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feat: add no-unassigned-vars rule (#19618)
* feat: add no-unassigned-vars rule * Apply suggestions from code review Co-authored-by: Tanuj Kanti <[email protected]> Co-authored-by: Kirk Waiblinger <[email protected]> * add typescript examples * Update docs/src/rules/no-unassigned-vars.md Co-authored-by: Nitin Kumar <[email protected]> * review feedback * more review feedback * Apply suggestions from code review Co-authored-by: Tanuj Kanti <[email protected]> * npm run build:site * fix missing directive * review feedback * review feedback * add no-unused-vars to related_rules and add back-links from all related rules * review feedback * add When Not To Use It per @snitin315 feedback --------- Co-authored-by: Tanuj Kanti <[email protected]> Co-authored-by: Kirk Waiblinger <[email protected]> Co-authored-by: Nitin Kumar <[email protected]>
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‎docs/src/_data/rules.json‎

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"frozen": false,
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"hasSuggestions": false
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},
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{
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"name": "no-unassigned-vars",
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"description": "Disallow `let` or `var` variables that are read but never assigned",
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"recommended": false,
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"fixable": false,
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"frozen": false,
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"hasSuggestions": false
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},
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{
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"name": "no-undef",
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"description": "Disallow the use of undeclared variables unless mentioned in `/*global */` comments",

‎docs/src/_data/rules_meta.json‎

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},
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"fixable": "whitespace"
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},
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"no-unassigned-vars": {
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"type": "problem",
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"dialects": [
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"typescript",
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"javascript"
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],
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"language": "javascript",
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"docs": {
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"description": "Disallow `let` or `var` variables that are read but never assigned",
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"recommended": false,
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"url": "https://eslint.org/docs/latest/rules/no-unassigned-vars"
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}
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},
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"no-undef": {
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"type": "problem",
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"defaultOptions": [

‎docs/src/rules/init-declarations.md‎

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---
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title: init-declarations
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rule_type: suggestion
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related_rules:
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- no-unassigned-vars
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---
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‎docs/src/rules/no-unassigned-vars.md‎

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---
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title: no-unassigned-vars
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rule_type: problem
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related_rules:
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- init-declarations
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- no-unused-vars
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- prefer-const
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---
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This rule flags `let` or `var` declarations that are never assigned a value but are still read or used in the code. Since these variables will always be `undefined`, their usage is likely a programming mistake.
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For example, if you check the value of a `status` variable, but it was never given a value, it will always be `undefined`:
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```js
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let status;
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// ...forgot to assign a value to status...
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if (status === 'ready') {
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console.log('Ready!');
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}
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```
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## Rule Details
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Examples of **incorrect** code for this rule:
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::: incorrect
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```js
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/*eslint no-unassigned-vars: "error"*/
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let status;
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if (status === 'ready') {
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console.log('Ready!');
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}
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let user;
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greet(user);
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function test() {
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let error;
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return error || "Unknown error";
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}
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let options;
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const { debug } = options || {};
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let flag;
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while (!flag) {
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// Do something...
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}
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let config;
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function init() {
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return config?.enabled;
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}
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```
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:::
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In TypeScript:
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::: incorrect
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```ts
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/*eslint no-unassigned-vars: "error"*/
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let value: number | undefined;
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console.log(value);
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```
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:::
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Examples of **correct** code for this rule:
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::: correct
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```js
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/*eslint no-unassigned-vars: "error"*/
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let message = "hello";
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console.log(message);
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let user;
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user = getUser();
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console.log(user.name);
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let count;
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count = 1;
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count++;
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// Variable is unused (should be reported by `no-unused-vars` only)
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let temp;
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let error;
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if (somethingWentWrong) {
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error = "Something went wrong";
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}
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console.log(error);
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let item;
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for (item of items) {
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process(item);
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}
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let config;
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function setup() {
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config = { debug: true };
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}
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setup();
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console.log(config);
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let one = undefined;
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if (one === two) {
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// Noop
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}
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```
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:::
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In TypeScript:
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::: correct
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```ts
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/*eslint no-unassigned-vars: "error"*/
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declare let value: number | undefined;
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console.log(value);
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```
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:::
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## When Not To Use It
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You can disable this rule if your code intentionally uses variables that are declared and used, but are never assigned a value. This might be the case in:
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- Legacy codebases where uninitialized variables are used as placeholders.
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- Certain TypeScript use cases where variables are declared with a type and intentionally left unassigned (though using `declare` is preferred).

