+Default]

async

[no LineTerminator here]

function

(

FormalParameters

[~Yield, +Await]

)

{

AsyncFunctionBody

}

AsyncFunctionExpression

async

[no LineTerminator here]

function

BindingIdentifier

[~Yield, +Await]

opt

(

FormalParameters

[~Yield, +Await]

)

{

AsyncFunctionBody

}

AsyncMethod

[Yield, Await]

async

[no LineTerminator here]

ClassElementName

[?Yield, ?Await]

(

UniqueFormalParameters

[~Yield, +Await]

)

{

AsyncFunctionBody

}

AsyncGeneratorDeclaration

[Yield, Await, Default]

async

[no LineTerminator here]

function

BindingIdentifier

[?Yield, ?Await]

(

FormalParameters

[+Yield, +Await]

)

{

AsyncGeneratorBody

}

[+Default]

async

[no LineTerminator here]

function

(

FormalParameters

[+Yield, +Await]

)

{

AsyncGeneratorBody

}

AsyncGeneratorExpression

async

[no LineTerminator here]

function

BindingIdentifier

[+Yield, +Await]

opt

(

FormalParameters

[+Yield, +Await]

)

{

AsyncGeneratorBody

}

AsyncGeneratorMethod

[Yield, Await]

async

[no LineTerminator here]

ClassElementName

[?Yield, ?Await]

(

UniqueFormalParameters

[+Yield, +Await]

)

{

AsyncGeneratorBody

}

AsyncArrowFunction

[In, Yield, Await]

async

[no LineTerminator here]

AsyncArrowBindingIdentifier

[?Yield]

[no LineTerminator here]

AsyncConciseBody

[?In]

CoverCallExpressionAndAsyncArrowHead

[?Yield, ?Await]

[no LineTerminator here]

AsyncConciseBody

[?In]

AsyncArrowHead

async

[no LineTerminator here]

ArrowFormalParameters

[~Yield, +Await]

The practical effect of these restricted productions is as follows:

When a ++ or -- token is encountered where the parser would treat it as a postfix operator, and at least one LineTerminator occurred between the preceding token and the ++ or -- token, then a semicolon is automatically inserted before the ++ or -- token.
When a continue, break, return, throw, or yield token is encountered and a LineTerminator is encountered before the next token, a semicolon is automatically inserted after the continue, break, return, throw, or yield token.
When arrow function parameter(s) are followed by a LineTerminator before a => token, a semicolon is automatically inserted and the punctuator causes a syntax error.
When an async token is followed by a LineTerminator before a function or IdentifierName or ( token, a semicolon is automatically inserted and the async token is not treated as part of the same expression or class element as the following tokens.
When an async token is followed by a LineTerminator before a * token, a semicolon is automatically inserted and the punctuator causes a syntax error.

The resulting practical advice to ECMAScript programmers is:

A postfix ++ or -- operator should be on the same line as its operand.
An Expression in a return or throw statement or an AssignmentExpression in a yield expression should start on the same line as the return, throw, or yield token.
A LabelIdentifier in a break or continue statement should be on the same line as the break or continue token.
The end of an arrow function's parameter(s) and its => should be on the same line.
The async token preceding an asynchronous function or method should be on the same line as the immediately following token.

12.10.2 Examples of Automatic Semicolon Insertion

This section is non-normative.

The source

{ 1 2 } 3

is not a valid sentence in the ECMAScript grammar, even with the automatic semicolon insertion rules. In contrast, the source

{ 1
2 } 3

is also not a valid ECMAScript sentence, but is transformed by automatic semicolon insertion into the following:

{ 1
;2 ;} 3;

which is a valid ECMAScript sentence.

The source

for (a; b
)

is not a valid ECMAScript sentence and is not altered by automatic semicolon insertion because the semicolon is needed for the header of a for statement. Automatic semicolon insertion never inserts one of the two semicolons in the header of a for statement.

The source

return
a + b

is transformed by automatic semicolon insertion into the following:

return;
a + b;

Note 1

The expression a + b is not treated as a value to be returned by the return statement, because a LineTerminator separates it from the token return.

The source

a = b
++c

is transformed by automatic semicolon insertion into the following:

a = b;
++c;

Note 2

The token ++ is not treated as a postfix operator applying to the variable b, because a LineTerminator occurs between b and ++.

The source

if (a > b)
else c = d

is not a valid ECMAScript sentence and is not altered by automatic semicolon insertion before the else token, even though no production of the grammar applies at that point, because an automatically inserted semicolon would then be parsed as an empty statement.

The source

a = b + c
(d + e).print()

is not transformed by automatic semicolon insertion, because the parenthesized expression that begins the second line can be interpreted as an argument list for a function call:

a = b + c(d + e).print()

In the circumstance that an assignment statement must begin with a left parenthesis, it is a good idea for the programmer to provide an explicit semicolon at the end of the preceding statement rather than to rely on automatic semicolon insertion.

12.10.3 Interesting Cases of Automatic Semicolon Insertion

This section is non-normative.

ECMAScript programs can be written in a style with very few semicolons by relying on automatic semicolon insertion. As described above, semicolons are not inserted at every newline, and automatic semicolon insertion can depend on multiple tokens across line terminators.

As new syntactic features are added to ECMAScript, additional grammar productions could be added that cause lines relying on automatic semicolon insertion preceding them to change grammar productions when parsed.

For the purposes of this section, a case of automatic semicolon insertion is considered interesting if it is a place where a semicolon may or may not be inserted, depending on the source text which precedes it. The rest of this section describes a number of interesting cases of automatic semicolon insertion in this version of ECMAScript.

12.10.3.1 Interesting Cases of Automatic Semicolon Insertion in Statement Lists

In a StatementList, many StatementListItems end in semicolons, which may be omitted using automatic semicolon insertion. As a consequence of the rules above, at the end of a line ending an expression, a semicolon is required if the following line begins with any of the following:

An opening parenthesis ((). Without a semicolon, the two lines together are treated as a CallExpression.
An opening square bracket ([). Without a semicolon, the two lines together are treated as property access, rather than an ArrayLiteral or ArrayAssignmentPattern.
A template literal (`). Without a semicolon, the two lines together are interpreted as a tagged Template (13.3.11), with the previous expression as the MemberExpression.
Unary + or -. Without a semicolon, the two lines together are interpreted as a usage of the corresponding binary operator.
A RegExp literal. Without a semicolon, the two lines together may be parsed instead as the / MultiplicativeOperator, for example if the RegExp has flags.

12.10.3.2 Cases of Automatic Semicolon Insertion and “[no LineTerminator here]”

This section is non-normative.

ECMAScript contains grammar productions which include “[no LineTerminator here]”. These productions are sometimes a means to have optional operands in the grammar. Introducing a LineTerminator in these locations would change the grammar production of a source text by using the grammar production without the optional operand.

The rest of this section describes a number of productions using “[no LineTerminator here]” in this version of ECMAScript.

12.10.3.2.1 List of Grammar Productions with Optional Operands and “[no LineTerminator here]”

UpdateExpression.
ContinueStatement.
BreakStatement.
ReturnStatement.
YieldExpression.
Async Function Definitions (15.8) with relation to Function Definitions (15.2)

13 ECMAScript Language: Expressions

13.1 Identifiers

Syntax

IdentifierReference

[Yield, Await]

Identifier

[~Yield]

yield

[~Await]

await

BindingIdentifier

[Yield, Await]

Identifier

yield

await

LabelIdentifier

[Yield, Await]

Identifier

[~Yield]

yield

[~Await]

await

Identifier

IdentifierName

but not ReservedWord

Note

yield and await are permitted as BindingIdentifier in the grammar, and prohibited with static semantics below, to prohibit automatic semicolon insertion in cases such as

let
await 0;

13.1.1 Static Semantics: Early Errors

BindingIdentifier

Identifier

It is a Syntax Error if the source text matched by this production is contained in strict mode code and the StringValue of Identifier is either "arguments" or "eval".

IdentifierReference

yield

BindingIdentifier

yield

LabelIdentifier

yield

It is a Syntax Error if the source text matched by this production is contained in strict mode code.

IdentifierReference

await

BindingIdentifier

await

LabelIdentifier

await

It is a Syntax Error if the goal symbol of the syntactic grammar is Module.

BindingIdentifier

[Yield, Await]

yield

It is a Syntax Error if this production has a _[Yield] parameter.

BindingIdentifier

[Yield, Await]

await

It is a Syntax Error if this production has an _[Await] parameter.

[Yield, Await]

[Yield, Await]

[Yield, Await]

It is a Syntax Error if this production has a _[Yield] parameter and StringValue of Identifier is "yield".
It is a Syntax Error if this production has an _[Await] parameter and StringValue of Identifier is "await".

Identifier

IdentifierName

but not ReservedWord

It is a Syntax Error if this phrase is contained in strict mode code and the StringValue of IdentifierName is one of "implements", "interface", "let", "package", "private", "protected", "public", "static", or "yield".
It is a Syntax Error if the goal symbol of the syntactic grammar is Module and the StringValue of IdentifierName is "await".
It is a Syntax Error if the StringValue of IdentifierName is the StringValue of any ReservedWord except for yield or await.

Note

StringValue of IdentifierName normalizes any Unicode escape sequences in IdentifierName hence such escapes cannot be used to write an Identifier whose code point sequence is the same as a ReservedWord.

13.1.2 Static Semantics: StringValue

The syntax-directed operation StringValue takes no arguments and returns a String. It is defined piecewise over the following productions:

Let idTextUnescaped be IdentifierCodePoints of IdentifierName.
Return CodePointsToString(idTextUnescaped).

IdentifierReference

yield

BindingIdentifier

yield

LabelIdentifier

yield

Return "yield".

IdentifierReference

await

BindingIdentifier

await

LabelIdentifier

await

Return "await".

Identifier

IdentifierName

but not ReservedWord

Return the StringValue of IdentifierName.

PrivateIdentifier

IdentifierName

Return the string-concatenation of 0x0023 (NUMBER SIGN) and the StringValue of IdentifierName.

ModuleExportName

StringLiteral

Return the SV of StringLiteral.

13.1.3 Runtime Semantics: Evaluation

IdentifierReference

Identifier

Return ? ResolveBinding(StringValue of Identifier).

IdentifierReference

yield

Return ? ResolveBinding("yield").

IdentifierReference

await

Return ? ResolveBinding("await").

Note 1

The result of evaluating an IdentifierReference is always a value of type Reference.

Note 2

In non-strict code, the keyword yield may be used as an identifier. Evaluating the IdentifierReference resolves the binding of yield as if it was an Identifier. Early Error restriction ensures that such an evaluation only can occur for non-strict code.

13.2 Primary Expression

Syntax

PrimaryExpression

[Yield, Await]

this

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

AsyncFunctionExpression

AsyncGeneratorExpression

RegularExpressionLiteral

TemplateLiteral

[?Yield, ?Await, ~Tagged]

CoverParenthesizedExpressionAndArrowParameterList

[?Yield, ?Await]

CoverParenthesizedExpressionAndArrowParameterList

[Yield, Await]

(

Expression

[+In, ?Yield, ?Await]

)

(

Expression

[+In, ?Yield, ?Await]

)

(

)

(

...

BindingIdentifier

[?Yield, ?Await]

)

(

...

BindingPattern

[?Yield, ?Await]

)

(

Expression

[+In, ?Yield, ?Await]

...

BindingIdentifier

[?Yield, ?Await]

)

(

Expression

[+In, ?Yield, ?Await]

...

BindingPattern

[?Yield, ?Await]

)

Supplemental Syntax

When processing an instance of the production
PrimaryExpression[Yield, Await] : CoverParenthesizedExpressionAndArrowParameterList[?Yield, ?Await]
the interpretation of CoverParenthesizedExpressionAndArrowParameterList is refined using the following grammar:

ParenthesizedExpression

[Yield, Await]

(

Expression

[+In, ?Yield, ?Await]

)

13.2.1 The `this` Keyword

13.2.1.1 Runtime Semantics: Evaluation

PrimaryExpression

this

Return ? ResolveThisBinding().

13.2.2 Identifier Reference

See 13.1 for IdentifierReference.

13.2.3 Literals

Syntax

13.2.3.1 Runtime Semantics: Evaluation

Literal

NullLiteral

Return null.

Literal

BooleanLiteral

If BooleanLiteral is the token false, return false.
If BooleanLiteral is the token true, return true.

Literal

NumericLiteral

Return the NumericValue of NumericLiteral as defined in 12.9.3.

Literal

StringLiteral

Return the SV of StringLiteral as defined in 12.9.4.2.

13.2.4 Array Initializer

Note

An ArrayLiteral is an expression describing the initialization of an Array, using a list, of zero or more expressions each of which represents an array element, enclosed in square brackets. The elements need not be literals; they are evaluated each time the array initializer is evaluated.

Array elements may be elided at the beginning, middle or end of the element list. Whenever a comma in the element list is not preceded by an AssignmentExpression (i.e., a comma at the beginning or after another comma), the missing array element contributes to the length of the Array and increases the index of subsequent elements. Elided array elements are not defined. If an element is elided at the end of an array, that element does not contribute to the length of the Array.

Syntax

ArrayLiteral

[Yield, Await]

[

Elision

opt

]

[

ElementList

[?Yield, ?Await]

]

[

ElementList

[?Yield, ?Await]

Elision

opt

]

ElementList

[Yield, Await]

Elision

opt

AssignmentExpression

[+In, ?Yield, ?Await]

opt

[?Yield, ?Await]

[?Yield, ?Await]

opt

[+In, ?Yield, ?Await]

[?Yield, ?Await]

opt

[?Yield, ?Await]

[Yield, Await]

...

AssignmentExpression

[+In, ?Yield, ?Await]

13.2.4.1 Runtime Semantics: ArrayAccumulation

The syntax-directed operation ArrayAccumulation takes arguments array (an Array) and nextIndex (an integer) and returns either a normal completion containing an integer or an abrupt completion. It is defined piecewise over the following productions:

Elision

Let len be nextIndex + 1.
Perform ? Set(array, "length", 𝔽(len), true).
NOTE: The above step throws if len exceeds 2³² - 1.
Return len.

Elision

Return ? ArrayAccumulation of Elision with arguments array and (nextIndex + 1).

ElementList

Elision

opt

AssignmentExpression

1. If Elision is present, then
1. Set nextIndex to ? ArrayAccumulation of Elision with arguments array and nextIndex.
Let initResult be ? Evaluation of AssignmentExpression.
Let initValue be ? GetValue(initResult).
Perform ! CreateDataPropertyOrThrow(array, ! ToString(𝔽(nextIndex)), initValue).
Return nextIndex + 1.

ElementList

Elision

opt

SpreadElement

1. If Elision is present, then
1. Set nextIndex to ? ArrayAccumulation of Elision with arguments array and nextIndex.
Return ? ArrayAccumulation of SpreadElement with arguments array and nextIndex.

ElementList

Elision

opt

AssignmentExpression

Set nextIndex to ? ArrayAccumulation of ElementList with arguments array and nextIndex.
2. If Elision is present, then
1. Set nextIndex to ? ArrayAccumulation of Elision with arguments array and nextIndex.
Let initResult be ? Evaluation of AssignmentExpression.
Let initValue be ? GetValue(initResult).
Perform ! CreateDataPropertyOrThrow(array, ! ToString(𝔽(nextIndex)), initValue).
Return nextIndex + 1.

ElementList

Elision

opt

SpreadElement

Set nextIndex to ? ArrayAccumulation of ElementList with arguments array and nextIndex.
2. If Elision is present, then
1. Set nextIndex to ? ArrayAccumulation of Elision with arguments array and nextIndex.
Return ? ArrayAccumulation of SpreadElement with arguments array and nextIndex.

SpreadElement

...

AssignmentExpression

Let spreadRef be ? Evaluation of AssignmentExpression.
Let spreadObj be ? GetValue(spreadRef).
Let iteratorRecord be ? GetIterator(spreadObj, sync).
4. Repeat,
1. Let next be ? IteratorStepValue(iteratorRecord).
2. If next is done, return nextIndex.
3. Perform ! CreateDataPropertyOrThrow(array, ! ToString(𝔽(nextIndex)), next).
4. Set nextIndex to nextIndex + 1.

Note

CreateDataPropertyOrThrow is used to ensure that own properties are defined for the array even if the standard built-in Array prototype object has been modified in a manner that would preclude the creation of new own properties using [[Set]].

13.2.4.2 Runtime Semantics: Evaluation

ArrayLiteral

[

Elision

opt

]

Let array be ! ArrayCreate(0).
2. If Elision is present, then
1. Perform ? ArrayAccumulation of Elision with arguments array and 0.
Return array.

ArrayLiteral

[

ElementList

]

Let array be ! ArrayCreate(0).
Perform ? ArrayAccumulation of ElementList with arguments array and 0.
Return array.

ArrayLiteral

[

ElementList

Elision

opt

]

Let array be ! ArrayCreate(0).
Let nextIndex be ? ArrayAccumulation of ElementList with arguments array and 0.
3. If Elision is present, then
1. Perform ? ArrayAccumulation of Elision with arguments array and nextIndex.
Return array.

13.2.5 Object Initializer

Note 1

An object initializer is an expression describing the initialization of an Object, written in a form resembling a literal. It is a list of zero or more pairs of property keys and associated values, enclosed in curly brackets. The values need not be literals; they are evaluated each time the object initializer is evaluated.

Syntax

ObjectLiteral

[Yield, Await]

{

}

{

PropertyDefinitionList

[?Yield, ?Await]

}

{

PropertyDefinitionList

[?Yield, ?Await]

}

PropertyDefinitionList

[Yield, Await]

PropertyDefinition

[?Yield, ?Await]

PropertyDefinitionList

[?Yield, ?Await]

[?Yield, ?Await]

[Yield, Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[+In, ?Yield, ?Await]

MethodDefinition

[?Yield, ?Await]

...

AssignmentExpression

[+In, ?Yield, ?Await]

[Yield, Await]

[?Yield, ?Await]

[Yield, Await]

[

AssignmentExpression

[+In, ?Yield, ?Await]

]

CoverInitializedName

[Yield, Await]

IdentifierReference

[?Yield, ?Await]

Initializer

[+In, ?Yield, ?Await]

Initializer

[In, Yield, Await]

AssignmentExpression

[?In, ?Yield, ?Await]

Note 2

MethodDefinition is defined in 15.4.

Note 3

In certain contexts, ObjectLiteral is used as a cover grammar for a more restricted secondary grammar. The CoverInitializedName production is necessary to fully cover these secondary grammars. However, use of this production results in an early Syntax Error in normal contexts where an actual ObjectLiteral is expected.

13.2.5.1 Static Semantics: Early Errors

PropertyDefinition

MethodDefinition

It is a Syntax Error if HasDirectSuper of MethodDefinition is true.
It is a Syntax Error if PrivateBoundIdentifiers of MethodDefinition is not empty.

In addition to describing an actual object initializer the ObjectLiteral productions are also used as a cover grammar for ObjectAssignmentPattern and may be recognized as part of a CoverParenthesizedExpressionAndArrowParameterList. When ObjectLiteral appears in a context where ObjectAssignmentPattern is required the following Early Error rules are not applied. In addition, they are not applied when initially parsing a CoverParenthesizedExpressionAndArrowParameterList or CoverCallExpressionAndAsyncArrowHead.

PropertyDefinition

CoverInitializedName

It is a Syntax Error if any source text is matched by this production.

Note 1

This production exists so that ObjectLiteral can serve as a cover grammar for ObjectAssignmentPattern. It cannot occur in an actual object initializer.

ObjectLiteral

{

PropertyDefinitionList

}

{

PropertyDefinitionList

}

It is a Syntax Error if PropertyNameList of PropertyDefinitionList contains any duplicate entries for "__proto__" and at least two of those entries were obtained from productions of the form PropertyDefinition : PropertyName : AssignmentExpression . This rule is not applied if this ObjectLiteral is contained within a Script that is being parsed for JSON.parse (see step 4 of JSON.parse).

Note 2

The List returned by PropertyNameList does not include property names defined using a ComputedPropertyName.

13.2.5.2 Static Semantics: IsComputedPropertyKey

The syntax-directed operation IsComputedPropertyKey takes no arguments and returns a Boolean. It is defined piecewise over the following productions:

PropertyName

LiteralPropertyName

Return false.

PropertyName

ComputedPropertyName

Return true.

13.2.5.3 Static Semantics: PropertyNameList

The syntax-directed operation PropertyNameList takes no arguments and returns a List of Strings. It is defined piecewise over the following productions:

PropertyDefinitionList

PropertyDefinition

Let propName be PropName of PropertyDefinition.
If propName is empty, return a new empty List.
Return « propName ».

PropertyDefinitionList

PropertyDefinition

Let list be PropertyNameList of PropertyDefinitionList.
Let propName be PropName of PropertyDefinition.
If propName is empty, return list.
Return the list-concatenation of list and « propName ».

13.2.5.4 Runtime Semantics: Evaluation

ObjectLiteral

{

}

Return OrdinaryObjectCreate(%Object.prototype%).

ObjectLiteral

{

PropertyDefinitionList

}

{

PropertyDefinitionList

}

Let obj be OrdinaryObjectCreate(%Object.prototype%).
Perform ? PropertyDefinitionEvaluation of PropertyDefinitionList with argument obj.
Return obj.

LiteralPropertyName

IdentifierName

Return StringValue of IdentifierName.

LiteralPropertyName

StringLiteral

Return the SV of StringLiteral.

LiteralPropertyName

NumericLiteral

Let nbr be the NumericValue of NumericLiteral.
Return ! ToString(nbr).

ComputedPropertyName

[

AssignmentExpression

]

Let exprValue be ? Evaluation of AssignmentExpression.
Let propName be ? GetValue(exprValue).
Return ? ToPropertyKey(propName).

13.2.5.5 Runtime Semantics: PropertyDefinitionEvaluation

The syntax-directed operation PropertyDefinitionEvaluation takes argument object (an Object) and returns either a normal completion containing unused or an abrupt completion. It is defined piecewise over the following productions:

PropertyDefinitionList

PropertyDefinition

Perform ? PropertyDefinitionEvaluation of PropertyDefinitionList with argument object.
Perform ? PropertyDefinitionEvaluation of PropertyDefinition with argument object.
Return unused.

PropertyDefinition

...

AssignmentExpression

Let exprValue be ? Evaluation of AssignmentExpression.
Let fromValue be ? GetValue(exprValue).
Let excludedNames be a new empty List.
Perform ? CopyDataProperties(object, fromValue, excludedNames).
Return unused.

PropertyDefinition

IdentifierReference

Let propName be StringValue of IdentifierReference.
Let exprValue be ? Evaluation of IdentifierReference.
Let propValue be ? GetValue(exprValue).
Assert: object is an ordinary, extensible object with no non-configurable properties.
Perform ! CreateDataPropertyOrThrow(object, propName, propValue).
Return unused.

PropertyDefinition

PropertyName

AssignmentExpression

Let propKey be ? Evaluation of PropertyName.
2. If this PropertyDefinition is contained within a Script that is being evaluated for JSON.parse (see step 7 of JSON.parse), then
1. Let isProtoSetter be false.
3. Else if propKey is "__proto__" and IsComputedPropertyKey of PropertyName is false, then
1. Let isProtoSetter be true.
4. Else,
1. Let isProtoSetter be false.
5. If IsAnonymousFunctionDefinition(AssignmentExpression) is true and isProtoSetter is false, then
1. Let propValue be ? NamedEvaluation of AssignmentExpression with argument propKey.
6. Else,
1. Let exprValueRef be ? Evaluation of AssignmentExpression.
2. Let propValue be ? GetValue(exprValueRef).
7. If isProtoSetter is true, then
1. a. If propValue is an Object or propValue is null, then
  1. Perform ! object.[[SetPrototypeOf]](propValue).
2. Return unused.
Assert: object is an ordinary, extensible object with no non-configurable properties.
Perform ! CreateDataPropertyOrThrow(object, propKey, propValue).
Return unused.

PropertyDefinition

MethodDefinition

Perform ? MethodDefinitionEvaluation of MethodDefinition with arguments object and true.
Return unused.

13.2.6 Function Defining Expressions

See 15.2 for PrimaryExpression : FunctionExpression .

See 15.5 for PrimaryExpression : GeneratorExpression .

See 15.7 for PrimaryExpression : ClassExpression .

See 15.8 for PrimaryExpression : AsyncFunctionExpression .

See 15.6 for PrimaryExpression : AsyncGeneratorExpression .

13.2.7 Regular Expression Literals

Syntax

See 12.9.5.

13.2.7.1 Static Semantics: Early Errors

PrimaryExpression

RegularExpressionLiteral

It is a Syntax Error if IsValidRegularExpressionLiteral(RegularExpressionLiteral) is false.

13.2.7.2 Static Semantics: IsValidRegularExpressionLiteral ( `literal` )

The abstract operation IsValidRegularExpressionLiteral takes argument literal (a RegularExpressionLiteral Parse Node) and returns a Boolean. It determines if its argument is a valid regular expression literal. It performs the following steps when called:

Let flags be FlagText of literal.
If flags contains any code points other than d, g, i, m, s, u, v, or y, or if flags contains any code point more than once, return false.
If flags contains u, let u be true; else let u be false.
If flags contains v, let v be true; else let v be false.
Let patternText be BodyText of literal.
6. If u is false and v is false, then
1. Let stringValue be CodePointsToString(patternText).
2. Set patternText to the sequence of code points resulting from interpreting each of the 16-bit elements of stringValue as a Unicode BMP code point. UTF-16 decoding is not applied to the elements.
Let parseResult be ParsePattern(patternText, u, v).
If parseResult is a Parse Node, return true; else return false.

13.2.7.3 Runtime Semantics: Evaluation

PrimaryExpression

RegularExpressionLiteral

Let pattern be CodePointsToString(BodyText of RegularExpressionLiteral).
Let flags be CodePointsToString(FlagText of RegularExpressionLiteral).
Return ! RegExpCreate(pattern, flags).

13.2.8 Template Literals

Syntax

TemplateLiteral

[Yield, Await, Tagged]

NoSubstitutionTemplate

SubstitutionTemplate

[?Yield, ?Await, ?Tagged]

SubstitutionTemplate

[Yield, Await, Tagged]

TemplateHead

Expression

[+In, ?Yield, ?Await]

TemplateSpans

[?Yield, ?Await, ?Tagged]

TemplateSpans

[Yield, Await, Tagged]

TemplateTail

TemplateMiddleList

[?Yield, ?Await, ?Tagged]

TemplateTail

TemplateMiddleList

[Yield, Await, Tagged]

TemplateMiddle

Expression

[+In, ?Yield, ?Await]

TemplateMiddleList

[?Yield, ?Await, ?Tagged]

TemplateMiddle

Expression

[+In, ?Yield, ?Await]

13.2.8.1 Static Semantics: Early Errors

TemplateLiteral

[Yield, Await, Tagged]

NoSubstitutionTemplate

It is a Syntax Error if the _[Tagged] parameter was not set and NoSubstitutionTemplate Contains NotEscapeSequence.

TemplateLiteral

[Yield, Await, Tagged]

SubstitutionTemplate

[?Yield, ?Await, ?Tagged]

It is a Syntax Error if the number of elements in the result of TemplateStrings of TemplateLiteral with argument false is greater than or equal to 2³².

SubstitutionTemplate

[Yield, Await, Tagged]

TemplateHead

Expression

[+In, ?Yield, ?Await]

TemplateSpans

[?Yield, ?Await, ?Tagged]

It is a Syntax Error if the _[Tagged] parameter was not set and TemplateHead Contains NotEscapeSequence.

TemplateSpans

[Yield, Await, Tagged]

TemplateTail

It is a Syntax Error if the _[Tagged] parameter was not set and TemplateTail Contains NotEscapeSequence.

TemplateMiddleList

[Yield, Await, Tagged]

TemplateMiddle

Expression

[+In, ?Yield, ?Await]

TemplateMiddleList

[?Yield, ?Await, ?Tagged]

TemplateMiddle

Expression

[+In, ?Yield, ?Await]

It is a Syntax Error if the _[Tagged] parameter was not set and TemplateMiddle Contains NotEscapeSequence.

13.2.8.2 Static Semantics: TemplateStrings

The syntax-directed operation TemplateStrings takes argument raw (a Boolean) and returns a List of either Strings or undefined. It is defined piecewise over the following productions:

TemplateLiteral

NoSubstitutionTemplate

Return « TemplateString(NoSubstitutionTemplate, raw) ».

Let head be « TemplateString(TemplateHead, raw) ».
Let tail be TemplateStrings of TemplateSpans with argument raw.
Return the list-concatenation of head and tail.

TemplateSpans

TemplateTail

Return « TemplateString(TemplateTail, raw) ».

TemplateSpans

TemplateMiddleList

TemplateTail

Let middle be TemplateStrings of TemplateMiddleList with argument raw.
Let tail be « TemplateString(TemplateTail, raw) ».
Return the list-concatenation of middle and tail.

TemplateMiddleList

TemplateMiddle

Expression

Return « TemplateString(TemplateMiddle, raw) ».

TemplateMiddleList

TemplateMiddle

Expression

Let front be TemplateStrings of TemplateMiddleList with argument raw.
Let last be « TemplateString(TemplateMiddle, raw) ».
Return the list-concatenation of front and last.

13.2.8.3 Static Semantics: TemplateString ( `templateToken`, `raw` )

The abstract operation TemplateString takes arguments templateToken (a NoSubstitutionTemplate Parse Node, a TemplateHead Parse Node, a TemplateMiddle Parse Node, or a TemplateTail Parse Node) and raw (a Boolean) and returns a String or undefined. It performs the following steps when called:

1. If raw is true, then
1. Let string be the TRV of templateToken.
2. Else,
1. Let string be the TV of templateToken.
Return string.

Note

This operation returns undefined if raw is false and templateToken contains a NotEscapeSequence. In all other cases, it returns a String.

13.2.8.4 GetTemplateObject ( `templateLiteral` )

The abstract operation GetTemplateObject takes argument templateLiteral (a Parse Node) and returns an Array. It performs the following steps when called:

Let realm be the current Realm Record.
Let templateRegistry be realm.[[TemplateMap]].
3. For each element e of templateRegistry, do
1. a. If e.[[Site]] is the same Parse Node as templateLiteral, then
  1. Return e.[[Array]].
Let rawStrings be TemplateStrings of templateLiteral with argument true.
Assert: rawStrings is a List of Strings.
Let cookedStrings be TemplateStrings of templateLiteral with argument false.
Let count be the number of elements in the List cookedStrings.
Assert: count ≤ 2³² - 1.
Let template be ! ArrayCreate(count).
Let rawObj be ! ArrayCreate(count).
Let index be 0.
12. Repeat, while index < count,
1. Let prop be ! ToString(𝔽(index)).
2. Let cookedValue be cookedStrings[index].
3. Perform ! DefinePropertyOrThrow(template, prop, PropertyDescriptor { [[Value]]: cookedValue, [[Writable]]: false, [[Enumerable]]: true, [[Configurable]]: false }).
4. Let rawValue be the String value rawStrings[index].
5. Perform ! DefinePropertyOrThrow(rawObj, prop, PropertyDescriptor { [[Value]]: rawValue, [[Writable]]: false, [[Enumerable]]: true, [[Configurable]]: false }).
6. Set index to index + 1.
Perform ! SetIntegrityLevel(rawObj, frozen).
Perform ! DefinePropertyOrThrow(template, "raw", PropertyDescriptor { [[Value]]: rawObj, [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }).
Perform ! SetIntegrityLevel(template, frozen).
Append the Record { [[Site]]: templateLiteral, [[Array]]: template } to realm.[[TemplateMap]].
Return template.

Note 1

The creation of a template object cannot result in an abrupt completion.

Note 2

Each TemplateLiteral in the program code of a realm is associated with a unique template object that is used in the evaluation of tagged Templates (13.2.8.6). The template objects are frozen and the same template object is used each time a specific tagged Template is evaluated. Whether template objects are created lazily upon first evaluation of the TemplateLiteral or eagerly prior to first evaluation is an implementation choice that is not observable to ECMAScript code.

Note 3

Future editions of this specification may define additional non-enumerable properties of template objects.

13.2.8.5 Runtime Semantics: SubstitutionEvaluation

The syntax-directed operation SubstitutionEvaluation takes no arguments and returns either a normal completion containing a List of ECMAScript language values or an abrupt completion. It is defined piecewise over the following productions:

TemplateSpans

TemplateTail

Return a new empty List.

TemplateSpans

TemplateMiddleList

TemplateTail

Return ? SubstitutionEvaluation of TemplateMiddleList.

TemplateMiddleList

TemplateMiddle

Expression

Let subRef be ? Evaluation of Expression.
Let sub be ? GetValue(subRef).
Return « sub ».

TemplateMiddleList

TemplateMiddle

Expression

Let preceding be ? SubstitutionEvaluation of TemplateMiddleList.
Let nextRef be ? Evaluation of Expression.
Let next be ? GetValue(nextRef).
Return the list-concatenation of preceding and « next ».

13.2.8.6 Runtime Semantics: Evaluation

TemplateLiteral

NoSubstitutionTemplate

Return the TV of NoSubstitutionTemplate as defined in 12.9.6.

Let head be the TV of TemplateHead as defined in 12.9.6.
Let subRef be ? Evaluation of Expression.
Let sub be ? GetValue(subRef).
Let middle be ? ToString(sub).
Let tail be ? Evaluation of TemplateSpans.
Return the string-concatenation of head, middle, and tail.

Note 1

The string conversion semantics applied to the Expression value are like String.prototype.concat rather than the + operator.

TemplateSpans

TemplateTail

Return the TV of TemplateTail as defined in 12.9.6.

TemplateSpans

TemplateMiddleList

TemplateTail

Let head be ? Evaluation of TemplateMiddleList.
Let tail be the TV of TemplateTail as defined in 12.9.6.
Return the string-concatenation of head and tail.

TemplateMiddleList

TemplateMiddle

Expression

Let head be the TV of TemplateMiddle as defined in 12.9.6.
Let subRef be ? Evaluation of Expression.
Let sub be ? GetValue(subRef).
Let middle be ? ToString(sub).
Return the string-concatenation of head and middle.

Note 2

The string conversion semantics applied to the Expression value are like String.prototype.concat rather than the + operator.

TemplateMiddleList

TemplateMiddle

Expression

Let rest be ? Evaluation of TemplateMiddleList.
Let middle be the TV of TemplateMiddle as defined in 12.9.6.
Let subRef be ? Evaluation of Expression.
Let sub be ? GetValue(subRef).
Let last be ? ToString(sub).
Return the string-concatenation of rest, middle, and last.

Note 3

The string conversion semantics applied to the Expression value are like String.prototype.concat rather than the + operator.

13.2.9 The Grouping Operator

13.2.9.1 Static Semantics: Early Errors

PrimaryExpression

CoverParenthesizedExpressionAndArrowParameterList

CoverParenthesizedExpressionAndArrowParameterList must cover a ParenthesizedExpression.

13.2.9.2 Runtime Semantics: Evaluation

PrimaryExpression

CoverParenthesizedExpressionAndArrowParameterList

Let expr be the ParenthesizedExpression that is covered by CoverParenthesizedExpressionAndArrowParameterList.
Return ? Evaluation of expr.

ParenthesizedExpression

(

Expression

)

Return ? Evaluation of Expression. This may be of type Reference.

Note

This algorithm does not apply GetValue to Evaluation of Expression. The principal motivation for this is so that operators such as delete and typeof may be applied to parenthesized expressions.

13.3 Left-Hand-Side Expressions

Syntax

MemberExpression

[Yield, Await]

PrimaryExpression

[?Yield, ?Await]

MemberExpression

[?Yield, ?Await]

[

Expression

[+In, ?Yield, ?Await]

]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await, +Tagged]

[?Yield, ?Await]

new

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[Yield, Await]

super

[

Expression

[+In, ?Yield, ?Await]

]

super

new

target

ImportMeta

import

meta

NewExpression

[Yield, Await]

MemberExpression

[?Yield, ?Await]

new

NewExpression

[?Yield, ?Await]

CallExpression

[Yield, Await]

CoverCallExpressionAndAsyncArrowHead

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[

Expression

[+In, ?Yield, ?Await]

]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await, +Tagged]

CallExpression

[?Yield, ?Await]

PrivateIdentifier

SuperCall

[Yield, Await]

super

Arguments

[?Yield, ?Await]

ImportCall

[Yield, Await]

import

(

AssignmentExpression

[+In, ?Yield, ?Await]

)

Arguments

[Yield, Await]

(

)

(

ArgumentList

[?Yield, ?Await]

)

(

ArgumentList

[?Yield, ?Await]

)

ArgumentList

[Yield, Await]

AssignmentExpression

[+In, ?Yield, ?Await]

...

AssignmentExpression

[+In, ?Yield, ?Await]

ArgumentList

[?Yield, ?Await]

AssignmentExpression

[+In, ?Yield, ?Await]

ArgumentList

[?Yield, ?Await]

...

AssignmentExpression

[+In, ?Yield, ?Await]

[Yield, Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[Yield, Await]

[?Yield, ?Await]

[

Expression

[+In, ?Yield, ?Await]

]

IdentifierName

TemplateLiteral

[?Yield, ?Await, +Tagged]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[

Expression

[+In, ?Yield, ?Await]

]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await, +Tagged]

OptionalChain

[?Yield, ?Await]

PrivateIdentifier

LeftHandSideExpression

[Yield, Await]

NewExpression

[?Yield, ?Await]

CallExpression

[?Yield, ?Await]

OptionalExpression

[?Yield, ?Await]

Supplemental Syntax

When processing an instance of the production
CallExpression : CoverCallExpressionAndAsyncArrowHead
the interpretation of CoverCallExpressionAndAsyncArrowHead is refined using the following grammar:

CallMemberExpression

[Yield, Await]

MemberExpression

[?Yield, ?Await]

Arguments

[?Yield, ?Await]

13.3.1 Static Semantics

13.3.1.1 Static Semantics: Early Errors

It is a Syntax Error if any source text is matched by this production.

Note

This production exists in order to prevent automatic semicolon insertion rules (12.10) from being applied to the following code:

a?.b
`c`

so that it would be interpreted as two valid statements. The purpose is to maintain consistency with similar code without optional chaining:

a.b
`c`

which is a valid statement and where automatic semicolon insertion does not apply.

ImportMeta

import

meta

It is a Syntax Error if the syntactic goal symbol is not Module.

13.3.2 Property Accessors

Note

Properties are accessed by name, using either the dot notation:

.

.

or the bracket notation:

[

]

[

]

The dot notation is explained by the following syntactic conversion:

MemberExpression

.

IdentifierName

is identical in its behaviour to

MemberExpression

[ <identifier-name-string> ]

and similarly

CallExpression

.

IdentifierName

is identical in its behaviour to

CallExpression

[ <identifier-name-string> ]

where <identifier-name-string> is the result of evaluating StringValue of IdentifierName.

13.3.2.1 Runtime Semantics: Evaluation

MemberExpression

[

Expression

]

Let baseReference be ? Evaluation of MemberExpression.
Let baseValue be ? GetValue(baseReference).
If the source text matched by this MemberExpression is strict mode code, let strict be true; else let strict be false.
Return ? EvaluatePropertyAccessWithExpressionKey(baseValue, Expression, strict).

MemberExpression

IdentifierName

Let baseReference be ? Evaluation of MemberExpression.
Let baseValue be ? GetValue(baseReference).
If the source text matched by this MemberExpression is strict mode code, let strict be true; else let strict be false.
Return EvaluatePropertyAccessWithIdentifierKey(baseValue, IdentifierName, strict).

MemberExpression

PrivateIdentifier

Let baseReference be ? Evaluation of MemberExpression.
Let baseValue be ? GetValue(baseReference).
Let fieldNameString be the StringValue of PrivateIdentifier.
Return MakePrivateReference(baseValue, fieldNameString).

CallExpression

[

Expression

]

Let baseReference be ? Evaluation of CallExpression.
Let baseValue be ? GetValue(baseReference).
If the source text matched by this CallExpression is strict mode code, let strict be true; else let strict be false.
Return ? EvaluatePropertyAccessWithExpressionKey(baseValue, Expression, strict).

CallExpression

IdentifierName

Let baseReference be ? Evaluation of CallExpression.
Let baseValue be ? GetValue(baseReference).
If the source text matched by this CallExpression is strict mode code, let strict be true; else let strict be false.
Return EvaluatePropertyAccessWithIdentifierKey(baseValue, IdentifierName, strict).

CallExpression

PrivateIdentifier

Let baseReference be ? Evaluation of CallExpression.
Let baseValue be ? GetValue(baseReference).
Let fieldNameString be the StringValue of PrivateIdentifier.
Return MakePrivateReference(baseValue, fieldNameString).

13.3.3 EvaluatePropertyAccessWithExpressionKey ( `baseValue`, `expression`, `strict` )

The abstract operation EvaluatePropertyAccessWithExpressionKey takes arguments baseValue (an ECMAScript language value), expression (an Expression Parse Node), and strict (a Boolean) and returns either a normal completion containing a Reference Record or an abrupt completion. It performs the following steps when called:

Let propertyNameReference be ? Evaluation of expression.
Let propertyNameValue be ? GetValue(propertyNameReference).
Let propertyKey be ? ToPropertyKey(propertyNameValue).
Return the Reference Record { [[Base]]: baseValue, [[ReferencedName]]: propertyKey, [[Strict]]: strict, [[ThisValue]]: empty }.

13.3.4 EvaluatePropertyAccessWithIdentifierKey ( `baseValue`, `identifierName`, `strict` )

The abstract operation EvaluatePropertyAccessWithIdentifierKey takes arguments baseValue (an ECMAScript language value), identifierName (an IdentifierName Parse Node), and strict (a Boolean) and returns a Reference Record. It performs the following steps when called:

Let propertyNameString be StringValue of identifierName.
Return the Reference Record { [[Base]]: baseValue, [[ReferencedName]]: propertyNameString, [[Strict]]: strict, [[ThisValue]]: empty }.

13.3.5 The `new` Operator

13.3.5.1 Runtime Semantics: Evaluation

NewExpression

new

NewExpression

Return ? EvaluateNew(NewExpression, empty).

MemberExpression

new

MemberExpression

Arguments

Return ? EvaluateNew(MemberExpression, Arguments).

13.3.5.1.1 EvaluateNew ( `constructExpr`, `arguments` )

The abstract operation EvaluateNew takes arguments constructExpr (a NewExpression Parse Node or a MemberExpression Parse Node) and arguments (empty or an Arguments Parse Node) and returns either a normal completion containing an ECMAScript language value or an abrupt completion. It performs the following steps when called:

Let ref be ? Evaluation of constructExpr.
Let constructor be ? GetValue(ref).
3. If arguments is empty, then
1. Let argList be a new empty List.
4. Else,
1. Let argList be ? ArgumentListEvaluation of arguments.
If IsConstructor(constructor) is false, throw a TypeError exception.
Return ? Construct(constructor, argList).