‎docs/src/rules/no-unused-vars.md‎

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title: no-unused-vars
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rule_type: problem
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related_rules:
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- no-unassigned-vars
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- no-useless-assignment
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---
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‎docs/src/rules/prefer-const.md‎

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rule_type: suggestion
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related_rules:
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- no-var
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- no-unassigned-vars
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- no-use-before-define
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---
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‎lib/rules/index.js‎

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"no-this-before-super": () => require("./no-this-before-super"),
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"no-throw-literal": () => require("./no-throw-literal"),
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"no-trailing-spaces": () => require("./no-trailing-spaces"),
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"no-unassigned-vars": () => require("./no-unassigned-vars"),
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"no-undef": () => require("./no-undef"),
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"no-undef-init": () => require("./no-undef-init"),
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"no-undefined": () => require("./no-undefined"),

‎lib/rules/no-unassigned-vars.js‎

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/**
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* @fileoverview Rule to flag variables that are never assigned
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* @author Jacob Bandes-Storch <https://github.com/jtbandes>
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*/
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"use strict";
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//------------------------------------------------------------------------------
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// Rule Definition
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//------------------------------------------------------------------------------
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/** @type {import('../types').Rule.RuleModule} */
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module.exports = {
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meta: {
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type: "problem",
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dialects: ["typescript", "javascript"],
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language: "javascript",
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docs: {
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description:
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"Disallow `let` or `var` variables that are read but never assigned",
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recommended: false,
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url: "https://eslint.org/docs/latest/rules/no-unassigned-vars",
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},
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schema: [],
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messages: {
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unassigned:
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"'{{name}}' is always 'undefined' because it's never assigned.",
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},
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},
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create(context) {
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const sourceCode = context.sourceCode;
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return {
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VariableDeclarator(node) {
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/** @type {import('estree').VariableDeclaration} */
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const declaration = node.parent;
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const shouldCheck =
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!node.init &&
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node.id.type === "Identifier" &&
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declaration.kind !== "const" &&
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!declaration.declare;
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if (!shouldCheck) {
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return;
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}
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const [variable] = sourceCode.getDeclaredVariables(node);
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if (!variable) {
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return;
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}
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let hasRead = false;
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for (const reference of variable.references) {
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if (reference.isWrite()) {
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return;
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}
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if (reference.isRead()) {
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hasRead = true;
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}
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}
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if (!hasRead) {
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// Variables that are never read should be flagged by no-unused-vars instead
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return;
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}
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context.report({
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node,
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messageId: "unassigned",
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data: { name: node.id.name },
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});
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},
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};
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},
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};

‎lib/types/rules.d.ts‎

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]
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>;
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/**
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* Rule to disallow `let` or `var` variables that are read but never assigned.
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*
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* @see https://eslint.org/docs/latest/rules/no-unassigned-vars
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*/
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"no-unassigned-vars": Linter.RuleEntry<[]>;
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/**
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* Rule to disallow the use of undeclared variables unless mentioned in \/*global *\/ comments.
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*

‎packages/eslint-config-eslint/base.js‎

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"no-sequences": "error",
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"no-shadow": "error",
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"no-throw-literal": "error",
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"no-unassigned-vars": "error",
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"no-undef": ["error", { typeof: true }],
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"no-undef-init": "error",
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"no-undefined": "error",

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