13.3.6 Function Calls

13.3.6.1 Runtime Semantics: Evaluation

CallExpression

CoverCallExpressionAndAsyncArrowHead

Let expr be the CallMemberExpression that is covered by CoverCallExpressionAndAsyncArrowHead.
Let memberExpr be the MemberExpression of expr.
Let arguments be the Arguments of expr.
Let ref be ? Evaluation of memberExpr.
Let func be ? GetValue(ref).
6. If ref is a Reference Record, IsPropertyReference(ref) is false, and ref.[[ReferencedName]] is "eval", then
1. a. If SameValue(func, %eval%) is true, then
  1. Let argList be ? ArgumentListEvaluation of arguments.
  2. If argList has no elements, return undefined.
  3. Let evalArg be the first element of argList.
  4. If the source text matched by this CallExpression is strict mode code, let strictCaller be true. Otherwise let strictCaller be false.
  5. Return ? PerformEval(evalArg, strictCaller, true).
Let thisCall be this CallExpression.
Let tailCall be IsInTailPosition(thisCall).
Return ? EvaluateCall(func, ref, arguments, tailCall).

A CallExpression evaluation that executes step 6.a.v is a direct eval.

CallExpression

Arguments

Let ref be ? Evaluation of CallExpression.
Let func be ? GetValue(ref).
Let thisCall be this CallExpression.
Let tailCall be IsInTailPosition(thisCall).
Return ? EvaluateCall(func, ref, Arguments, tailCall).

13.3.6.2 EvaluateCall ( `func`, `ref`, `arguments`, `tailPosition` )

The abstract operation EvaluateCall takes arguments func (an ECMAScript language value), ref (an ECMAScript language value or a Reference Record), arguments (a Parse Node), and tailPosition (a Boolean) and returns either a normal completion containing an ECMAScript language value or an abrupt completion. It performs the following steps when called:

1. If ref is a Reference Record, then
1. a. If IsPropertyReference(ref) is true, then
  1. Let thisValue be GetThisValue(ref).
2. b. Else,
  1. Let refEnv be ref.[[Base]].
  2. Assert: refEnv is an Environment Record.
  3. Let thisValue be refEnv.WithBaseObject().
2. Else,
1. Let thisValue be undefined.
Let argList be ? ArgumentListEvaluation of arguments.
If func is not an Object, throw a TypeError exception.
If IsCallable(func) is false, throw a TypeError exception.
If tailPosition is true, perform PrepareForTailCall().
Return ? Call(func, thisValue, argList).

13.3.7 The `super` Keyword

13.3.7.1 Runtime Semantics: Evaluation

SuperProperty

super

[

Expression

]

Let env be GetThisEnvironment().
Let actualThis be ? env.GetThisBinding().
Let propertyNameReference be ? Evaluation of Expression.
Let propertyNameValue be ? GetValue(propertyNameReference).
Let propertyKey be ? ToPropertyKey(propertyNameValue).
If the source text matched by this SuperProperty is strict mode code, let strict be true; else let strict be false.
Return ? MakeSuperPropertyReference(actualThis, propertyKey, strict).

SuperProperty

super

IdentifierName

Let env be GetThisEnvironment().
Let actualThis be ? env.GetThisBinding().
Let propertyKey be StringValue of IdentifierName.
If the source text matched by this SuperProperty is strict mode code, let strict be true; else let strict be false.
Return ? MakeSuperPropertyReference(actualThis, propertyKey, strict).

SuperCall

super

Arguments

Let newTarget be GetNewTarget().
Assert: newTarget is an Object.
Let func be GetSuperConstructor().
Let argList be ? ArgumentListEvaluation of Arguments.
If IsConstructor(func) is false, throw a TypeError exception.
Let result be ? Construct(func, argList, newTarget).
Let thisER be GetThisEnvironment().
Perform ? thisER.BindThisValue(result).
Let F be thisER.[[FunctionObject]].
Assert: F is an ECMAScript function object.
Perform ? InitializeInstanceElements(result, F).
Return result.

13.3.7.2 GetSuperConstructor ( )

The abstract operation GetSuperConstructor takes no arguments and returns an ECMAScript language value. It performs the following steps when called:

Let envRec be GetThisEnvironment().
Assert: envRec is a Function Environment Record.
Let activeFunction be envRec.[[FunctionObject]].
Assert: activeFunction is an ECMAScript function object.
Let superConstructor be ! activeFunction.[[GetPrototypeOf]]().
Return superConstructor.

13.3.7.3 MakeSuperPropertyReference ( `actualThis`, `propertyKey`, `strict` )

The abstract operation MakeSuperPropertyReference takes arguments actualThis (an ECMAScript language value), propertyKey (a property key), and strict (a Boolean) and returns either a normal completion containing a Super Reference Record or a throw completion. It performs the following steps when called:

Let env be GetThisEnvironment().
Assert: env.HasSuperBinding() is true.
Let baseValue be ? env.GetSuperBase().
Return the Reference Record { [[Base]]: baseValue, [[ReferencedName]]: propertyKey, [[Strict]]: strict, [[ThisValue]]: actualThis }.

13.3.8 Argument Lists

Note

The evaluation of an argument list produces a List of values.

13.3.8.1 Runtime Semantics: ArgumentListEvaluation

The syntax-directed operation ArgumentListEvaluation takes no arguments and returns either a normal completion containing a List of ECMAScript language values or an abrupt completion. It is defined piecewise over the following productions:

Arguments

(

)

Return a new empty List.

ArgumentList

AssignmentExpression

Let ref be ? Evaluation of AssignmentExpression.
Let arg be ? GetValue(ref).
Return « arg ».

ArgumentList

...

AssignmentExpression

Let list be a new empty List.
Let spreadRef be ? Evaluation of AssignmentExpression.
Let spreadObj be ? GetValue(spreadRef).
Let iteratorRecord be ? GetIterator(spreadObj, sync).
5. Repeat,
1. Let next be ? IteratorStepValue(iteratorRecord).
2. If next is done, return list.
3. Append next to list.

ArgumentList

AssignmentExpression

Let precedingArgs be ? ArgumentListEvaluation of ArgumentList.
Let ref be ? Evaluation of AssignmentExpression.
Let arg be ? GetValue(ref).
Return the list-concatenation of precedingArgs and « arg ».

ArgumentList

...

AssignmentExpression

Let precedingArgs be ? ArgumentListEvaluation of ArgumentList.
Let spreadRef be ? Evaluation of AssignmentExpression.
Let iteratorRecord be ? GetIterator(? GetValue(spreadRef), sync).
4. Repeat,
1. Let next be ? IteratorStepValue(iteratorRecord).
2. If next is done, return precedingArgs.
3. Append next to precedingArgs.

TemplateLiteral

NoSubstitutionTemplate

Let templateLiteral be this TemplateLiteral.
Let siteObj be GetTemplateObject(templateLiteral).
Return « siteObj ».

TemplateLiteral

SubstitutionTemplate

Let templateLiteral be this TemplateLiteral.
Let siteObj be GetTemplateObject(templateLiteral).
Let remaining be ? ArgumentListEvaluation of SubstitutionTemplate.
Return the list-concatenation of « siteObj » and remaining.

Let firstSubRef be ? Evaluation of Expression.
Let firstSub be ? GetValue(firstSubRef).
Let restSub be ? SubstitutionEvaluation of TemplateSpans.
Assert: restSub is a possibly empty List.
Return the list-concatenation of « firstSub » and restSub.

13.3.9 Optional Chains

Note

An optional chain is a chain of one or more property accesses and function calls, the first of which begins with the token ?..

13.3.9.1 Runtime Semantics: Evaluation

OptionalExpression

MemberExpression

OptionalChain

Let baseReference be ? Evaluation of MemberExpression.
Let baseValue be ? GetValue(baseReference).
3. If baseValue is either undefined or null, then
1. Return undefined.
Return ? ChainEvaluation of OptionalChain with arguments baseValue and baseReference.

OptionalExpression

CallExpression

OptionalChain

Let baseReference be ? Evaluation of CallExpression.
Let baseValue be ? GetValue(baseReference).
3. If baseValue is either undefined or null, then
1. Return undefined.
Return ? ChainEvaluation of OptionalChain with arguments baseValue and baseReference.

OptionalExpression

OptionalChain

Let baseReference be ? Evaluation of OptionalExpression.
Let baseValue be ? GetValue(baseReference).
3. If baseValue is either undefined or null, then
1. Return undefined.
Return ? ChainEvaluation of OptionalChain with arguments baseValue and baseReference.

13.3.9.2 Runtime Semantics: ChainEvaluation

The syntax-directed operation ChainEvaluation takes arguments baseValue (an ECMAScript language value) and baseReference (an ECMAScript language value or a Reference Record) and returns either a normal completion containing either an ECMAScript language value or a Reference Record, or an abrupt completion. It is defined piecewise over the following productions:

OptionalChain

Arguments

Let thisChain be this OptionalChain.
Let tailCall be IsInTailPosition(thisChain).
Return ? EvaluateCall(baseValue, baseReference, Arguments, tailCall).

OptionalChain

[

Expression

]

If the source text matched by this OptionalChain is strict mode code, let strict be true; else let strict be false.
Return ? EvaluatePropertyAccessWithExpressionKey(baseValue, Expression, strict).

OptionalChain

IdentifierName

If the source text matched by this OptionalChain is strict mode code, let strict be true; else let strict be false.
Return EvaluatePropertyAccessWithIdentifierKey(baseValue, IdentifierName, strict).

OptionalChain

PrivateIdentifier

Let fieldNameString be the StringValue of PrivateIdentifier.
Return MakePrivateReference(baseValue, fieldNameString).

OptionalChain

Arguments

Let optionalChain be OptionalChain.
Let newReference be ? ChainEvaluation of optionalChain with arguments baseValue and baseReference.
Let newValue be ? GetValue(newReference).
Let thisChain be this OptionalChain.
Let tailCall be IsInTailPosition(thisChain).
Return ? EvaluateCall(newValue, newReference, Arguments, tailCall).

OptionalChain

[

Expression

]

Let optionalChain be OptionalChain.
Let newReference be ? ChainEvaluation of optionalChain with arguments baseValue and baseReference.
Let newValue be ? GetValue(newReference).
If the source text matched by this OptionalChain is strict mode code, let strict be true; else let strict be false.
Return ? EvaluatePropertyAccessWithExpressionKey(newValue, Expression, strict).

OptionalChain

IdentifierName

Let optionalChain be OptionalChain.
Let newReference be ? ChainEvaluation of optionalChain with arguments baseValue and baseReference.
Let newValue be ? GetValue(newReference).
If the source text matched by this OptionalChain is strict mode code, let strict be true; else let strict be false.
Return EvaluatePropertyAccessWithIdentifierKey(newValue, IdentifierName, strict).

OptionalChain

PrivateIdentifier

Let optionalChain be OptionalChain.
Let newReference be ? ChainEvaluation of optionalChain with arguments baseValue and baseReference.
Let newValue be ? GetValue(newReference).
Let fieldNameString be the StringValue of PrivateIdentifier.
Return MakePrivateReference(newValue, fieldNameString).

13.3.10 Import Calls

13.3.10.1 Runtime Semantics: Evaluation

ImportCall

import

(

AssignmentExpression

)

Let referrer be GetActiveScriptOrModule().
If referrer is null, set referrer to the current Realm Record.
Let argRef be ? Evaluation of AssignmentExpression.
Let specifier be ? GetValue(argRef).
Let promiseCapability be ! NewPromiseCapability(%Promise%).
Let specifierString be Completion(ToString(specifier)).
IfAbruptRejectPromise(specifierString, promiseCapability).
Perform HostLoadImportedModule(referrer, specifierString, empty, promiseCapability).
Return promiseCapability.[[Promise]].

13.3.10.1.1 ContinueDynamicImport ( `promiseCapability`, `moduleCompletion` )

The abstract operation ContinueDynamicImport takes arguments promiseCapability (a PromiseCapability Record) and moduleCompletion (either a normal completion containing a Module Record or a throw completion) and returns unused. It completes the process of a dynamic import originally started by an import() call, resolving or rejecting the promise returned by that call as appropriate. It performs the following steps when called:

1. If moduleCompletion is an abrupt completion, then
1. Perform ! Call(promiseCapability.[[Reject]], undefined, « moduleCompletion.[[Value]] »).
2. Return unused.
Let module be moduleCompletion.[[Value]].
Let loadPromise be module.LoadRequestedModules().
4. Let rejectedClosure be a new Abstract Closure with parameters (reason) that captures promiseCapability and performs the following steps when called:
1. Perform ! Call(promiseCapability.[[Reject]], undefined, « reason »).
2. Return unused.
Let onRejected be CreateBuiltinFunction(rejectedClosure, 1, "", « »).
6. Let linkAndEvaluateClosure be a new Abstract Closure with no parameters that captures module, promiseCapability, and onRejected and performs the following steps when called:
1. Let link be Completion(module.Link()).
2. b. If link is an abrupt completion, then
  1. Perform ! Call(promiseCapability.[[Reject]], undefined, « link.[[Value]] »).
  2. Return unused.
3. Let evaluatePromise be module.Evaluate().
4. d. Let fulfilledClosure be a new Abstract Closure with no parameters that captures module and promiseCapability and performs the following steps when called:
  1. Let namespace be GetModuleNamespace(module).
  2. Perform ! Call(promiseCapability.[[Resolve]], undefined, « namespace »).
  3. Return unused.
5. Let onFulfilled be CreateBuiltinFunction(fulfilledClosure, 0, "", « »).
6. Perform PerformPromiseThen(evaluatePromise, onFulfilled, onRejected).
7. Return unused.
Let linkAndEvaluate be CreateBuiltinFunction(linkAndEvaluateClosure, 0, "", « »).
Perform PerformPromiseThen(loadPromise, linkAndEvaluate, onRejected).
Return unused.

13.3.11 Tagged Templates

Note

A tagged template is a function call where the arguments of the call are derived from a TemplateLiteral (13.2.8). The actual arguments include a template object (13.2.8.4) and the values produced by evaluating the expressions embedded within the TemplateLiteral.

13.3.11.1 Runtime Semantics: Evaluation

MemberExpression

TemplateLiteral

Let tagRef be ? Evaluation of MemberExpression.
Let tagFunc be ? GetValue(tagRef).
Let thisCall be this MemberExpression.
Let tailCall be IsInTailPosition(thisCall).
Return ? EvaluateCall(tagFunc, tagRef, TemplateLiteral, tailCall).

CallExpression

TemplateLiteral

Let tagRef be ? Evaluation of CallExpression.
Let tagFunc be ? GetValue(tagRef).
Let thisCall be this CallExpression.
Let tailCall be IsInTailPosition(thisCall).
Return ? EvaluateCall(tagFunc, tagRef, TemplateLiteral, tailCall).

13.3.12 Meta Properties

13.3.12.1 Runtime Semantics: Evaluation

NewTarget

new

target

Return GetNewTarget().

ImportMeta

import

meta

Let module be GetActiveScriptOrModule().
Assert: module is a Source Text Module Record.
Let importMeta be module.[[ImportMeta]].
4. If importMeta is empty, then
1. Set importMeta to OrdinaryObjectCreate(null).
2. Let importMetaValues be HostGetImportMetaProperties(module).
3. c. For each Record { [[Key]], [[Value]] } p of importMetaValues, do
  1. Perform ! CreateDataPropertyOrThrow(importMeta, p.[[Key]], p.[[Value]]).
4. Perform HostFinalizeImportMeta(importMeta, module).
5. Set module.[[ImportMeta]] to importMeta.
6. Return importMeta.
5. Else,
1. Assert: importMeta is an Object.
2. Return importMeta.

13.3.12.1.1 HostGetImportMetaProperties ( `moduleRecord` )

The host-defined abstract operation HostGetImportMetaProperties takes argument moduleRecord (a Module Record) and returns a List of Records with fields [[Key]] (a property key) and [[Value]] (an ECMAScript language value). It allows hosts to provide property keys and values for the object returned from import.meta.

The default implementation of HostGetImportMetaProperties is to return a new empty List.

13.3.12.1.2 HostFinalizeImportMeta ( `importMeta`, `moduleRecord` )

The host-defined abstract operation HostFinalizeImportMeta takes arguments importMeta (an Object) and moduleRecord (a Module Record) and returns unused. It allows hosts to perform any extraordinary operations to prepare the object returned from import.meta.

Most hosts will be able to simply define HostGetImportMetaProperties, and leave HostFinalizeImportMeta with its default behaviour. However, HostFinalizeImportMeta provides an "escape hatch" for hosts which need to directly manipulate the object before it is exposed to ECMAScript code.

The default implementation of HostFinalizeImportMeta is to return unused.

13.4 Update Expressions

Syntax

UpdateExpression

[Yield, Await]

LeftHandSideExpression

[?Yield, ?Await]

LeftHandSideExpression

[?Yield, ?Await]

[no LineTerminator here]

LeftHandSideExpression

[?Yield, ?Await]

[no LineTerminator here]

UnaryExpression

[?Yield, ?Await]

UnaryExpression

[?Yield, ?Await]

13.4.1 Static Semantics: Early Errors

UpdateExpression

LeftHandSideExpression

It is an early Syntax Error if AssignmentTargetType of LeftHandSideExpression is not simple.

UpdateExpression

UnaryExpression

It is an early Syntax Error if AssignmentTargetType of UnaryExpression is not simple.

13.4.2 Postfix Increment Operator

13.4.2.1 Runtime Semantics: Evaluation

UpdateExpression

LeftHandSideExpression

Let lhs be ? Evaluation of LeftHandSideExpression.
Let oldValue be ? ToNumeric(? GetValue(lhs)).
3. If oldValue is a Number, then
1. Let newValue be Number::add(oldValue, 1_𝔽).
4. Else,
1. Assert: oldValue is a BigInt.
2. Let newValue be BigInt::add(oldValue, 1_ℤ).
Perform ? PutValue(lhs, newValue).
Return oldValue.

13.4.3 Postfix Decrement Operator

13.4.3.1 Runtime Semantics: Evaluation

UpdateExpression

LeftHandSideExpression

Let lhs be ? Evaluation of LeftHandSideExpression.
Let oldValue be ? ToNumeric(? GetValue(lhs)).
3. If oldValue is a Number, then
1. Let newValue be Number::subtract(oldValue, 1_𝔽).
4. Else,
1. Assert: oldValue is a BigInt.
2. Let newValue be BigInt::subtract(oldValue, 1_ℤ).
Perform ? PutValue(lhs, newValue).
Return oldValue.

13.4.4 Prefix Increment Operator

13.4.4.1 Runtime Semantics: Evaluation

UpdateExpression

UnaryExpression

Let expr be ? Evaluation of UnaryExpression.
Let oldValue be ? ToNumeric(? GetValue(expr)).
3. If oldValue is a Number, then
1. Let newValue be Number::add(oldValue, 1_𝔽).
4. Else,
1. Assert: oldValue is a BigInt.
2. Let newValue be BigInt::add(oldValue, 1_ℤ).
Perform ? PutValue(expr, newValue).
Return newValue.

13.4.5 Prefix Decrement Operator

13.4.5.1 Runtime Semantics: Evaluation

UpdateExpression

UnaryExpression

Let expr be ? Evaluation of UnaryExpression.
Let oldValue be ? ToNumeric(? GetValue(expr)).
3. If oldValue is a Number, then
1. Let newValue be Number::subtract(oldValue, 1_𝔽).
4. Else,
1. Assert: oldValue is a BigInt.
2. Let newValue be BigInt::subtract(oldValue, 1_ℤ).
Perform ? PutValue(expr, newValue).
Return newValue.

13.5 Unary Operators

Syntax

UnaryExpression

[Yield, Await]

UpdateExpression

[?Yield, ?Await]

delete

UnaryExpression

[?Yield, ?Await]

void

UnaryExpression

[?Yield, ?Await]

typeof

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[+Await]

AwaitExpression

[?Yield]

13.5.1 The `delete` Operator

13.5.1.1 Static Semantics: Early Errors

UnaryExpression

delete

UnaryExpression

It is a Syntax Error if the UnaryExpression is contained in strict mode code and the derived UnaryExpression is PrimaryExpression : IdentifierReference , MemberExpression : MemberExpression . PrivateIdentifier , CallExpression : CallExpression . PrivateIdentifier , OptionalChain : ?. PrivateIdentifier , or OptionalChain : OptionalChain . PrivateIdentifier .
It is a Syntax Error if the derived UnaryExpression is
PrimaryExpression : CoverParenthesizedExpressionAndArrowParameterList
and CoverParenthesizedExpressionAndArrowParameterList ultimately derives a phrase that, if used in place of UnaryExpression, would produce a Syntax Error according to these rules. This rule is recursively applied.

Note

The last rule means that expressions such as delete (((foo))) produce early errors because of recursive application of the first rule.

13.5.1.2 Runtime Semantics: Evaluation

UnaryExpression

delete

UnaryExpression

Let ref be ? Evaluation of UnaryExpression.
If ref is not a Reference Record, return true.
3. If IsUnresolvableReference(ref) is true, then
1. Assert: ref.[[Strict]] is false.
2. Return true.
4. If IsPropertyReference(ref) is true, then
1. Assert: IsPrivateReference(ref) is false.
2. If IsSuperReference(ref) is true, throw a ReferenceError exception.
3. Let baseObj be ? ToObject(ref.[[Base]]).
4. Let deleteStatus be ? baseObj.[[Delete]](ref.[[ReferencedName]]).
5. If deleteStatus is false and ref.[[Strict]] is true, throw a TypeError exception.
6. Return deleteStatus.
5. Else,
1. Let base be ref.[[Base]].
2. Assert: base is an Environment Record.
3. Return ? base.DeleteBinding(ref.[[ReferencedName]]).

Note 1

When a delete operator occurs within strict mode code, a SyntaxError exception is thrown if its UnaryExpression is a direct reference to a variable, function argument, or function name. In addition, if a delete operator occurs within strict mode code and the property to be deleted has the attribute { [[Configurable]]: false } (or otherwise cannot be deleted), a TypeError exception is thrown.

Note 2

The object that may be created in step 4.c is not accessible outside of the above abstract operation and the ordinary object [[Delete]] internal method. An implementation might choose to avoid the actual creation of that object.

13.5.2 The `void` Operator

13.5.2.1 Runtime Semantics: Evaluation

UnaryExpression

void

UnaryExpression

Let expr be ? Evaluation of UnaryExpression.
Perform ? GetValue(expr).
Return undefined.

Note

GetValue must be called even though its value is not used because it may have observable side-effects.

13.5.3 The `typeof` Operator

13.5.3.1 Runtime Semantics: Evaluation

UnaryExpression

typeof

UnaryExpression

Let val be ? Evaluation of UnaryExpression.
2. If val is a Reference Record, then
1. If IsUnresolvableReference(val) is true, return "undefined".
Set val to ? GetValue(val).
If val is undefined, return "undefined".
If val is null, return "object".
If val is a String, return "string".
If val is a Symbol, return "symbol".
If val is a Boolean, return "boolean".
If val is a Number, return "number".
If val is a BigInt, return "bigint".
Assert: val is an Object.
NOTE: This step is replaced in section B.3.6.3.
If val has a [[Call]] internal slot, return "function".
Return "object".

13.5.4 Unary `+` Operator

Note

The unary + operator converts its operand to Number type.

13.5.4.1 Runtime Semantics: Evaluation

UnaryExpression

Let expr be ? Evaluation of UnaryExpression.
Return ? ToNumber(? GetValue(expr)).

13.5.5 Unary `-` Operator

Note

The unary - operator converts its operand to a numeric value and then negates it. Negating +0_𝔽 produces -0_𝔽, and negating -0_𝔽 produces +0_𝔽.

13.5.5.1 Runtime Semantics: Evaluation

UnaryExpression

Let expr be ? Evaluation of UnaryExpression.
Let oldValue be ? ToNumeric(? GetValue(expr)).
3. If oldValue is a Number, then
1. Return Number::unaryMinus(oldValue).
4. Else,
1. Assert: oldValue is a BigInt.
2. Return BigInt::unaryMinus(oldValue).

13.5.6 Bitwise NOT Operator ( `~` )

13.5.6.1 Runtime Semantics: Evaluation

UnaryExpression

Let expr be ? Evaluation of UnaryExpression.
Let oldValue be ? ToNumeric(? GetValue(expr)).
3. If oldValue is a Number, then
1. Return Number::bitwiseNOT(oldValue).
4. Else,
1. Assert: oldValue is a BigInt.
2. Return BigInt::bitwiseNOT(oldValue).

13.5.7 Logical NOT Operator ( `!` )

13.5.7.1 Runtime Semantics: Evaluation

UnaryExpression

Let expr be ? Evaluation of UnaryExpression.
Let oldValue be ToBoolean(? GetValue(expr)).
If oldValue is true, return false.
Return true.

13.6 Exponentiation Operator

Syntax

ExponentiationExpression

[Yield, Await]

UnaryExpression

[?Yield, ?Await]

UpdateExpression

[?Yield, ?Await]

ExponentiationExpression

[?Yield, ?Await]

13.6.1 Runtime Semantics: Evaluation

ExponentiationExpression

UpdateExpression

ExponentiationExpression

Return ? EvaluateStringOrNumericBinaryExpression(UpdateExpression, **, ExponentiationExpression).

13.7 Multiplicative Operators

Syntax

MultiplicativeExpression

[Yield, Await]

ExponentiationExpression

[?Yield, ?Await]

MultiplicativeExpression

[?Yield, ?Await]

MultiplicativeOperator

ExponentiationExpression

[?Yield, ?Await]

MultiplicativeOperator

one of

Note

The * operator performs multiplication, producing the product of its operands.
The / operator performs division, producing the quotient of its operands.
The % operator yields the remainder of its operands from an implied division.

13.7.1 Runtime Semantics: Evaluation

MultiplicativeExpression

MultiplicativeOperator

ExponentiationExpression

Let opText be the source text matched by MultiplicativeOperator.
Return ? EvaluateStringOrNumericBinaryExpression(MultiplicativeExpression, opText, ExponentiationExpression).

13.8 Additive Operators

Syntax

AdditiveExpression

[Yield, Await]

MultiplicativeExpression

[?Yield, ?Await]

AdditiveExpression

[?Yield, ?Await]

MultiplicativeExpression

[?Yield, ?Await]

AdditiveExpression

[?Yield, ?Await]

MultiplicativeExpression

[?Yield, ?Await]

13.8.1 The Addition Operator ( `+` )

Note

The addition operator either performs string concatenation or numeric addition.

13.8.1.1 Runtime Semantics: Evaluation

AdditiveExpression

MultiplicativeExpression

Return ? EvaluateStringOrNumericBinaryExpression(AdditiveExpression, +, MultiplicativeExpression).

13.8.2 The Subtraction Operator ( `-` )

Note

The - operator performs subtraction, producing the difference of its operands.

13.8.2.1 Runtime Semantics: Evaluation

AdditiveExpression

MultiplicativeExpression

Return ? EvaluateStringOrNumericBinaryExpression(AdditiveExpression, -, MultiplicativeExpression).

13.9 Bitwise Shift Operators

Syntax

[Yield, Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

>>>

AdditiveExpression

[?Yield, ?Await]

13.9.1 The Left Shift Operator ( `<<` )

Note

Performs a bitwise left shift operation on the left operand by the amount specified by the right operand.

13.9.1.1 Runtime Semantics: Evaluation

ShiftExpression

AdditiveExpression

Return ? EvaluateStringOrNumericBinaryExpression(ShiftExpression, <<, AdditiveExpression).

13.9.2 The Signed Right Shift Operator ( `>>` )

Note

Performs a sign-filling bitwise right shift operation on the left operand by the amount specified by the right operand.

13.9.2.1 Runtime Semantics: Evaluation

ShiftExpression

AdditiveExpression

Return ? EvaluateStringOrNumericBinaryExpression(ShiftExpression, >>, AdditiveExpression).

13.9.3 The Unsigned Right Shift Operator ( `>>>` )

Note

Performs a zero-filling bitwise right shift operation on the left operand by the amount specified by the right operand.

13.9.3.1 Runtime Semantics: Evaluation

ShiftExpression

>>>

AdditiveExpression

Return ? EvaluateStringOrNumericBinaryExpression(ShiftExpression, >>>, AdditiveExpression).

13.10 Relational Operators

Note 1

The result of evaluating a relational operator is always of type Boolean, reflecting whether the relationship named by the operator holds between its two operands.

Syntax

RelationalExpression

[In, Yield, Await]

ShiftExpression

[?Yield, ?Await]

RelationalExpression

[?In, ?Yield, ?Await]

ShiftExpression

[?Yield, ?Await]

RelationalExpression

[?In, ?Yield, ?Await]

ShiftExpression

[?Yield, ?Await]

RelationalExpression

[?In, ?Yield, ?Await]

ShiftExpression

[?Yield, ?Await]

RelationalExpression

[?In, ?Yield, ?Await]

ShiftExpression

[?Yield, ?Await]

RelationalExpression

[?In, ?Yield, ?Await]

instanceof

ShiftExpression

[?Yield, ?Await]

[+In]

RelationalExpression

[+In, ?Yield, ?Await]

ShiftExpression

[?Yield, ?Await]

[+In]

PrivateIdentifier

ShiftExpression

[?Yield, ?Await]

Note 2

The _[In] grammar parameter is needed to avoid confusing the in operator in a relational expression with the in operator in a for statement.

13.10.1 Runtime Semantics: Evaluation

RelationalExpression

ShiftExpression

Let lref be ? Evaluation of RelationalExpression.
Let lval be ? GetValue(lref).
Let rref be ? Evaluation of ShiftExpression.
Let rval be ? GetValue(rref).
Let r be ? IsLessThan(lval, rval, true).
If r is undefined, return false. Otherwise, return r.

RelationalExpression

ShiftExpression

Let lref be ? Evaluation of RelationalExpression.
Let lval be ? GetValue(lref).
Let rref be ? Evaluation of ShiftExpression.
Let rval be ? GetValue(rref).
Let r be ? IsLessThan(rval, lval, false).
If r is undefined, return false. Otherwise, return r.

RelationalExpression

ShiftExpression

Let lref be ? Evaluation of RelationalExpression.
Let lval be ? GetValue(lref).
Let rref be ? Evaluation of ShiftExpression.
Let rval be ? GetValue(rref).
Let r be ? IsLessThan(rval, lval, false).
If r is either true or undefined, return false. Otherwise, return true.

RelationalExpression

ShiftExpression

Let lref be ? Evaluation of RelationalExpression.
Let lval be ? GetValue(lref).
Let rref be ? Evaluation of ShiftExpression.
Let rval be ? GetValue(rref).
Let r be ? IsLessThan(lval, rval, true).
If r is either true or undefined, return false. Otherwise, return true.

RelationalExpression

instanceof

ShiftExpression

Let lref be ? Evaluation of RelationalExpression.
Let lval be ? GetValue(lref).
Let rref be ? Evaluation of ShiftExpression.
Let rval be ? GetValue(rref).
Return ? InstanceofOperator(lval, rval).

RelationalExpression

ShiftExpression

Let lref be ? Evaluation of RelationalExpression.
Let lval be ? GetValue(lref).
Let rref be ? Evaluation of ShiftExpression.
Let rval be ? GetValue(rref).
If rval is not an Object, throw a TypeError exception.
Return ? HasProperty(rval, ? ToPropertyKey(lval)).

RelationalExpression

PrivateIdentifier

ShiftExpression

Let privateIdentifier be the StringValue of PrivateIdentifier.
Let rref be ? Evaluation of ShiftExpression.
Let rval be ? GetValue(rref).
If rval is not an Object, throw a TypeError exception.
Let privateEnv be the running execution context's PrivateEnvironment.
Let privateName be ResolvePrivateIdentifier(privateEnv, privateIdentifier).
If PrivateElementFind(rval, privateName) is not empty, return true.
Return false.

13.10.2 InstanceofOperator ( `V`, `target` )

The abstract operation InstanceofOperator takes arguments V (an ECMAScript language value) and target (an ECMAScript language value) and returns either a normal completion containing a Boolean or a throw completion. It implements the generic algorithm for determining if V is an instance of target either by consulting target's @@hasInstance method or, if absent, determining whether the value of target's "prototype" property is present in V's prototype chain. It performs the following steps when called:

If target is not an Object, throw a TypeError exception.
Let instOfHandler be ? GetMethod(target, @@hasInstance).
3. If instOfHandler is not undefined, then
1. Return ToBoolean(? Call(instOfHandler, target, « V »)).
If IsCallable(target) is false, throw a TypeError exception.
Return ? OrdinaryHasInstance(target, V).

Note

Steps 4 and 5 provide compatibility with previous editions of ECMAScript that did not use a @@hasInstance method to define the instanceof operator semantics. If an object does not define or inherit @@hasInstance it uses the default instanceof semantics.

13.11 Equality Operators

Note

The result of evaluating an equality operator is always of type Boolean, reflecting whether the relationship named by the operator holds between its two operands.

Syntax

EqualityExpression

[In, Yield, Await]

RelationalExpression

[?In, ?Yield, ?Await]

EqualityExpression

[?In, ?Yield, ?Await]

RelationalExpression

[?In, ?Yield, ?Await]

EqualityExpression

[?In, ?Yield, ?Await]

RelationalExpression

[?In, ?Yield, ?Await]

EqualityExpression

[?In, ?Yield, ?Await]

===

RelationalExpression

[?In, ?Yield, ?Await]

EqualityExpression

[?In, ?Yield, ?Await]

!==

RelationalExpression

[?In, ?Yield, ?Await]

13.11.1 Runtime Semantics: Evaluation

EqualityExpression

RelationalExpression

Let lref be ? Evaluation of EqualityExpression.
Let lval be ? GetValue(lref).
Let rref be ? Evaluation of RelationalExpression.
Let rval be ? GetValue(rref).
Return ? IsLooselyEqual(rval, lval).

EqualityExpression

RelationalExpression

Let lref be ? Evaluation of EqualityExpression.
Let lval be ? GetValue(lref).
Let rref be ? Evaluation of RelationalExpression.
Let rval be ? GetValue(rref).
Let r be ? IsLooselyEqual(rval, lval).
If r is true, return false. Otherwise, return true.

EqualityExpression

===

RelationalExpression

Let lref be ? Evaluation of EqualityExpression.
Let lval be ? GetValue(lref).
Let rref be ? Evaluation of RelationalExpression.
Let rval be ? GetValue(rref).
Return IsStrictlyEqual(rval, lval).

EqualityExpression

!==

RelationalExpression

Let lref be ? Evaluation of EqualityExpression.
Let lval be ? GetValue(lref).
Let rref be ? Evaluation of RelationalExpression.
Let rval be ? GetValue(rref).
Let r be IsStrictlyEqual(rval, lval).
If r is true, return false. Otherwise, return true.

Note 1

Given the above definition of equality:

String comparison can be forced by: `${a}` == `${b}`.
Numeric comparison can be forced by: +a == +b.
Boolean comparison can be forced by: !a == !b.

Note 2

The equality operators maintain the following invariants:

A != B is equivalent to !(A == B).
A == B is equivalent to B == A, except in the order of evaluation of A and B.

Note 3

The equality operator is not always transitive. For example, there might be two distinct String objects, each representing the same String value; each String object would be considered equal to the String value by the == operator, but the two String objects would not be equal to each other. For example:

new String("a") == "a" and "a" == new String("a") are both true.
new String("a") == new String("a") is false.

Note 4

Comparison of Strings uses a simple equality test on sequences of code unit values. There is no attempt to use the more complex, semantically oriented definitions of character or string equality and collating order defined in the Unicode specification. Therefore Strings values that are canonically equal according to the Unicode Standard could test as unequal. In effect this algorithm assumes that both Strings are already in normalized form.

13.12 Binary Bitwise Operators

Syntax

BitwiseANDExpression

[In, Yield, Await]

EqualityExpression

[?In, ?Yield, ?Await]

BitwiseANDExpression

[?In, ?Yield, ?Await]

EqualityExpression

[?In, ?Yield, ?Await]

BitwiseXORExpression

[In, Yield, Await]

BitwiseANDExpression

[?In, ?Yield, ?Await]

BitwiseXORExpression

[?In, ?Yield, ?Await]

BitwiseANDExpression

[?In, ?Yield, ?Await]

BitwiseORExpression

[In, Yield, Await]

BitwiseXORExpression

[?In, ?Yield, ?Await]

BitwiseORExpression

[?In, ?Yield, ?Await]

BitwiseXORExpression

[?In, ?Yield, ?Await]

13.12.1 Runtime Semantics: Evaluation

BitwiseANDExpression

EqualityExpression

Return ? EvaluateStringOrNumericBinaryExpression(BitwiseANDExpression, &, EqualityExpression).

BitwiseXORExpression

BitwiseANDExpression

Return ? EvaluateStringOrNumericBinaryExpression(BitwiseXORExpression, ^, BitwiseANDExpression).

BitwiseORExpression

BitwiseXORExpression

Return ? EvaluateStringOrNumericBinaryExpression(BitwiseORExpression, |, BitwiseXORExpression).

13.13 Binary Logical Operators

Syntax

LogicalANDExpression

[In, Yield, Await]

BitwiseORExpression

[?In, ?Yield, ?Await]

LogicalANDExpression

[?In, ?Yield, ?Await]

BitwiseORExpression

[?In, ?Yield, ?Await]

LogicalORExpression

[In, Yield, Await]

LogicalANDExpression

[?In, ?Yield, ?Await]

LogicalORExpression

[?In, ?Yield, ?Await]

LogicalANDExpression

[?In, ?Yield, ?Await]

CoalesceExpression

[In, Yield, Await]

CoalesceExpressionHead

[?In, ?Yield, ?Await]

BitwiseORExpression

[?In, ?Yield, ?Await]

CoalesceExpressionHead

[In, Yield, Await]

CoalesceExpression

[?In, ?Yield, ?Await]

BitwiseORExpression

[?In, ?Yield, ?Await]

ShortCircuitExpression

[In, Yield, Await]

LogicalORExpression

[?In, ?Yield, ?Await]

CoalesceExpression

[?In, ?Yield, ?Await]

Note

The value produced by a && or || operator is not necessarily of type Boolean. The value produced will always be the value of one of the two operand expressions.

13.13.1 Runtime Semantics: Evaluation

LogicalANDExpression

BitwiseORExpression

Let lref be ? Evaluation of LogicalANDExpression.
Let lval be ? GetValue(lref).
Let lbool be ToBoolean(lval).
If lbool is false, return lval.
Let rref be ? Evaluation of BitwiseORExpression.
Return ? GetValue(rref).

LogicalORExpression

LogicalANDExpression

Let lref be ? Evaluation of LogicalORExpression.
Let lval be ? GetValue(lref).
Let lbool be ToBoolean(lval).
If lbool is true, return lval.
Let rref be ? Evaluation of LogicalANDExpression.
Return ? GetValue(rref).

CoalesceExpression

CoalesceExpressionHead

BitwiseORExpression

Let lref be ? Evaluation of CoalesceExpressionHead.
Let lval be ? GetValue(lref).
3. If lval is either undefined or null, then
1. Let rref be ? Evaluation of BitwiseORExpression.
2. Return ? GetValue(rref).
4. Else,
1. Return lval.

13.14 Conditional Operator ( `? :` )

Syntax

ConditionalExpression

[In, Yield, Await]

ShortCircuitExpression

[?In, ?Yield, ?Await]

ShortCircuitExpression

[?In, ?Yield, ?Await]

AssignmentExpression

[+In, ?Yield, ?Await]

AssignmentExpression

[?In, ?Yield, ?Await]

Note

The grammar for a ConditionalExpression in ECMAScript is slightly different from that in C and Java, which each allow the second subexpression to be an Expression but restrict the third expression to be a ConditionalExpression. The motivation for this difference in ECMAScript is to allow an assignment expression to be governed by either arm of a conditional and to eliminate the confusing and fairly useless case of a comma expression as the centre expression.

13.14.1 Runtime Semantics: Evaluation

ConditionalExpression

ShortCircuitExpression

AssignmentExpression

Let lref be ? Evaluation of ShortCircuitExpression.
Let lval be ToBoolean(? GetValue(lref)).
3. If lval is true, then
1. Let trueRef be ? Evaluation of the first AssignmentExpression.
2. Return ? GetValue(trueRef).
4. Else,
1. Let falseRef be ? Evaluation of the second AssignmentExpression.
2. Return ? GetValue(falseRef).

13.15 Assignment Operators

Syntax

AssignmentExpression

[In, Yield, Await]

ConditionalExpression

[?In, ?Yield, ?Await]

[+Yield]

YieldExpression

[?In, ?Await]

ArrowFunction

[?In, ?Yield, ?Await]

AsyncArrowFunction

[?In, ?Yield, ?Await]

LeftHandSideExpression

[?Yield, ?Await]

AssignmentExpression

[?In, ?Yield, ?Await]

LeftHandSideExpression

[?Yield, ?Await]

AssignmentOperator

AssignmentExpression

[?In, ?Yield, ?Await]

LeftHandSideExpression

[?Yield, ?Await]

&&=

AssignmentExpression

[?In, ?Yield, ?Await]

LeftHandSideExpression

[?Yield, ?Await]

||=

AssignmentExpression

[?In, ?Yield, ?Await]

LeftHandSideExpression

[?Yield, ?Await]

??=

AssignmentExpression

[?In, ?Yield, ?Await]

AssignmentOperator

one of

<<=

>>=

>>>=

**=

13.15.1 Static Semantics: Early Errors

AssignmentExpression

LeftHandSideExpression

AssignmentExpression

If LeftHandSideExpression is either an ObjectLiteral or an ArrayLiteral, the following Early Error rules are applied:

LeftHandSideExpression must cover an AssignmentPattern.

If LeftHandSideExpression is neither an ObjectLiteral nor an ArrayLiteral, the following Early Error rule is applied:

It is a Syntax Error if AssignmentTargetType of LeftHandSideExpression is not simple.

AssignmentExpression

LeftHandSideExpression

AssignmentOperator

AssignmentExpression

LeftHandSideExpression

&&=

AssignmentExpression

LeftHandSideExpression

||=

AssignmentExpression

LeftHandSideExpression

??=

AssignmentExpression

It is a Syntax Error if AssignmentTargetType of LeftHandSideExpression is not simple.

13.15.2 Runtime Semantics: Evaluation

AssignmentExpression

LeftHandSideExpression

AssignmentExpression

1. If LeftHandSideExpression is neither an ObjectLiteral nor an ArrayLiteral, then
1. Let lref be ? Evaluation of LeftHandSideExpression.
2. b. If IsAnonymousFunctionDefinition(AssignmentExpression) and IsIdentifierRef of LeftHandSideExpression are both true, then
  1. Let rval be ? NamedEvaluation of AssignmentExpression with argument lref.[[ReferencedName]].
3. c. Else,
  1. Let rref be ? Evaluation of AssignmentExpression.
  2. Let rval be ? GetValue(rref).
4. Perform ? PutValue(lref, rval).
5. Return rval.
Let assignmentPattern be the AssignmentPattern that is covered by LeftHandSideExpression.
Let rref be ? Evaluation of AssignmentExpression.
Let rval be ? GetValue(rref).
Perform ? DestructuringAssignmentEvaluation of assignmentPattern with argument rval.
Return rval.

AssignmentExpression

LeftHandSideExpression

AssignmentOperator

AssignmentExpression

Let lref be ? Evaluation of LeftHandSideExpression.
Let lval be ? GetValue(lref).
Let rref be ? Evaluation of AssignmentExpression.
Let rval be ? GetValue(rref).
Let assignmentOpText be the source text matched by AssignmentOperator.

Let opText be the sequence of Unicode code points associated with assignmentOpText in the following table:

`assignmentOpText`	`opText`
`**=`	`**`
`*=`	`*`
`/=`	`/`
`%=`	`%`
`+=`	`+`
`-=`	`-`
`<<=`	`<<`
`>>=`	`>>`
`>>>=`	`>>>`
`&=`	`&`
`^=`	`^`
`\|=`	`\|`

Let r be ? ApplyStringOrNumericBinaryOperator(lval, opText, rval).
Perform ? PutValue(lref, r).
Return r.

AssignmentExpression

LeftHandSideExpression

&&=

AssignmentExpression

Let lref be ? Evaluation of LeftHandSideExpression.
Let lval be ? GetValue(lref).
Let lbool be ToBoolean(lval).
If lbool is false, return lval.
5. If IsAnonymousFunctionDefinition(AssignmentExpression) is true and IsIdentifierRef of LeftHandSideExpression is true, then
1. Let rval be ? NamedEvaluation of AssignmentExpression with argument lref.[[ReferencedName]].
6. Else,
1. Let rref be ? Evaluation of AssignmentExpression.
2. Let rval be ? GetValue(rref).
Perform ? PutValue(lref, rval).
Return rval.

AssignmentExpression

LeftHandSideExpression

||=

AssignmentExpression

Let lref be ? Evaluation of LeftHandSideExpression.
Let lval be ? GetValue(lref).
Let lbool be ToBoolean(lval).
If lbool is true, return lval.
5. If IsAnonymousFunctionDefinition(AssignmentExpression) is true and IsIdentifierRef of LeftHandSideExpression is true, then
1. Let rval be ? NamedEvaluation of AssignmentExpression with argument lref.[[ReferencedName]].
6. Else,
1. Let rref be ? Evaluation of AssignmentExpression.
2. Let rval be ? GetValue(rref).
Perform ? PutValue(lref, rval).
Return rval.

AssignmentExpression

LeftHandSideExpression

??=

AssignmentExpression

Let lref be ? Evaluation of LeftHandSideExpression.
Let lval be ? GetValue(lref).
If lval is neither undefined nor null, return lval.
4. If IsAnonymousFunctionDefinition(AssignmentExpression) is true and IsIdentifierRef of LeftHandSideExpression is true, then
1. Let rval be ? NamedEvaluation of AssignmentExpression with argument lref.[[ReferencedName]].
5. Else,
1. Let rref be ? Evaluation of AssignmentExpression.
2. Let rval be ? GetValue(rref).
Perform ? PutValue(lref, rval).
Return rval.

Note

When this expression occurs within strict mode code, it is a runtime error if lref in step 1.d, 2, 2, 2, 2 is an unresolvable reference. If it is, a ReferenceError exception is thrown. Additionally, it is a runtime error if the lref in step 8, 7, 7, 6 is a reference to a data property with the attribute value { [[Writable]]: false }, to an accessor property with the attribute value { [[Set]]: undefined }, or to a non-existent property of an object for which the IsExtensible predicate returns the value false. In these cases a TypeError exception is thrown.

13.15.3 ApplyStringOrNumericBinaryOperator ( `lval`, `opText`, `rval` )

The abstract operation ApplyStringOrNumericBinaryOperator takes arguments lval (an ECMAScript language value), opText (**, *, /, %, +, -, <<, >>, >>>, &, ^, or |), and rval (an ECMAScript language value) and returns either a normal completion containing either a String, a BigInt, or a Number, or a throw completion. It performs the following steps when called:

1. If opText is +, then
1. Let lprim be ? ToPrimitive(lval).
2. Let rprim be ? ToPrimitive(rval).
3. c. If lprim is a String or rprim is a String, then
  1. Let lstr be ? ToString(lprim).
  2. Let rstr be ? ToString(rprim).
  3. Return the string-concatenation of lstr and rstr.
4. Set lval to lprim.
5. Set rval to rprim.
NOTE: At this point, it must be a numeric operation.
Let lnum be ? ToNumeric(lval).
Let rnum be ? ToNumeric(rval).
If Type(lnum) is not Type(rnum), throw a TypeError exception.
6. If lnum is a BigInt, then
1. If opText is **, return ? BigInt::exponentiate(lnum, rnum).
2. If opText is /, return ? BigInt::divide(lnum, rnum).
3. If opText is %, return ? BigInt::remainder(lnum, rnum).
4. If opText is >>>, return ? BigInt::unsignedRightShift(lnum, rnum).

Let operation be the abstract operation associated with opText and Type(lnum) in the following table:

`opText`	Type(`lnum`)	`operation`
`**`	Number	Number::exponentiate
`*`	Number	Number::multiply
`*`	BigInt	BigInt::multiply
`/`	Number	Number::divide
`%`	Number	Number::remainder
`+`	Number	Number::add
`+`	BigInt	BigInt::add
`-`	Number	Number::subtract
`-`	BigInt	BigInt::subtract
`<<`	Number	Number::leftShift
`<<`	BigInt	BigInt::leftShift
`>>`	Number	Number::signedRightShift
`>>`	BigInt	BigInt::signedRightShift
`>>>`	Number	Number::unsignedRightShift
`&`	Number	Number::bitwiseAND
`&`	BigInt	BigInt::bitwiseAND
`^`	Number	Number::bitwiseXOR
`^`	BigInt	BigInt::bitwiseXOR
`\|`	Number	Number::bitwiseOR
`\|`	BigInt	BigInt::bitwiseOR

Return operation(lnum, rnum).

Note 1

No hint is provided in the calls to ToPrimitive in steps 1.a and 1.b. All standard objects except Dates handle the absence of a hint as if number were given; Dates handle the absence of a hint as if string were given. Exotic objects may handle the absence of a hint in some other manner.

Note 2

Step 1.c differs from step 3 of the IsLessThan algorithm, by using the logical-or operation instead of the logical-and operation.

13.15.4 EvaluateStringOrNumericBinaryExpression ( `leftOperand`, `opText`, `rightOperand` )

The abstract operation EvaluateStringOrNumericBinaryExpression takes arguments leftOperand (a Parse Node), opText (a sequence of Unicode code points), and rightOperand (a Parse Node) and returns either a normal completion containing either a String, a BigInt, or a Number, or an abrupt completion. It performs the following steps when called:

Let lref be ? Evaluation of leftOperand.
Let lval be ? GetValue(lref).
Let rref be ? Evaluation of rightOperand.
Let rval be ? GetValue(rref).
Return ? ApplyStringOrNumericBinaryOperator(lval, opText, rval).

13.15.5 Destructuring Assignment

Supplemental Syntax

In certain circumstances when processing an instance of the production
AssignmentExpression : LeftHandSideExpression = AssignmentExpression
the interpretation of LeftHandSideExpression is refined using the following grammar:

AssignmentPattern

[Yield, Await]

ObjectAssignmentPattern

[?Yield, ?Await]

ArrayAssignmentPattern

[?Yield, ?Await]

ObjectAssignmentPattern

[Yield, Await]

{

}

{

AssignmentRestProperty

[?Yield, ?Await]

}

{

AssignmentPropertyList

[?Yield, ?Await]

}

{

AssignmentPropertyList

[?Yield, ?Await]

AssignmentRestProperty

[?Yield, ?Await]

opt

}

ArrayAssignmentPattern

[Yield, Await]

[

Elision

opt

AssignmentRestElement

[?Yield, ?Await]

opt

]

[

AssignmentElementList

[?Yield, ?Await]

]

[

AssignmentElementList

[?Yield, ?Await]

Elision

opt

AssignmentRestElement

[?Yield, ?Await]

opt

]

AssignmentRestProperty

[Yield, Await]

...

DestructuringAssignmentTarget

[?Yield, ?Await]

AssignmentPropertyList

[Yield, Await]

AssignmentProperty

[?Yield, ?Await]

AssignmentPropertyList

[?Yield, ?Await]

AssignmentProperty

[?Yield, ?Await]

AssignmentElementList

[Yield, Await]

AssignmentElisionElement

[?Yield, ?Await]

AssignmentElementList

[?Yield, ?Await]

AssignmentElisionElement

[?Yield, ?Await]

AssignmentElisionElement

[Yield, Await]

opt

[?Yield, ?Await]

[Yield, Await]

[?Yield, ?Await]

[+In, ?Yield, ?Await]

opt

PropertyName

[?Yield, ?Await]

AssignmentElement

[?Yield, ?Await]

AssignmentElement

[Yield, Await]

DestructuringAssignmentTarget

[?Yield, ?Await]

Initializer

[+In, ?Yield, ?Await]

opt

AssignmentRestElement

[Yield, Await]

...

DestructuringAssignmentTarget

[?Yield, ?Await]

DestructuringAssignmentTarget

[Yield, Await]

LeftHandSideExpression

[?Yield, ?Await]

13.15.5.1 Static Semantics: Early Errors

AssignmentProperty

IdentifierReference

Initializer

opt

It is a Syntax Error if AssignmentTargetType of IdentifierReference is not simple.

AssignmentRestProperty

...

DestructuringAssignmentTarget

It is a Syntax Error if DestructuringAssignmentTarget is either an ArrayLiteral or an ObjectLiteral.

DestructuringAssignmentTarget

LeftHandSideExpression

If LeftHandSideExpression is either an ObjectLiteral or an ArrayLiteral, the following Early Error rules are applied:

LeftHandSideExpression must cover an AssignmentPattern.

If LeftHandSideExpression is neither an ObjectLiteral nor an ArrayLiteral, the following Early Error rule is applied:

It is a Syntax Error if AssignmentTargetType of LeftHandSideExpression is not simple.

13.15.5.2 Runtime Semantics: DestructuringAssignmentEvaluation

The syntax-directed operation DestructuringAssignmentEvaluation takes argument value (an ECMAScript language value) and returns either a normal completion containing unused or an abrupt completion. It is defined piecewise over the following productions:

ObjectAssignmentPattern

{

}

Perform ? RequireObjectCoercible(value).
Return unused.

ObjectAssignmentPattern

{

AssignmentPropertyList

}

{

AssignmentPropertyList

}

Perform ? RequireObjectCoercible(value).
Perform ? PropertyDestructuringAssignmentEvaluation of AssignmentPropertyList with argument value.
Return unused.

ObjectAssignmentPattern

{

AssignmentRestProperty

}

Perform ? RequireObjectCoercible(value).
Let excludedNames be a new empty List.
Return ? RestDestructuringAssignmentEvaluation of AssignmentRestProperty with arguments value and excludedNames.

ObjectAssignmentPattern

{

AssignmentPropertyList

AssignmentRestProperty

}

Perform ? RequireObjectCoercible(value).
Let excludedNames be ? PropertyDestructuringAssignmentEvaluation of AssignmentPropertyList with argument value.
Return ? RestDestructuringAssignmentEvaluation of AssignmentRestProperty with arguments value and excludedNames.

ArrayAssignmentPattern

[

]

Let iteratorRecord be ? GetIterator(value, sync).
Return ? IteratorClose(iteratorRecord, NormalCompletion(unused)).

ArrayAssignmentPattern

[

Elision

]

Let iteratorRecord be ? GetIterator(value, sync).
Let result be Completion(IteratorDestructuringAssignmentEvaluation of Elision with argument iteratorRecord).
If iteratorRecord.[[Done]] is false, return ? IteratorClose(iteratorRecord, result).
Return result.

ArrayAssignmentPattern

[

Elision

opt

AssignmentRestElement

]

Let iteratorRecord be ? GetIterator(value, sync).
2. If Elision is present, then
1. Let status be Completion(IteratorDestructuringAssignmentEvaluation of Elision with argument iteratorRecord).
2. b. If status is an abrupt completion, then
  1. Assert: iteratorRecord.[[Done]] is true.
  2. Return ? status.
Let result be Completion(IteratorDestructuringAssignmentEvaluation of AssignmentRestElement with argument iteratorRecord).
If iteratorRecord.[[Done]] is false, return ? IteratorClose(iteratorRecord, result).
Return result.

ArrayAssignmentPattern

[

AssignmentElementList

]

Let iteratorRecord be ? GetIterator(value, sync).
Let result be Completion(IteratorDestructuringAssignmentEvaluation of AssignmentElementList with argument iteratorRecord).
If iteratorRecord.[[Done]] is false, return ? IteratorClose(iteratorRecord, result).
Return result.

ArrayAssignmentPattern

[

AssignmentElementList

Elision

opt

AssignmentRestElement

opt

]

Let iteratorRecord be ? GetIterator(value, sync).
Let status be Completion(IteratorDestructuringAssignmentEvaluation of AssignmentElementList with argument iteratorRecord).
3. If status is an abrupt completion, then
1. If iteratorRecord.[[Done]] is false, return ? IteratorClose(iteratorRecord, status).
2. Return ? status.
4. If Elision is present, then
1. Set status to Completion(IteratorDestructuringAssignmentEvaluation of Elision with argument iteratorRecord).
2. b. If status is an abrupt completion, then
  1. Assert: iteratorRecord.[[Done]] is true.
  2. Return ? status.
5. If AssignmentRestElement is present, then
1. Set status to Completion(IteratorDestructuringAssignmentEvaluation of AssignmentRestElement with argument iteratorRecord).
If iteratorRecord.[[Done]] is false, return ? IteratorClose(iteratorRecord, status).
Return ? status.

13.15.5.3 Runtime Semantics: PropertyDestructuringAssignmentEvaluation

The syntax-directed operation PropertyDestructuringAssignmentEvaluation takes argument value (an ECMAScript language value) and returns either a normal completion containing a List of property keys or an abrupt completion. It collects a list of all destructured property keys. It is defined piecewise over the following productions:

AssignmentPropertyList

AssignmentProperty

Let propertyNames be ? PropertyDestructuringAssignmentEvaluation of AssignmentPropertyList with argument value.
Let nextNames be ? PropertyDestructuringAssignmentEvaluation of AssignmentProperty with argument value.
Return the list-concatenation of propertyNames and nextNames.

AssignmentProperty

IdentifierReference

Initializer

opt

Let P be StringValue of IdentifierReference.
Let lref be ? ResolveBinding(P).
Let v be ? GetV(value, P).
4. If Initializer is present and v is undefined, then
1. a. If IsAnonymousFunctionDefinition(Initializer) is true, then
  1. Set v to ? NamedEvaluation of Initializer with argument P.
2. b. Else,
  1. Let defaultValue be ? Evaluation of Initializer.
  2. Set v to ? GetValue(defaultValue).
Perform ? PutValue(lref, v).
Return « P ».

AssignmentProperty

PropertyName

AssignmentElement

Let name be ? Evaluation of PropertyName.
Perform ? KeyedDestructuringAssignmentEvaluation of AssignmentElement with arguments value and name.
Return « name ».

13.15.5.4 Runtime Semantics: RestDestructuringAssignmentEvaluation

The syntax-directed operation RestDestructuringAssignmentEvaluation takes arguments value (an ECMAScript language value) and excludedNames (a List of property keys) and returns either a normal completion containing unused or an abrupt completion. It is defined piecewise over the following productions:

AssignmentRestProperty

...

DestructuringAssignmentTarget

Let lref be ? Evaluation of DestructuringAssignmentTarget.
Let restObj be OrdinaryObjectCreate(%Object.prototype%).
Perform ? CopyDataProperties(restObj, value, excludedNames).
Return ? PutValue(lref, restObj).

13.15.5.5 Runtime Semantics: IteratorDestructuringAssignmentEvaluation

The syntax-directed operation IteratorDestructuringAssignmentEvaluation takes argument iteratorRecord (an Iterator Record) and returns either a normal completion containing unused or an abrupt completion. It is defined piecewise over the following productions:

AssignmentElementList

AssignmentElisionElement

Return ? IteratorDestructuringAssignmentEvaluation of AssignmentElisionElement with argument iteratorRecord.

AssignmentElementList

AssignmentElisionElement

Perform ? IteratorDestructuringAssignmentEvaluation of AssignmentElementList with argument iteratorRecord.
Return ? IteratorDestructuringAssignmentEvaluation of AssignmentElisionElement with argument iteratorRecord.

AssignmentElisionElement

AssignmentElement

Return ? IteratorDestructuringAssignmentEvaluation of AssignmentElement with argument iteratorRecord.

AssignmentElisionElement

Elision

AssignmentElement

Perform ? IteratorDestructuringAssignmentEvaluation of Elision with argument iteratorRecord.
Return ? IteratorDestructuringAssignmentEvaluation of AssignmentElement with argument iteratorRecord.

Elision

1. If iteratorRecord.[[Done]] is false, then
1. Let next be Completion(IteratorStep(iteratorRecord)).
2. If next is an abrupt completion, set iteratorRecord.[[Done]] to true.
3. ReturnIfAbrupt(next).
4. If next is false, set iteratorRecord.[[Done]] to true.
Return unused.

Elision

Perform ? IteratorDestructuringAssignmentEvaluation of Elision with argument iteratorRecord.
2. If iteratorRecord.[[Done]] is false, then
1. Let next be Completion(IteratorStep(iteratorRecord)).
2. If next is an abrupt completion, set iteratorRecord.[[Done]] to true.
3. ReturnIfAbrupt(next).
4. If next is false, set iteratorRecord.[[Done]] to true.
Return unused.

AssignmentElement

DestructuringAssignmentTarget

Initializer

opt

1. If DestructuringAssignmentTarget is neither an ObjectLiteral nor an ArrayLiteral, then
1. Let lref be ? Evaluation of DestructuringAssignmentTarget.
Let value be undefined.
3. If iteratorRecord.[[Done]] is false, then
1. Let next be ? IteratorStepValue(iteratorRecord).
2. b. If next is not done, then
  1. Set value to next.
4. If Initializer is present and value is undefined, then
1. a. If IsAnonymousFunctionDefinition(Initializer) is true and IsIdentifierRef of DestructuringAssignmentTarget is true, then
  1. Let v be ? NamedEvaluation of Initializer with argument lref.[[ReferencedName]].
2. b. Else,
  1. Let defaultValue be ? Evaluation of Initializer.
  2. Let v be ? GetValue(defaultValue).
5. Else,
1. Let v be value.
6. If DestructuringAssignmentTarget is either an ObjectLiteral or an ArrayLiteral, then
1. Let nestedAssignmentPattern be the AssignmentPattern that is covered by DestructuringAssignmentTarget.
2. Return ? DestructuringAssignmentEvaluation of nestedAssignmentPattern with argument v.
Return ? PutValue(lref, v).

Note

Left to right evaluation order is maintained by evaluating a DestructuringAssignmentTarget that is not a destructuring pattern prior to accessing the iterator or evaluating the Initializer.

AssignmentRestElement

...

DestructuringAssignmentTarget

1. If DestructuringAssignmentTarget is neither an ObjectLiteral nor an ArrayLiteral, then
1. Let lref be ? Evaluation of DestructuringAssignmentTarget.
Let A be ! ArrayCreate(0).
Let n be 0.
4. Repeat, while iteratorRecord.[[Done]] is false,
1. Let next be ? IteratorStepValue(iteratorRecord).
2. b. If next is not done, then
  1. Perform ! CreateDataPropertyOrThrow(A, ! ToString(𝔽(n)), next).
  2. Set n to n + 1.
5. If DestructuringAssignmentTarget is neither an ObjectLiteral nor an ArrayLiteral, then
1. Return ? PutValue(lref, A).
Let nestedAssignmentPattern be the AssignmentPattern that is covered by DestructuringAssignmentTarget.
Return ? DestructuringAssignmentEvaluation of nestedAssignmentPattern with argument A.

13.15.5.6 Runtime Semantics: KeyedDestructuringAssignmentEvaluation

The syntax-directed operation KeyedDestructuringAssignmentEvaluation takes arguments value (an ECMAScript language value) and propertyName (a property key) and returns either a normal completion containing unused or an abrupt completion. It is defined piecewise over the following productions:

AssignmentElement

DestructuringAssignmentTarget

Initializer

opt

1. If DestructuringAssignmentTarget is neither an ObjectLiteral nor an ArrayLiteral, then
1. Let lref be ? Evaluation of DestructuringAssignmentTarget.
Let v be ? GetV(value, propertyName).
3. If Initializer is present and v is undefined, then
1. a. If IsAnonymousFunctionDefinition(Initializer) and IsIdentifierRef of DestructuringAssignmentTarget are both true, then
  1. Let rhsValue be ? NamedEvaluation of Initializer with argument lref.[[ReferencedName]].
2. b. Else,
  1. Let defaultValue be ? Evaluation of Initializer.
  2. Let rhsValue be ? GetValue(defaultValue).
4. Else,
1. Let rhsValue be v.
5. If DestructuringAssignmentTarget is either an ObjectLiteral or an ArrayLiteral, then
1. Let assignmentPattern be the AssignmentPattern that is covered by DestructuringAssignmentTarget.
2. Return ? DestructuringAssignmentEvaluation of assignmentPattern with argument rhsValue.
Return ? PutValue(lref, rhsValue).

13.16 Comma Operator ( `,` )

Syntax

Expression

[In, Yield, Await]

AssignmentExpression

[?In, ?Yield, ?Await]

Expression

[?In, ?Yield, ?Await]

AssignmentExpression

[?In, ?Yield, ?Await]

13.16.1 Runtime Semantics: Evaluation

Expression

AssignmentExpression

Let lref be ? Evaluation of Expression.
Perform ? GetValue(lref).
Let rref be ? Evaluation of AssignmentExpression.
Return ? GetValue(rref).

Note

GetValue must be called even though its value is not used because it may have observable side-effects.

14 ECMAScript Language: Statements and Declarations

Syntax

Statement

[Yield, Await, Return]

BlockStatement

[?Yield, ?Await, ?Return]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await, ?Return]

BreakableStatement

[?Yield, ?Await, ?Return]

ContinueStatement

[?Yield, ?Await]

BreakStatement

[?Yield, ?Await]

[+Return]

ReturnStatement

[?Yield, ?Await]

WithStatement

[?Yield, ?Await, ?Return]

LabelledStatement

[?Yield, ?Await, ?Return]

ThrowStatement

[?Yield, ?Await]

TryStatement

[?Yield, ?Await, ?Return]

DebuggerStatement

Declaration

[Yield, Await]

HoistableDeclaration

[?Yield, ?Await, ~Default]

ClassDeclaration

[?Yield, ?Await, ~Default]

LexicalDeclaration

[+In, ?Yield, ?Await]

HoistableDeclaration

[Yield, Await, Default]

FunctionDeclaration

[?Yield, ?Await, ?Default]

GeneratorDeclaration

[?Yield, ?Await, ?Default]

AsyncFunctionDeclaration

[?Yield, ?Await, ?Default]

AsyncGeneratorDeclaration

[?Yield, ?Await, ?Default]

BreakableStatement

[Yield, Await, Return]

IterationStatement

[?Yield, ?Await, ?Return]

SwitchStatement

[?Yield, ?Await, ?Return]

14.1 Statement Semantics

14.1.1 Runtime Semantics: Evaluation

HoistableDeclaration

GeneratorDeclaration

AsyncFunctionDeclaration

AsyncGeneratorDeclaration

Return empty.

HoistableDeclaration

FunctionDeclaration

Return ? Evaluation of FunctionDeclaration.

BreakableStatement

IterationStatement

SwitchStatement

Let newLabelSet be a new empty List.
Return ? LabelledEvaluation of this BreakableStatement with argument newLabelSet.

14.2 Block

Syntax

BlockStatement

[Yield, Await, Return]

Block

[?Yield, ?Await, ?Return]

Block

[Yield, Await, Return]

{

StatementList

[?Yield, ?Await, ?Return]

opt

}

StatementList

[Yield, Await, Return]

StatementListItem

[?Yield, ?Await, ?Return]

StatementList

[?Yield, ?Await, ?Return]

StatementListItem

[?Yield, ?Await, ?Return]

StatementListItem

[Yield, Await, Return]

Statement

[?Yield, ?Await, ?Return]

Declaration

[?Yield, ?Await]

14.2.1 Static Semantics: Early Errors

Block

{

StatementList

}

It is a Syntax Error if the LexicallyDeclaredNames of StatementList contains any duplicate entries.
It is a Syntax Error if any element of the LexicallyDeclaredNames of StatementList also occurs in the VarDeclaredNames of StatementList.

14.2.2 Runtime Semantics: Evaluation

Block

{

}

Return empty.

Block

{

StatementList

}

Let oldEnv be the running execution context's LexicalEnvironment.
Let blockEnv be NewDeclarativeEnvironment(oldEnv).
Perform BlockDeclarationInstantiation(StatementList, blockEnv).
Set the running execution context's LexicalEnvironment to blockEnv.
Let blockValue be Completion(Evaluation of StatementList).
Set the running execution context's LexicalEnvironment to oldEnv.
Return ? blockValue.

Note 1

No matter how control leaves the Block the LexicalEnvironment is always restored to its former state.

StatementList

StatementListItem

Let sl be ? Evaluation of StatementList.
Let s be Completion(Evaluation of StatementListItem).
Return ? UpdateEmpty(s, sl).

Note 2

The value of a StatementList is the value of the last value-producing item in the StatementList. For example, the following calls to the eval function all return the value 1:

eval("1;;;;;")
eval("1;{}")
eval("1;var a;")

14.2.3 BlockDeclarationInstantiation ( `code`, `env` )

The abstract operation BlockDeclarationInstantiation takes arguments code (a Parse Node) and env (a Declarative Environment Record) and returns unused. code is the Parse Node corresponding to the body of the block. env is the Environment Record in which bindings are to be created.

Note

When a Block or CaseBlock is evaluated a new Declarative Environment Record is created and bindings for each block scoped variable, constant, function, or class declared in the block are instantiated in the Environment Record.

It performs the following steps when called:

Let declarations be the LexicallyScopedDeclarations of code.
Let privateEnv be the running execution context's PrivateEnvironment.
3. For each element d of declarations, do
1. a. For each element dn of the BoundNames of d, do
  1. i. If IsConstantDeclaration of d is true, then
    1. Perform ! env.CreateImmutableBinding(dn, true).
  2. ii. Else,
    1. Perform ! env.CreateMutableBinding(dn, false). NOTE: This step is replaced in section B.3.2.6.
2. b. If d is either a FunctionDeclaration, a GeneratorDeclaration, an AsyncFunctionDeclaration, or an AsyncGeneratorDeclaration, then
  1. Let fn be the sole element of the BoundNames of d.
  2. Let fo be InstantiateFunctionObject of d with arguments env and privateEnv.
  3. Perform ! env.InitializeBinding(fn, fo). NOTE: This step is replaced in section B.3.2.6.
Return unused.

14.3 Declarations and the Variable Statement

14.3.1 Let and Const Declarations

Note

let and const declarations define variables that are scoped to the running execution context's LexicalEnvironment. The variables are created when their containing Environment Record is instantiated but may not be accessed in any way until the variable's LexicalBinding is evaluated. A variable defined by a LexicalBinding with an Initializer is assigned the value of its Initializer's AssignmentExpression when the LexicalBinding is evaluated, not when the variable is created. If a LexicalBinding in a let declaration does not have an Initializer the variable is assigned the value undefined when the LexicalBinding is evaluated.

Syntax

LexicalDeclaration

[In, Yield, Await]

LetOrConst

BindingList

[?In, ?Yield, ?Await]

;

LetOrConst

let

const

BindingList

[In, Yield, Await]

LexicalBinding

[?In, ?Yield, ?Await]

BindingList

[?In, ?Yield, ?Await]

LexicalBinding

[?In, ?Yield, ?Await]

LexicalBinding

[In, Yield, Await]

BindingIdentifier

[?Yield, ?Await]

Initializer

[?In, ?Yield, ?Await]

opt

BindingPattern

[?Yield, ?Await]

Initializer

[?In, ?Yield, ?Await]

14.3.1.1 Static Semantics: Early Errors

LexicalDeclaration

LetOrConst

BindingList

;

It is a Syntax Error if the BoundNames of BindingList contains "let".
It is a Syntax Error if the BoundNames of BindingList contains any duplicate entries.

LexicalBinding

BindingIdentifier

Initializer

opt

It is a Syntax Error if Initializer is not present and IsConstantDeclaration of the LexicalDeclaration containing this LexicalBinding is true.

14.3.1.2 Runtime Semantics: Evaluation

LexicalDeclaration

LetOrConst

BindingList

;

Perform ? Evaluation of BindingList.
Return empty.

BindingList

LexicalBinding

Perform ? Evaluation of BindingList.
Return ? Evaluation of LexicalBinding.

LexicalBinding

BindingIdentifier

Let lhs be ! ResolveBinding(StringValue of BindingIdentifier).
Perform ! InitializeReferencedBinding(lhs, undefined).
Return empty.

Note

A static semantics rule ensures that this form of LexicalBinding never occurs in a const declaration.

LexicalBinding

BindingIdentifier

Initializer

Let bindingId be StringValue of BindingIdentifier.
Let lhs be ! ResolveBinding(bindingId).
3. If IsAnonymousFunctionDefinition(Initializer) is true, then
1. Let value be ? NamedEvaluation of Initializer with argument bindingId.
4. Else,
1. Let rhs be ? Evaluation of Initializer.
2. Let value be ? GetValue(rhs).
Perform ! InitializeReferencedBinding(lhs, value).
Return empty.

LexicalBinding

BindingPattern

Initializer

Let rhs be ? Evaluation of Initializer.
Let value be ? GetValue(rhs).
Let env be the running execution context's LexicalEnvironment.
Return ? BindingInitialization of BindingPattern with arguments value and env.

14.3.2 Variable Statement

Note

A var statement declares variables that are scoped to the running execution context's VariableEnvironment. Var variables are created when their containing Environment Record is instantiated and are initialized to undefined when created. Within the scope of any VariableEnvironment a common BindingIdentifier may appear in more than one VariableDeclaration but those declarations collectively define only one variable. A variable defined by a VariableDeclaration with an Initializer is assigned the value of its Initializer's AssignmentExpression when the VariableDeclaration is executed, not when the variable is created.

Syntax

VariableStatement

[Yield, Await]

var

VariableDeclarationList

[+In, ?Yield, ?Await]

;

VariableDeclarationList

[In, Yield, Await]

VariableDeclaration

[?In, ?Yield, ?Await]

VariableDeclarationList

[?In, ?Yield, ?Await]

VariableDeclaration

[?In, ?Yield, ?Await]

VariableDeclaration

[In, Yield, Await]

BindingIdentifier

[?Yield, ?Await]

Initializer

[?In, ?Yield, ?Await]

opt

BindingPattern

[?Yield, ?Await]

Initializer

[?In, ?Yield, ?Await]

14.3.2.1 Runtime Semantics: Evaluation

VariableStatement

var

VariableDeclarationList

;

Perform ? Evaluation of VariableDeclarationList.
Return empty.

VariableDeclarationList

VariableDeclaration

Perform ? Evaluation of VariableDeclarationList.
Return ? Evaluation of VariableDeclaration.

VariableDeclaration

BindingIdentifier

Return empty.

VariableDeclaration

BindingIdentifier

Initializer

Let bindingId be StringValue of BindingIdentifier.
Let lhs be ? ResolveBinding(bindingId).
3. If IsAnonymousFunctionDefinition(Initializer) is true, then
1. Let value be ? NamedEvaluation of Initializer with argument bindingId.
4. Else,
1. Let rhs be ? Evaluation of Initializer.
2. Let value be ? GetValue(rhs).
Perform ? PutValue(lhs, value).
Return empty.

Note

If a VariableDeclaration is nested within a with statement and the BindingIdentifier in the VariableDeclaration is the same as a property name of the binding object of the with statement's Object Environment Record, then step 5 will assign value to the property instead of assigning to the VariableEnvironment binding of the Identifier.

VariableDeclaration

BindingPattern

Initializer

Let rhs be ? Evaluation of Initializer.
Let rval be ? GetValue(rhs).
Return ? BindingInitialization of BindingPattern with arguments rval and undefined.

14.3.3 Destructuring Binding Patterns

Syntax

[Yield, Await]

[?Yield, ?Await]

[?Yield, ?Await]

[Yield, Await]

{

}

{

BindingRestProperty

[?Yield, ?Await]

}

{

BindingPropertyList

[?Yield, ?Await]

}

{

BindingPropertyList

[?Yield, ?Await]

BindingRestProperty

[?Yield, ?Await]

opt

}

ArrayBindingPattern

[Yield, Await]

[

Elision

opt

BindingRestElement

[?Yield, ?Await]

opt

]

[

BindingElementList

[?Yield, ?Await]

]

[

BindingElementList

[?Yield, ?Await]

Elision

opt

BindingRestElement

[?Yield, ?Await]

opt

]

BindingRestProperty

[Yield, Await]

...

[?Yield, ?Await]

[Yield, Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[Yield, Await]

BindingElisionElement

[?Yield, ?Await]

BindingElementList

[?Yield, ?Await]

BindingElisionElement

[?Yield, ?Await]

BindingElisionElement

[Yield, Await]

opt

[?Yield, ?Await]

[Yield, Await]

[?Yield, ?Await]

[?Yield, ?Await]

[?Yield, ?Await]

[Yield, Await]

[?Yield, ?Await]

[?Yield, ?Await]

[+In, ?Yield, ?Await]

opt

SingleNameBinding

[Yield, Await]

BindingIdentifier

[?Yield, ?Await]

Initializer

[+In, ?Yield, ?Await]

opt

BindingRestElement

[Yield, Await]

...

BindingIdentifier

[?Yield, ?Await]

...

BindingPattern

[?Yield, ?Await]

14.3.3.1 Runtime Semantics: PropertyBindingInitialization

The syntax-directed operation PropertyBindingInitialization takes arguments value (an ECMAScript language value) and environment (an Environment Record or undefined) and returns either a normal completion containing a List of property keys or an abrupt completion. It collects a list of all bound property names. It is defined piecewise over the following productions:

BindingPropertyList

BindingProperty

Let boundNames be ? PropertyBindingInitialization of BindingPropertyList with arguments value and environment.
Let nextNames be ? PropertyBindingInitialization of BindingProperty with arguments value and environment.
Return the list-concatenation of boundNames and nextNames.

BindingProperty

SingleNameBinding

Let name be the sole element of the BoundNames of SingleNameBinding.
Perform ? KeyedBindingInitialization of SingleNameBinding with arguments value, environment, and name.
Return « name ».

BindingProperty

PropertyName

BindingElement

Let P be ? Evaluation of PropertyName.
Perform ? KeyedBindingInitialization of BindingElement with arguments value, environment, and P.
Return « P ».

14.3.3.2 Runtime Semantics: RestBindingInitialization

The syntax-directed operation RestBindingInitialization takes arguments value (an ECMAScript language value), environment (an Environment Record or undefined), and excludedNames (a List of property keys) and returns either a normal completion containing unused or an abrupt completion. It is defined piecewise over the following productions:

BindingRestProperty

...

BindingIdentifier

Let lhs be ? ResolveBinding(StringValue of BindingIdentifier, environment).
Let restObj be OrdinaryObjectCreate(%Object.prototype%).
Perform ? CopyDataProperties(restObj, value, excludedNames).
If environment is undefined, return ? PutValue(lhs, restObj).
Return ? InitializeReferencedBinding(lhs, restObj).

14.3.3.3 Runtime Semantics: KeyedBindingInitialization

The syntax-directed operation KeyedBindingInitialization takes arguments value (an ECMAScript language value), environment (an Environment Record or undefined), and propertyName (a property key) and returns either a normal completion containing unused or an abrupt completion.

Note

It is defined piecewise over the following productions:

BindingElement

BindingPattern

Initializer

opt

Let v be ? GetV(value, propertyName).
2. If Initializer is present and v is undefined, then
1. Let defaultValue be ? Evaluation of Initializer.
2. Set v to ? GetValue(defaultValue).
Return ? BindingInitialization of BindingPattern with arguments v and environment.

SingleNameBinding

BindingIdentifier

Initializer

opt

Let bindingId be StringValue of BindingIdentifier.
Let lhs be ? ResolveBinding(bindingId, environment).
Let v be ? GetV(value, propertyName).
4. If Initializer is present and v is undefined, then
1. a. If IsAnonymousFunctionDefinition(Initializer) is true, then
  1. Set v to ? NamedEvaluation of Initializer with argument bindingId.
2. b. Else,
  1. Let defaultValue be ? Evaluation of Initializer.
  2. Set v to ? GetValue(defaultValue).
If environment is undefined, return ? PutValue(lhs, v).
Return ? InitializeReferencedBinding(lhs, v).

14.4 Empty Statement

Syntax

EmptyStatement

;

14.4.1 Runtime Semantics: Evaluation

EmptyStatement

;

Return empty.

14.5 Expression Statement

Syntax

ExpressionStatement

[Yield, Await]

[lookahead ∉ { {, function, async

[no LineTerminator here]

function, class, let [ }]

Expression

[+In, ?Yield, ?Await]

;

Note

An ExpressionStatement cannot start with a U+007B (LEFT CURLY BRACKET) because that might make it ambiguous with a Block. An ExpressionStatement cannot start with the function or class keywords because that would make it ambiguous with a FunctionDeclaration, a GeneratorDeclaration, or a ClassDeclaration. An ExpressionStatement cannot start with async function because that would make it ambiguous with an AsyncFunctionDeclaration or a AsyncGeneratorDeclaration. An ExpressionStatement cannot start with the two token sequence let [ because that would make it ambiguous with a let LexicalDeclaration whose first LexicalBinding was an ArrayBindingPattern.

14.5.1 Runtime Semantics: Evaluation

ExpressionStatement

Expression

;

Let exprRef be ? Evaluation of Expression.
Return ? GetValue(exprRef).

14.6 The `if` Statement

Syntax

IfStatement

[Yield, Await, Return]

(

Expression

[+In, ?Yield, ?Await]

)

Statement

[?Yield, ?Await, ?Return]

else

Statement

[?Yield, ?Await, ?Return]

(

Expression

[+In, ?Yield, ?Await]

)

Statement

[?Yield, ?Await, ?Return]

[lookahead ≠ else]

Note

The lookahead-restriction [lookahead ≠ else] resolves the classic "dangling else" problem in the usual way. That is, when the choice of associated if is otherwise ambiguous, the else is associated with the nearest (innermost) of the candidate ifs

14.6.1 Static Semantics: Early Errors

(

)

else

It is a Syntax Error if IsLabelledFunction(the first Statement) is true.
It is a Syntax Error if IsLabelledFunction(the second Statement) is true.

IfStatement

(

Expression

)

Statement

It is a Syntax Error if IsLabelledFunction(Statement) is true.

Note

It is only necessary to apply this rule if the extension specified in B.3.1 is implemented.

14.6.2 Runtime Semantics: Evaluation

(

)

else

Let exprRef be ? Evaluation of Expression.
Let exprValue be ToBoolean(? GetValue(exprRef)).
3. If exprValue is true, then
1. Let stmtCompletion be Completion(Evaluation of the first Statement).
4. Else,
1. Let stmtCompletion be Completion(Evaluation of the second Statement).
Return ? UpdateEmpty(stmtCompletion, undefined).

IfStatement

(

Expression

)

Statement

Let exprRef be ? Evaluation of Expression.
Let exprValue be ToBoolean(? GetValue(exprRef)).
3. If exprValue is false, then
1. Return undefined.
4. Else,
1. Let stmtCompletion be Completion(Evaluation of Statement).
2. Return ? UpdateEmpty(stmtCompletion, undefined).

14.7 Iteration Statements

Syntax

IterationStatement

[Yield, Await, Return]

DoWhileStatement

[?Yield, ?Await, ?Return]

WhileStatement

[?Yield, ?Await, ?Return]

ForStatement

[?Yield, ?Await, ?Return]

ForInOfStatement

[?Yield, ?Await, ?Return]

14.7.1 Semantics

14.7.1.1 LoopContinues ( `completion`, `labelSet` )

The abstract operation LoopContinues takes arguments completion (a Completion Record) and labelSet (a List of Strings) and returns a Boolean. It performs the following steps when called:

If completion is a normal completion, return true.
If completion is not a continue completion, return false.
If completion.[[Target]] is empty, return true.
If labelSet contains completion.[[Target]], return true.
Return false.

Note

Within the Statement part of an IterationStatement a ContinueStatement may be used to begin a new iteration.

14.7.1.2 Runtime Semantics: LoopEvaluation

The syntax-directed operation LoopEvaluation takes argument labelSet (a List of Strings) and returns either a normal completion containing an ECMAScript language value or an abrupt completion. It is defined piecewise over the following productions:

IterationStatement

DoWhileStatement

Return ? DoWhileLoopEvaluation of DoWhileStatement with argument labelSet.

IterationStatement

WhileStatement

Return ? WhileLoopEvaluation of WhileStatement with argument labelSet.

IterationStatement

ForStatement

Return ? ForLoopEvaluation of ForStatement with argument labelSet.

IterationStatement

ForInOfStatement

Return ? ForInOfLoopEvaluation of ForInOfStatement with argument labelSet.

14.7.2 The `do`-`while` Statement

Syntax

DoWhileStatement

[Yield, Await, Return]

Statement

[?Yield, ?Await, ?Return]

while

(

Expression

[+In, ?Yield, ?Await]

)

;

14.7.2.1 Static Semantics: Early Errors

DoWhileStatement

Statement

while

(

Expression

)

;

It is a Syntax Error if IsLabelledFunction(Statement) is true.

Note

It is only necessary to apply this rule if the extension specified in B.3.1 is implemented.

14.7.2.2 Runtime Semantics: DoWhileLoopEvaluation

The syntax-directed operation DoWhileLoopEvaluation takes argument labelSet (a List of Strings) and returns either a normal completion containing an ECMAScript language value or an abrupt completion. It is defined piecewise over the following productions:

DoWhileStatement

Statement

while

(

Expression

)

;

Let V be undefined.
2. Repeat,
1. Let stmtResult be Completion(Evaluation of Statement).
2. If LoopContinues(stmtResult, labelSet) is false, return ? UpdateEmpty(stmtResult, V).
3. If stmtResult.[[Value]] is not empty, set V to stmtResult.[[Value]].
4. Let exprRef be ? Evaluation of Expression.
5. Let exprValue be ? GetValue(exprRef).
6. If ToBoolean(exprValue) is false, return V.

14.7.3 The `while` Statement

Syntax

WhileStatement

[Yield, Await, Return]

while

(

Expression

[+In, ?Yield, ?Await]

)

Statement

[?Yield, ?Await, ?Return]

14.7.3.1 Static Semantics: Early Errors

WhileStatement

while

(

Expression

)

Statement

It is a Syntax Error if IsLabelledFunction(Statement) is true.

Note

It is only necessary to apply this rule if the extension specified in B.3.1 is implemented.

14.7.3.2 Runtime Semantics: WhileLoopEvaluation

The syntax-directed operation WhileLoopEvaluation takes argument labelSet (a List of Strings) and returns either a normal completion containing an ECMAScript language value or an abrupt completion. It is defined piecewise over the following productions:

WhileStatement

while

(

Expression

)

Statement

Let V be undefined.
2. Repeat,
1. Let exprRef be ? Evaluation of Expression.
2. Let exprValue be ? GetValue(exprRef).
3. If ToBoolean(exprValue) is false, return V.
4. Let stmtResult be Completion(Evaluation of Statement).
5. If LoopContinues(stmtResult, labelSet) is false, return ? UpdateEmpty(stmtResult, V).
6. If stmtResult.[[Value]] is not empty, set V to stmtResult.[[Value]].

14.7.4 The `for` Statement

Syntax

ForStatement

[Yield, Await, Return]

for

(

[lookahead ≠ let []

Expression

[~In, ?Yield, ?Await]

opt

;

Expression

[+In, ?Yield, ?Await]

opt

;

Expression

[+In, ?Yield, ?Await]

opt

)

Statement

[?Yield, ?Await, ?Return]

for

(

var

VariableDeclarationList

[~In, ?Yield, ?Await]

;

Expression

[+In, ?Yield, ?Await]

opt

;

Expression

[+In, ?Yield, ?Await]

opt

)

Statement

[?Yield, ?Await, ?Return]

for

(

LexicalDeclaration

[~In, ?Yield, ?Await]

Expression

[+In, ?Yield, ?Await]

opt

;

Expression

[+In, ?Yield, ?Await]

opt

)

Statement

[?Yield, ?Await, ?Return]

14.7.4.1 Static Semantics: Early Errors

ForStatement

for

(

Expression

opt

;

Expression

opt

;

Expression

opt

)

Statement

for

(

var

VariableDeclarationList

;

Expression

opt

;

Expression

opt

)

Statement

for

(

LexicalDeclaration

Expression

opt

;

Expression

opt

)

Statement

It is a Syntax Error if IsLabelledFunction(Statement) is true.

Note

It is only necessary to apply this rule if the extension specified in B.3.1 is implemented.

ForStatement

for

(

LexicalDeclaration

Expression

opt

;

Expression

opt

)

Statement

It is a Syntax Error if any element of the BoundNames of LexicalDeclaration also occurs in the VarDeclaredNames of Statement.

14.7.4.2 Runtime Semantics: ForLoopEvaluation

The syntax-directed operation ForLoopEvaluation takes argument labelSet (a List of Strings) and returns either a normal completion containing an ECMAScript language value or an abrupt completion. It is defined piecewise over the following productions:

ForStatement

for

(

Expression

opt

;

Expression

opt

;

Expression

opt

)

Statement

1. If the first Expression is present, then
1. Let exprRef be ? Evaluation of the first Expression.
2. Perform ? GetValue(exprRef).
If the second Expression is present, let test be the second Expression; otherwise, let test be empty.
If the third Expression is present, let increment be the third Expression; otherwise, let increment be empty.
Return ? ForBodyEvaluation(test, increment, Statement, « », labelSet).

ForStatement

for

(

var

VariableDeclarationList

;

Expression

opt

;

Expression

opt

)

Statement

Perform ? Evaluation of VariableDeclarationList.
If the first Expression is present, let test be the first Expression; otherwise, let test be empty.
If the second Expression is present, let increment be the second Expression; otherwise, let increment be empty.
Return ? ForBodyEvaluation(test, increment, Statement, « », labelSet).

ForStatement

for

(

LexicalDeclaration

Expression

opt

;

Expression

opt

)

Statement

Let oldEnv be the running execution context's LexicalEnvironment.
Let loopEnv be NewDeclarativeEnvironment(oldEnv).
Let isConst be IsConstantDeclaration of LexicalDeclaration.
Let boundNames be the BoundNames of LexicalDeclaration.
5. For each element dn of boundNames, do
1. a. If isConst is true, then
  1. Perform ! loopEnv.CreateImmutableBinding(dn, true).
2. b. Else,
  1. Perform ! loopEnv.CreateMutableBinding(dn, false).
Set the running execution context's LexicalEnvironment to loopEnv.
Let forDcl be Completion(Evaluation of LexicalDeclaration).
8. If forDcl is an abrupt completion, then
1. Set the running execution context's LexicalEnvironment to oldEnv.
2. Return ? forDcl.
If isConst is false, let perIterationLets be boundNames; otherwise let perIterationLets be a new empty List.
If the first Expression is present, let test be the first Expression; otherwise, let test be empty.
If the second Expression is present, let increment be the second Expression; otherwise, let increment be empty.
Let bodyResult be Completion(ForBodyEvaluation(test, increment, Statement, perIterationLets, labelSet)).
Set the running execution context's LexicalEnvironment to oldEnv.
Return ? bodyResult.

14.7.4.3 ForBodyEvaluation ( `test`, `increment`, `stmt`, `perIterationBindings`, `labelSet` )

The abstract operation ForBodyEvaluation takes arguments test (an Expression Parse Node or empty), increment (an Expression Parse Node or empty), stmt (a Statement Parse Node), perIterationBindings (a List of Strings), and labelSet (a List of Strings) and returns either a normal completion containing an ECMAScript language value or an abrupt completion. It performs the following steps when called:

Let V be undefined.
Perform ? CreatePerIterationEnvironment(perIterationBindings).
3. Repeat,
1. a. If test is not empty, then
  1. Let testRef be ? Evaluation of test.
  2. Let testValue be ? GetValue(testRef).
  3. If ToBoolean(testValue) is false, return V.
2. Let result be Completion(Evaluation of stmt).
3. If LoopContinues(result, labelSet) is false, return ? UpdateEmpty(result, V).
4. If result.[[Value]] is not empty, set V to result.[[Value]].
5. Perform ? CreatePerIterationEnvironment(perIterationBindings).
6. f. If increment is not empty, then
  1. Let incRef be ? Evaluation of increment.
  2. Perform ? GetValue(incRef).

14.7.4.4 CreatePerIterationEnvironment ( `perIterationBindings` )

The abstract operation CreatePerIterationEnvironment takes argument perIterationBindings (a List of Strings) and returns either a normal completion containing unused or a throw completion. It performs the following steps when called:

1. If perIterationBindings has any elements, then
1. Let lastIterationEnv be the running execution context's LexicalEnvironment.
2. Let outer be lastIterationEnv.[[OuterEnv]].
3. Assert: outer is not null.
4. Let thisIterationEnv be NewDeclarativeEnvironment(outer).
5. e. For each element bn of perIterationBindings, do
  1. Perform ! thisIterationEnv.CreateMutableBinding(bn, false).
  2. Let lastValue be ? lastIterationEnv.GetBindingValue(bn, true).
  3. Perform ! thisIterationEnv.InitializeBinding(bn, lastValue).
6. Set the running execution context's LexicalEnvironment to thisIterationEnv.
Return unused.

14.7.5 The `for`-`in`, `for`-`of`, and `for`-`await`-`of` Statements

Syntax

ForInOfStatement

[Yield, Await, Return]

for

(

[lookahead ≠ let []

LeftHandSideExpression

[?Yield, ?Await]

Expression

[+In, ?Yield, ?Await]

)

Statement

[?Yield, ?Await, ?Return]

for

(

var

ForBinding

[?Yield, ?Await]

Expression

[+In, ?Yield, ?Await]

)

Statement

[?Yield, ?Await, ?Return]

for

(

ForDeclaration

[?Yield, ?Await]

Expression

[+In, ?Yield, ?Await]

)

Statement

[?Yield, ?Await, ?Return]

for

(

[lookahead ∉ { let, async of }]

LeftHandSideExpression

[?Yield, ?Await]

AssignmentExpression

[+In, ?Yield, ?Await]

)

Statement

[?Yield, ?Await, ?Return]

for

(

var

ForBinding

[?Yield, ?Await]

AssignmentExpression

[+In, ?Yield, ?Await]

)

Statement

[?Yield, ?Await, ?Return]

for

(

ForDeclaration

[?Yield, ?Await]

AssignmentExpression

[+In, ?Yield, ?Await]

)

Statement

[?Yield, ?Await, ?Return]

[+Await]

for

await

(

[lookahead ≠ let]

LeftHandSideExpression

[?Yield, ?Await]

AssignmentExpression

[+In, ?Yield, ?Await]

)

Statement

[?Yield, ?Await, ?Return]

[+Await]

for

await

(

var

ForBinding

[?Yield, ?Await]

AssignmentExpression

[+In, ?Yield, ?Await]

)

Statement

[?Yield, ?Await, ?Return]

[+Await]

for

await

(

ForDeclaration

[?Yield, ?Await]

AssignmentExpression

[+In, ?Yield, ?Await]

)

Statement

[?Yield, ?Await, ?Return]

[Yield, Await]

[?Yield, ?Await]

[Yield, Await]

[?Yield, ?Await]

[?Yield, ?Await]

Note

This section is extended by Annex B.3.5.

14.7.5.1 Static Semantics: Early Errors

ForInOfStatement

for

(

LeftHandSideExpression

Expression

)

Statement

for

(

var

ForBinding

Expression

)

Statement

for

(

ForDeclaration

Expression

)

Statement

for

(

LeftHandSideExpression

AssignmentExpression

)

Statement

for

(

var

ForBinding

AssignmentExpression

)

Statement

for

(

ForDeclaration

AssignmentExpression

)

Statement

for

await

(

LeftHandSideExpression

AssignmentExpression

)

Statement

for

await

(

var

ForBinding

AssignmentExpression

)

Statement

for

await

(

ForDeclaration

AssignmentExpression

)

Statement

It is a Syntax Error if IsLabelledFunction(Statement) is true.

Note

It is only necessary to apply this rule if the extension specified in B.3.1 is implemented.

ForInOfStatement

for

(

LeftHandSideExpression

Expression

)

Statement

for

(

LeftHandSideExpression

AssignmentExpression

)

Statement

for

await

(

LeftHandSideExpression

AssignmentExpression

)

Statement

If LeftHandSideExpression is either an ObjectLiteral or an ArrayLiteral, the following Early Error rules are applied:

LeftHandSideExpression must cover an AssignmentPattern.

If LeftHandSideExpression is neither an ObjectLiteral nor an ArrayLiteral, the following Early Error rule is applied:

It is a Syntax Error if AssignmentTargetType of LeftHandSideExpression is not simple.

ForInOfStatement

for

(

ForDeclaration

Expression

)

Statement

for

(

ForDeclaration

AssignmentExpression

)

Statement

for

await

(

ForDeclaration

AssignmentExpression

)

Statement

It is a Syntax Error if the BoundNames of ForDeclaration contains "let".
It is a Syntax Error if any element of the BoundNames of ForDeclaration also occurs in the VarDeclaredNames of Statement.
It is a Syntax Error if the BoundNames of ForDeclaration contains any duplicate entries.

14.7.5.2 Static Semantics: IsDestructuring

The syntax-directed operation IsDestructuring takes no arguments and returns a Boolean. It is defined piecewise over the following productions:

MemberExpression

PrimaryExpression

If PrimaryExpression is either an ObjectLiteral or an ArrayLiteral, return true.
Return false.

[

]

new

new

LeftHandSideExpression

CallExpression

OptionalExpression

Return false.

ForDeclaration

LetOrConst

ForBinding

Return IsDestructuring of ForBinding.

ForBinding

BindingIdentifier

Return false.

ForBinding

BindingPattern

Return true.

Note

This section is extended by Annex B.3.5.

14.7.5.3 Runtime Semantics: ForDeclarationBindingInitialization

The syntax-directed operation ForDeclarationBindingInitialization takes arguments value (an ECMAScript language value) and environment (an Environment Record or undefined) and returns either a normal completion containing unused or an abrupt completion.

Note

undefined is passed for environment to indicate that a PutValue operation should be used to assign the initialization value. This is the case for var statements and the formal parameter lists of some non-strict functions (see 10.2.11). In those cases a lexical binding is hoisted and preinitialized prior to evaluation of its initializer.

It is defined piecewise over the following productions:

ForDeclaration

LetOrConst

ForBinding

Return ? BindingInitialization of ForBinding with arguments value and environment.

14.7.5.4 Runtime Semantics: ForDeclarationBindingInstantiation

The syntax-directed operation ForDeclarationBindingInstantiation takes argument environment (a Declarative Environment Record) and returns unused. It is defined piecewise over the following productions:

ForDeclaration

LetOrConst

ForBinding

1. For each element name of the BoundNames of ForBinding, do
1. a. If IsConstantDeclaration of LetOrConst is true, then
  1. Perform ! environment.CreateImmutableBinding(name, true).
2. b. Else,
  1. Perform ! environment.CreateMutableBinding(name, false).
Return unused.

14.7.5.5 Runtime Semantics: ForInOfLoopEvaluation

The syntax-directed operation ForInOfLoopEvaluation takes argument labelSet (a List of Strings) and returns either a normal completion containing an ECMAScript language value or an abrupt completion. It is defined piecewise over the following productions:

ForInOfStatement

for

(

LeftHandSideExpression

Expression

)

Statement

Let keyResult be ? ForIn/OfHeadEvaluation(« », Expression, enumerate).
Return ? ForIn/OfBodyEvaluation(LeftHandSideExpression, Statement, keyResult, enumerate, assignment, labelSet).

ForInOfStatement

for

(

var

ForBinding

Expression

)

Statement

Let keyResult be ? ForIn/OfHeadEvaluation(« », Expression, enumerate).
Return ? ForIn/OfBodyEvaluation(ForBinding, Statement, keyResult, enumerate, var-binding, labelSet).

ForInOfStatement

for

(

ForDeclaration

Expression

)

Statement

Let keyResult be ? ForIn/OfHeadEvaluation(BoundNames of ForDeclaration, Expression, enumerate).
Return ? ForIn/OfBodyEvaluation(ForDeclaration, Statement, keyResult, enumerate, lexical-binding, labelSet).

ForInOfStatement

for

(

LeftHandSideExpression

AssignmentExpression

)

Statement

Let keyResult be ? ForIn/OfHeadEvaluation(« », AssignmentExpression, iterate).
Return ? ForIn/OfBodyEvaluation(LeftHandSideExpression, Statement, keyResult, iterate, assignment, labelSet).

ForInOfStatement

for

(

var

ForBinding

AssignmentExpression

)

Statement

Let keyResult be ? ForIn/OfHeadEvaluation(« », AssignmentExpression, iterate).
Return ? ForIn/OfBodyEvaluation(ForBinding, Statement, keyResult, iterate, var-binding, labelSet).

ForInOfStatement

for

(

ForDeclaration

AssignmentExpression

)

Statement

Let keyResult be ? ForIn/OfHeadEvaluation(BoundNames of ForDeclaration, AssignmentExpression, iterate).
Return ? ForIn/OfBodyEvaluation(ForDeclaration, Statement, keyResult, iterate, lexical-binding, labelSet).

ForInOfStatement

for

await

(

LeftHandSideExpression

AssignmentExpression

)

Statement

Let keyResult be ? ForIn/OfHeadEvaluation(« », AssignmentExpression, async-iterate).
Return ? ForIn/OfBodyEvaluation(LeftHandSideExpression, Statement, keyResult, iterate, assignment, labelSet, async).

ForInOfStatement

for

await

(

var

ForBinding

AssignmentExpression

)

Statement

Let keyResult be ? ForIn/OfHeadEvaluation(« », AssignmentExpression, async-iterate).
Return ? ForIn/OfBodyEvaluation(ForBinding, Statement, keyResult, iterate, var-binding, labelSet, async).

ForInOfStatement

for

await

(

ForDeclaration

AssignmentExpression

)

Statement

Let keyResult be ? ForIn/OfHeadEvaluation(BoundNames of ForDeclaration, AssignmentExpression, async-iterate).
Return ? ForIn/OfBodyEvaluation(ForDeclaration, Statement, keyResult, iterate, lexical-binding, labelSet, async).

Note

This section is extended by Annex B.3.5.

14.7.5.6 ForIn/OfHeadEvaluation ( `uninitializedBoundNames`, `expr`, `iterationKind` )

The abstract operation ForIn/OfHeadEvaluation takes arguments uninitializedBoundNames (a List of Strings), expr (an Expression Parse Node or an AssignmentExpression Parse Node), and iterationKind (enumerate, iterate, or async-iterate) and returns either a normal completion containing an Iterator Record or an abrupt completion. It performs the following steps when called:

Let oldEnv be the running execution context's LexicalEnvironment.
2. If uninitializedBoundNames is not empty, then
1. Assert: uninitializedBoundNames has no duplicate entries.
2. Let newEnv be NewDeclarativeEnvironment(oldEnv).
3. c. For each String name of uninitializedBoundNames, do
  1. Perform ! newEnv.CreateMutableBinding(name, false).
4. Set the running execution context's LexicalEnvironment to newEnv.
Let exprRef be Completion(Evaluation of expr).
Set the running execution context's LexicalEnvironment to oldEnv.
Let exprValue be ? GetValue(? exprRef).
6. If iterationKind is enumerate, then
1. a. If exprValue is either undefined or null, then
  1. Return Completion Record { [[Type]]: break, [[Value]]: empty, [[Target]]: empty }.
2. Let obj be ! ToObject(exprValue).
3. Let iterator be EnumerateObjectProperties(obj).
4. Let nextMethod be ! GetV(iterator, "next").
5. Return the Iterator Record { [[Iterator]]: iterator, [[NextMethod]]: nextMethod, [[Done]]: false }.
7. Else,
1. Assert: iterationKind is either iterate or async-iterate.
2. If iterationKind is async-iterate, let iteratorKind be async.
3. Else, let iteratorKind be sync.
4. Return ? GetIterator(exprValue, iteratorKind).

14.7.5.7 ForIn/OfBodyEvaluation ( `lhs`, `stmt`, `iteratorRecord`, `iterationKind`, `lhsKind`, `labelSet` [ , `iteratorKind` ] )

The abstract operation ForIn/OfBodyEvaluation takes arguments lhs (a Parse Node), stmt (a Statement Parse Node), iteratorRecord (an Iterator Record), iterationKind (enumerate or iterate), lhsKind (assignment, var-binding, or lexical-binding), and labelSet (a List of Strings) and optional argument iteratorKind (sync or async) and returns either a normal completion containing an ECMAScript language value or an abrupt completion. It performs the following steps when called:

If iteratorKind is not present, set iteratorKind to sync.
Let oldEnv be the running execution context's LexicalEnvironment.
Let V be undefined.
Let destructuring be IsDestructuring of lhs.
5. If destructuring is true and lhsKind is assignment, then
1. Assert: lhs is a LeftHandSideExpression.
2. Let assignmentPattern be the AssignmentPattern that is covered by lhs.
6. Repeat,
1. Let nextResult be ? Call(iteratorRecord.[[NextMethod]], iteratorRecord.[[Iterator]]).
2. If iteratorKind is async, set nextResult to ? Await(nextResult).
3. If nextResult is not an Object, throw a TypeError exception.
4. Let done be ? IteratorComplete(nextResult).
5. If done is true, return V.
6. Let nextValue be ? IteratorValue(nextResult).
7. g. If lhsKind is either assignment or var-binding, then
  1. i. If destructuring is true, then
    1. 1. If lhsKind is assignment, then
      1. Let status be Completion(DestructuringAssignmentEvaluation of assignmentPattern with argument nextValue).
    2. 2. Else,
      1. Assert: lhsKind is var-binding.
      2. Assert: lhs is a ForBinding.
      3. Let status be Completion(BindingInitialization of lhs with arguments nextValue and undefined).
  2. ii. Else,
    1. Let lhsRef be Completion(Evaluation of lhs). (It may be evaluated repeatedly.)
    2. 2. If lhsRef is an abrupt completion, then
      1. Let status be lhsRef.
    3. 3. Else,
      1. Let status be Completion(PutValue(lhsRef.[[Value]], nextValue)).
8. h. Else,
  1. Assert: lhsKind is lexical-binding.
  2. Assert: lhs is a ForDeclaration.
  3. Let iterationEnv be NewDeclarativeEnvironment(oldEnv).
  4. Perform ForDeclarationBindingInstantiation of lhs with argument iterationEnv.
  5. Set the running execution context's LexicalEnvironment to iterationEnv.
  6. vi. If destructuring is true, then
    1. Let status be Completion(ForDeclarationBindingInitialization of lhs with arguments nextValue and iterationEnv).
  7. vii. Else,
    1. Assert: lhs binds a single name.
    2. Let lhsName be the sole element of BoundNames of lhs.
    3. Let lhsRef be ! ResolveBinding(lhsName).
    4. Let status be Completion(InitializeReferencedBinding(lhsRef, nextValue)).
9. i. If status is an abrupt completion, then
  1. Set the running execution context's LexicalEnvironment to oldEnv.
  2. If iteratorKind is async, return ? AsyncIteratorClose(iteratorRecord, status).
  3. iii. If iterationKind is enumerate, then
    1. Return ? status.
  4. iv. Else,
    1. Assert: iterationKind is iterate.
    2. Return ? IteratorClose(iteratorRecord, status).
10. Let result be Completion(Evaluation of stmt).
11. Set the running execution context's LexicalEnvironment to oldEnv.
12. l. If LoopContinues(result, labelSet) is false, then
  1. i. If iterationKind is enumerate, then
    1. Return ? UpdateEmpty(result, V).
  2. ii. Else,
    1. Assert: iterationKind is iterate.
    2. Set status to Completion(UpdateEmpty(result, V)).
    3. If iteratorKind is async, return ? AsyncIteratorClose(iteratorRecord, status).
    4. Return ? IteratorClose(iteratorRecord, status).
13. If result.[[Value]] is not empty, set V to result.[[Value]].

14.7.5.8 Runtime Semantics: Evaluation

BindingIdentifier

Identifier

yield

await

Let bindingId be StringValue of BindingIdentifier.
Return ? ResolveBinding(bindingId).

14.7.5.9 EnumerateObjectProperties ( `O` )

The abstract operation EnumerateObjectProperties takes argument O (an Object) and returns an Iterator. It performs the following steps when called:

Return an Iterator object (27.1.1.2) whose next method iterates over all the String-valued keys of enumerable properties of O. The iterator object is never directly accessible to ECMAScript code. The mechanics and order of enumerating the properties is not specified but must conform to the rules specified below.

The iterator's throw and return methods are null and are never invoked. The iterator's next method processes object properties to determine whether the property key should be returned as an iterator value. Returned property keys do not include keys that are Symbols. Properties of the target object may be deleted during enumeration. A property that is deleted before it is processed by the iterator's next method is ignored. If new properties are added to the target object during enumeration, the newly added properties are not guaranteed to be processed in the active enumeration. A property name will be returned by the iterator's next method at most once in any enumeration.

Enumerating the properties of the target object includes enumerating properties of its prototype, and the prototype of the prototype, and so on, recursively; but a property of a prototype is not processed if it has the same name as a property that has already been processed by the iterator's next method. The values of [[Enumerable]] attributes are not considered when determining if a property of a prototype object has already been processed. The enumerable property names of prototype objects must be obtained by invoking EnumerateObjectProperties passing the prototype object as the argument. EnumerateObjectProperties must obtain the own property keys of the target object by calling its [[OwnPropertyKeys]] internal method. Property attributes of the target object must be obtained by calling its [[GetOwnProperty]] internal method.

In addition, if neither O nor any object in its prototype chain is a Proxy exotic object, TypedArray, module namespace exotic object, or implementation provided exotic object, then the iterator must behave as would the iterator given by CreateForInIterator(O) until one of the following occurs:

the value of the [[Prototype]] internal slot of O or an object in its prototype chain changes,
a property is removed from O or an object in its prototype chain,
a property is added to an object in O's prototype chain, or
the value of the [[Enumerable]] attribute of a property of O or an object in its prototype chain changes.

Note 1

ECMAScript implementations are not required to implement the algorithm in 14.7.5.10.2.1 directly. They may choose any implementation whose behaviour will not deviate from that algorithm unless one of the constraints in the previous paragraph is violated.

The following is an informative definition of an ECMAScript generator function that conforms to these rules:

function* EnumerateObjectProperties(obj) {
  const visited = new Set();
  for (const key of Reflect.ownKeys(obj)) {
    if (typeof key === "symbol") continue;
    const desc = Reflect.getOwnPropertyDescriptor(obj, key);
    if (desc) {
      visited.add(key);
      if (desc.enumerable) yield key;
    }
  }
  const proto = Reflect.getPrototypeOf(obj);
  if (proto === null) return;
  for (const protoKey of EnumerateObjectProperties(proto)) {
    if (!visited.has(protoKey)) yield protoKey;
  }
}

Note 2

The list of exotic objects for which implementations are not required to match CreateForInIterator was chosen because implementations historically differed in behaviour for those cases, and agreed in all others.

14.7.5.10 For-In Iterator Objects

A For-In Iterator is an object that represents a specific iteration over some specific object. For-In Iterator objects are never directly accessible to ECMAScript code; they exist solely to illustrate the behaviour of EnumerateObjectProperties.

14.7.5.10.1 CreateForInIterator ( `object` )

The abstract operation CreateForInIterator takes argument object (an Object) and returns a For-In Iterator. It is used to create a For-In Iterator object which iterates over the own and inherited enumerable string properties of object in a specific order. It performs the following steps when called:

Let iterator be OrdinaryObjectCreate(%ForInIteratorPrototype%, « [[Object]], [[ObjectWasVisited]], [[VisitedKeys]], [[RemainingKeys]] »).
Set iterator.[[Object]] to object.
Set iterator.[[ObjectWasVisited]] to false.
Set iterator.[[VisitedKeys]] to a new empty List.
Set iterator.[[RemainingKeys]] to a new empty List.
Return iterator.

14.7.5.10.2 The %ForInIteratorPrototype% Object

The %ForInIteratorPrototype% object:

has properties that are inherited by all For-In Iterator Objects.
is an ordinary object.
has a [[Prototype]] internal slot whose value is %IteratorPrototype%.
is never directly accessible to ECMAScript code.
has the following properties:

14.7.5.10.2.1 %ForInIteratorPrototype%.next ( )

Let O be the this value.
Assert: O is an Object.
Assert: O has all of the internal slots of a For-In Iterator Instance (14.7.5.10.3).
Let object be O.[[Object]].
5. Repeat,
1. a. If O.[[ObjectWasVisited]] is false, then
  1. Let keys be ? object.[[OwnPropertyKeys]]().
  2. ii. For each element key of keys, do
    1. 1. If key is a String, then
      1. Append key to O.[[RemainingKeys]].
  3. Set O.[[ObjectWasVisited]] to true.
2. b. Repeat, while O.[[RemainingKeys]] is not empty,
  1. Let r be the first element of O.[[RemainingKeys]].
  2. Remove the first element from O.[[RemainingKeys]].
  3. iii. If there does not exist an element v of O.[[VisitedKeys]] such that SameValue(r, v) is true, then
    1. Let desc be ? object.[[GetOwnProperty]](r).
    2. 2. If desc is not undefined, then
      1. Append r to O.[[VisitedKeys]].
      2. If desc.[[Enumerable]] is true, return CreateIterResultObject(r, false).
3. Set object to ? object.[[GetPrototypeOf]]().
4. Set O.[[Object]] to object.
5. Set O.[[ObjectWasVisited]] to false.
6. If object is null, return CreateIterResultObject(undefined, true).

14.7.5.10.3 Properties of For-In Iterator Instances

For-In Iterator instances are ordinary objects that inherit properties from the %ForInIteratorPrototype% intrinsic object. For-In Iterator instances are initially created with the internal slots listed in Table 39.

Table 39: Internal Slots of For-In Iterator Instances

Internal Slot	Type	Description
`[[Object]]`	an Object	The Object value whose properties are being iterated.
`[[ObjectWasVisited]]`	a Boolean	true if the iterator has invoked `[[OwnPropertyKeys]]` on `[[Object]]`, false otherwise.
`[[VisitedKeys]]`	a List of Strings	The values that have been emitted by this iterator thus far.
`[[RemainingKeys]]`	a List of Strings	The values remaining to be emitted for the current object, before iterating the properties of its prototype (if its prototype is not null).

14.8 The `continue` Statement

Syntax

ContinueStatement

[Yield, Await]

continue

;

continue

[no LineTerminator here]

LabelIdentifier

[?Yield, ?Await]

;

14.8.1 Static Semantics: Early Errors

ContinueStatement

continue

;

continue

LabelIdentifier

;

It is a Syntax Error if this ContinueStatement is not nested, directly or indirectly (but not crossing function or static initialization block boundaries), within an IterationStatement.

14.8.2 Runtime Semantics: Evaluation

ContinueStatement

continue

;

Return Completion Record { [[Type]]: continue, [[Value]]: empty, [[Target]]: empty }.

ContinueStatement

continue

LabelIdentifier

;

Let label be the StringValue of LabelIdentifier.
Return Completion Record { [[Type]]: continue, [[Value]]: empty, [[Target]]: label }.

14.9 The `break` Statement

Syntax

BreakStatement

[Yield, Await]

break

;

break

[no LineTerminator here]

LabelIdentifier

[?Yield, ?Await]

;

14.9.1 Static Semantics: Early Errors

BreakStatement

break

;

It is a Syntax Error if this BreakStatement is not nested, directly or indirectly (but not crossing function or static initialization block boundaries), within an IterationStatement or a SwitchStatement.

14.9.2 Runtime Semantics: Evaluation

BreakStatement

break

;

Return Completion Record { [[Type]]: break, [[Value]]: empty, [[Target]]: empty }.

BreakStatement

break

LabelIdentifier

;

Let label be the StringValue of LabelIdentifier.
Return Completion Record { [[Type]]: break, [[Value]]: empty, [[Target]]: label }.

14.10 The `return` Statement

Syntax

ReturnStatement

[Yield, Await]

return

;

return

[no LineTerminator here]

Expression

[+In, ?Yield, ?Await]

;

Note

A return statement causes a function to cease execution and, in most cases, returns a value to the caller. If Expression is omitted, the return value is undefined. Otherwise, the return value is the value of Expression. A return statement may not actually return a value to the caller depending on surrounding context. For example, in a try block, a return statement's Completion Record may be replaced with another Completion Record during evaluation of the finally block.

14.10.1 Runtime Semantics: Evaluation

ReturnStatement

return

;

Return Completion Record { [[Type]]: return, [[Value]]: undefined, [[Target]]: empty }.

ReturnStatement

return

Expression

;

Let exprRef be ? Evaluation of Expression.
Let exprValue be ? GetValue(exprRef).
If GetGeneratorKind() is async, set exprValue to ? Await(exprValue).
Return Completion Record { [[Type]]: return, [[Value]]: exprValue, [[Target]]: empty }.

Legacy

14.11 The `with` Statement

Note 1

Use of the Legacy with statement is discouraged in new ECMAScript code. Consider alternatives that are permitted in both strict mode code and non-strict code, such as destructuring assignment.

Syntax

WithStatement

[Yield, Await, Return]

with

(

Expression

[+In, ?Yield, ?Await]

)

Statement

[?Yield, ?Await, ?Return]

Note 2

The with statement adds an Object Environment Record for a computed object to the lexical environment of the running execution context. It then executes a statement using this augmented lexical environment. Finally, it restores the original lexical environment.

14.11.1 Static Semantics: Early Errors

WithStatement

with

(

Expression

)

Statement

It is a Syntax Error if the source text matched by this production is contained in strict mode code.
It is a Syntax Error if IsLabelledFunction(Statement) is true.

Note

It is only necessary to apply the second rule if the extension specified in B.3.1 is implemented.

14.11.2 Runtime Semantics: Evaluation

WithStatement

with

(

Expression

)

Statement

Let val be ? Evaluation of Expression.
Let obj be ? ToObject(? GetValue(val)).
Let oldEnv be the running execution context's LexicalEnvironment.
Let newEnv be NewObjectEnvironment(obj, true, oldEnv).
Set the running execution context's LexicalEnvironment to newEnv.
Let C be Completion(Evaluation of Statement).
Set the running execution context's LexicalEnvironment to oldEnv.
Return ? UpdateEmpty(C, undefined).

Note

No matter how control leaves the embedded Statement, whether normally or by some form of abrupt completion or exception, the LexicalEnvironment is always restored to its former state.

14.12 The `switch` Statement

Syntax

SwitchStatement

[Yield, Await, Return]

switch

(

Expression

[+In, ?Yield, ?Await]

)

CaseBlock

[?Yield, ?Await, ?Return]

CaseBlock

[Yield, Await, Return]

{

CaseClauses

[?Yield, ?Await, ?Return]

opt

}

{

CaseClauses

[?Yield, ?Await, ?Return]

opt

DefaultClause

[?Yield, ?Await, ?Return]

CaseClauses

[?Yield, ?Await, ?Return]

opt

}

CaseClauses

[Yield, Await, Return]

CaseClause

[?Yield, ?Await, ?Return]

CaseClauses

[?Yield, ?Await, ?Return]

CaseClause

[?Yield, ?Await, ?Return]

CaseClause

[Yield, Await, Return]

case

Expression

[+In, ?Yield, ?Await]

StatementList

[?Yield, ?Await, ?Return]

opt

DefaultClause

[Yield, Await, Return]

default

StatementList

[?Yield, ?Await, ?Return]

opt

14.12.1 Static Semantics: Early Errors

SwitchStatement

switch

(

Expression

)

CaseBlock

It is a Syntax Error if the LexicallyDeclaredNames of CaseBlock contains any duplicate entries.
It is a Syntax Error if any element of the LexicallyDeclaredNames of CaseBlock also occurs in the VarDeclaredNames of CaseBlock.

14.12.2 Runtime Semantics: CaseBlockEvaluation

The syntax-directed operation CaseBlockEvaluation takes argument input (an ECMAScript language value) and returns either a normal completion containing an ECMAScript language value or an abrupt completion. It is defined piecewise over the following productions:

CaseBlock

{

}

Return undefined.

CaseBlock

{

CaseClauses

}

Let V be undefined.
Let A be the List of CaseClause items in CaseClauses, in source text order.
Let found be false.
4. For each CaseClause C of A, do
1. a. If found is false, then
  1. Set found to ? CaseClauseIsSelected(C, input).
2. b. If found is true, then
  1. Let R be Completion(Evaluation of C).
  2. If R.[[Value]] is not empty, set V to R.[[Value]].
  3. If R is an abrupt completion, return ? UpdateEmpty(R, V).
Return V.

{

opt

opt

}

Let V be undefined.
2. If the first CaseClauses is present, then
1. Let A be the List of CaseClause items in the first CaseClauses, in source text order.
3. Else,
1. Let A be a new empty List.
Let found be false.
5. For each CaseClause C of A, do
1. a. If found is false, then
  1. Set found to ? CaseClauseIsSelected(C, input).
2. b. If found is true, then
  1. Let R be Completion(Evaluation of C).
  2. If R.[[Value]] is not empty, set V to R.[[Value]].
  3. If R is an abrupt completion, return ? UpdateEmpty(R, V).
Let foundInB be false.
7. If the second CaseClauses is present, then
1. Let B be the List of CaseClause items in the second CaseClauses, in source text order.
8. Else,
1. Let B be a new empty List.
9. If found is false, then
1. a. For each CaseClause C of B, do
  1. i. If foundInB is false, then
    1. Set foundInB to ? CaseClauseIsSelected(C, input).
  2. ii. If foundInB is true, then
    1. Let R be Completion(Evaluation of CaseClause C).
    2. If R.[[Value]] is not empty, set V to R.[[Value]].
    3. If R is an abrupt completion, return ? UpdateEmpty(R, V).
If foundInB is true, return V.
Let defaultR be Completion(Evaluation of DefaultClause).
If defaultR.[[Value]] is not empty, set V to defaultR.[[Value]].
If defaultR is an abrupt completion, return ? UpdateEmpty(defaultR, V).
NOTE: The following is another complete iteration of the second CaseClauses.
15. For each CaseClause C of B, do
1. Let R be Completion(Evaluation of CaseClause C).
2. If R.[[Value]] is not empty, set V to R.[[Value]].
3. If R is an abrupt completion, return ? UpdateEmpty(R, V).
Return V.

14.12.3 CaseClauseIsSelected ( `C`, `input` )

The abstract operation CaseClauseIsSelected takes arguments C (a CaseClause Parse Node) and input (an ECMAScript language value) and returns either a normal completion containing a Boolean or an abrupt completion. It determines whether C matches input. It performs the following steps when called:

Assert: C is an instance of the production CaseClause : case Expression : StatementListopt .
Let exprRef be ? Evaluation of the Expression of C.
Let clauseSelector be ? GetValue(exprRef).
Return IsStrictlyEqual(input, clauseSelector).

Note

This operation does not execute C's StatementList (if any). The CaseBlock algorithm uses its return value to determine which StatementList to start executing.

14.12.4 Runtime Semantics: Evaluation

SwitchStatement

switch

(

Expression

)

CaseBlock

Let exprRef be ? Evaluation of Expression.
Let switchValue be ? GetValue(exprRef).
Let oldEnv be the running execution context's LexicalEnvironment.
Let blockEnv be NewDeclarativeEnvironment(oldEnv).
Perform BlockDeclarationInstantiation(CaseBlock, blockEnv).
Set the running execution context's LexicalEnvironment to blockEnv.
Let R be Completion(CaseBlockEvaluation of CaseBlock with argument switchValue).
Set the running execution context's LexicalEnvironment to oldEnv.
Return R.

Note

No matter how control leaves the SwitchStatement the LexicalEnvironment is always restored to its former state.

CaseClause

case

Expression

Return empty.

CaseClause

case

Expression

StatementList

Return ? Evaluation of StatementList.

DefaultClause

default

Return empty.

DefaultClause

default

StatementList

Return ? Evaluation of StatementList.

14.13 Labelled Statements

Syntax

LabelledStatement

[Yield, Await, Return]

LabelIdentifier

[?Yield, ?Await]

LabelledItem

[?Yield, ?Await, ?Return]

LabelledItem

[Yield, Await, Return]

Statement

[?Yield, ?Await, ?Return]

FunctionDeclaration

[?Yield, ?Await, ~Default]

Note

A Statement may be prefixed by a label. Labelled statements are only used in conjunction with labelled break and continue statements. ECMAScript has no goto statement. A Statement can be part of a LabelledStatement, which itself can be part of a LabelledStatement, and so on. The labels introduced this way are collectively referred to as the “current label set” when describing the semantics of individual statements.

14.13.1 Static Semantics: Early Errors

LabelledItem

FunctionDeclaration

It is a Syntax Error if any source text is matched by this production.

Note

An alternative definition for this rule is provided in B.3.1.

14.13.2 Static Semantics: IsLabelledFunction ( `stmt` )

The abstract operation IsLabelledFunction takes argument stmt (a Statement Parse Node) and returns a Boolean. It performs the following steps when called:

If stmt is not a LabelledStatement, return false.
Let item be the LabelledItem of stmt.
If item is LabelledItem : FunctionDeclaration , return true.
Let subStmt be the Statement of item.
Return IsLabelledFunction(subStmt).

14.13.3 Runtime Semantics: Evaluation

ECMA-262, 15th edition, June 2024 ECMAScript® 2024 Language Specification

About this Specification

Contributing to this Specification

Introduction

1 Scope

2 Conformance

2.1 Example Normative Optional Clause Heading

2.2 Example Legacy Clause Heading

2.3 Example Legacy Normative Optional Clause Heading

3 Normative References

4 Overview

4.1 Web Scripting

4.2 Hosts and Implementations

4.3 ECMAScript Overview

4.3.1 Objects

4.3.2 The Strict Variant of ECMAScript

4.4 Terms and Definitions

4.4.1 implementation-approximated

4.4.2 implementation-defined

4.4.3 host-defined

4.4.4 type

4.4.5 primitive value

4.4.6 object

4.4.7 constructor

4.4.8 prototype

4.4.9 ordinary object

4.4.10 exotic object

4.4.11 standard object

4.4.12 built-in object

4.4.13 undefined value

4.4.14 Undefined type

4.4.15 null value

4.4.16 Null type

4.4.17 Boolean value

4.4.18 Boolean type

4.4.19 Boolean object

4.4.20 String value

4.4.21 String type

4.4.22 String object

4.4.23 Number value

4.4.24 Number type

4.4.25 Number object

4.4.26 Infinity

4.4.27 NaN

4.4.28 BigInt value

4.4.29 BigInt type

4.4.30 BigInt object

4.4.31 Symbol value

4.4.32 Symbol type

4.4.33 Symbol object

4.4.34 function

4.4.35 built-in function

4.4.36 built-in constructor

4.4.37 property

4.4.38 method

4.4.39 built-in method

4.4.40 attribute

4.4.41 own property

4.4.42 inherited property

4.5 Organization of This Specification

5 Notational Conventions

5.1 Syntactic and Lexical Grammars

5.1.1 Context-Free Grammars

5.1.2 The Lexical and RegExp Grammars

5.1.3 The Numeric String Grammar

5.1.4 The Syntactic Grammar

5.1.5 Grammar Notation

5.1.5.1 Terminal Symbols

5.1.5.2 Nonterminal Symbols and Productions

5.1.5.3 Optional Symbols

5.1.5.4 Grammatical Parameters

5.1.5.5 one of

5.1.5.6 [empty]

5.1.5.7 Lookahead Restrictions

5.1.5.8 [no LineTerminator here]

5.1.5.9 but not

5.1.5.10 Descriptive Phrases

5.2 Algorithm Conventions

5.2.1 Abstract Operations

5.2.2 Syntax-Directed Operations

ECMA-262, 15^th edition, June 2024
ECMAScript® 2024 Language Specification

5.2.3.1 Completion ( `completionRecord` )

6.1.4.1 StringIndexOf ( `string`, `searchValue`, `fromIndex` )

6.1.6.1.1 Number::unaryMinus ( `x` )

6.1.6.1.2 Number::bitwiseNOT ( `x` )

6.1.6.1.3 Number::exponentiate ( `base`, `exponent` )

6.1.6.1.4 Number::multiply ( `x`, `y` )

6.1.6.1.5 Number::divide ( `x`, `y` )

6.1.6.1.6 Number::remainder ( `n`, `d` )

6.1.6.1.7 Number::add ( `x`, `y` )

6.1.6.1.8 Number::subtract ( `x`, `y` )

6.1.6.1.9 Number::leftShift ( `x`, `y` )

6.1.6.1.10 Number::signedRightShift ( `x`, `y` )

6.1.6.1.11 Number::unsignedRightShift ( `x`, `y` )

6.1.6.1.12 Number::lessThan ( `x`, `y` )

6.1.6.1.13 Number::equal ( `x`, `y` )

6.1.6.1.14 Number::sameValue ( `x`, `y` )

6.1.6.1.15 Number::sameValueZero ( `x`, `y` )

6.1.6.1.16 NumberBitwiseOp ( `op`, `x`, `y` )

6.1.6.1.17 Number::bitwiseAND ( `x`, `y` )

6.1.6.1.18 Number::bitwiseXOR ( `x`, `y` )

6.1.6.1.19 Number::bitwiseOR ( `x`, `y` )

6.1.6.1.20 Number::toString ( `x`, `radix` )

6.1.6.2.1 BigInt::unaryMinus ( `x` )

6.1.6.2.2 BigInt::bitwiseNOT ( `x` )

6.1.6.2.3 BigInt::exponentiate ( `base`, `exponent` )

6.1.6.2.4 BigInt::multiply ( `x`, `y` )

6.1.6.2.5 BigInt::divide ( `x`, `y` )

6.1.6.2.6 BigInt::remainder ( `n`, `d` )

6.1.6.2.7 BigInt::add ( `x`, `y` )

6.1.6.2.8 BigInt::subtract ( `x`, `y` )

6.1.6.2.9 BigInt::leftShift ( `x`, `y` )

6.1.6.2.10 BigInt::signedRightShift ( `x`, `y` )

6.1.6.2.11 BigInt::unsignedRightShift ( `x`, `y` )

6.1.6.2.12 BigInt::lessThan ( `x`, `y` )

6.1.6.2.13 BigInt::equal ( `x`, `y` )

6.1.6.2.14 BinaryAnd ( `x`, `y` )

6.1.6.2.15 BinaryOr ( `x`, `y` )

6.1.6.2.16 BinaryXor ( `x`, `y` )

6.1.6.2.17 BigIntBitwiseOp ( `op`, `x`, `y` )

6.1.6.2.18 BigInt::bitwiseAND ( `x`, `y` )

6.1.6.2.19 BigInt::bitwiseXOR ( `x`, `y` )

6.1.6.2.20 BigInt::bitwiseOR ( `x`, `y` )

6.1.6.2.21 BigInt::toString ( `x`, `radix` )

`[[GetPrototypeOf]]` ( )