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fix: anonymous default export name fix-up in ES5 environment
When `output.environment.const` is false and the default export slot is used, the harmony default export template assigns directly to `__webpack_exports__["default"]` without declaring any local `__WEBPACK_DEFAULT_EXPORT__` binding. The anonymous-default `.name` fix-up introduced in #20773 still referenced that undeclared binding, causing a `ReferenceError` at module load time. Also replaces `Reflect.*` with `Object.*` so the emitted code does not depend on ES2015 `Reflect` in ES5 target runtimes, and guards against an undefined descriptor so Node 10 (and other legacy V8 versions that do not set an own `name` property on anonymous class expressions assigned to a MemberExpression) does not throw. Adds regression tests covering anonymous arrow / async arrow / class / class-extends default exports under ES5 environment, plus an anonymous function default export under module concatenation. Fixes #20793
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Lines changed: 170 additions & 8 deletions

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‎.changeset/fix-anonymous-default-export-es5-regression.md‎

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---
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"webpack": patch
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---
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Fix two ES5-environment regressions in the anonymous default export `.name` fix-up: the generated code referenced an undeclared `__WEBPACK_DEFAULT_EXPORT__` binding causing `ReferenceError`, and used `Reflect.defineProperty` which is not available in pre-ES2015 runtimes. The fix-up now references the real assignment target and uses `Object.defineProperty` / `Object.getOwnPropertyDescriptor`.

‎lib/Module.js‎

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@@ -120,14 +120,11 @@ const makeSerializable = require("./util/makeSerializable");
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* @property {[{ shareScope: string, initStage: number, init: string }]} share-init share-init for modules federation
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*/
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/* eslint-disable jsdoc/type-formatting */
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/**
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* @template {string} K
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* @typedef {K extends keyof AllCodeGenerationSchemas ? AllCodeGenerationSchemas[K] : EXPECTED_ANY} CodeGenValue
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*/
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/* eslint-enable jsdoc/type-formatting */
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/* eslint-disable jsdoc/require-template */
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/**
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* @typedef {object} CodeGenMapOverloads
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* @property {<K extends string>(key: K) => CodeGenValue<K> | undefined} get

‎lib/dependencies/HarmonyExportExpressionDependency.js‎

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@@ -163,7 +163,7 @@ HarmonyExportExpressionDependency.Template = class HarmonyExportDependencyTempla
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// see test/test262-cases/test/language/module-code/instn-named-bndng-dflt-fun-anon.js cspell:disable-line
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initFragments.push(
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new InitFragment(
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`Reflect.defineProperty(${name}, "name", { value: "default", configurable: true });\n`,
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`Object.defineProperty(${name}, "name", { value: "default", configurable: true });\n`,
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InitFragment.STAGE_HARMONY_EXPORTS,
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2
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)
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} else {
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/** @type {string} */
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let content;
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const name = ConcatenationScope.DEFAULT_EXPORT;
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let name = ConcatenationScope.DEFAULT_EXPORT;
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if (runtimeTemplate.supportsConst()) {
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content = `/* harmony default export */ const ${name} = `;
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if (concatenationScope) {
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if (used) {
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runtimeRequirements.add(RuntimeGlobals.exports);
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// This is a little bit incorrect as TDZ is not correct, but we can't use const.
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content = `/* harmony default export */ ${exportsName}${propertyAccess(
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// No local `__WEBPACK_DEFAULT_EXPORT__` binding is created in this path,
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// so the anonymous-default `.name` fix-up below must reference the actual
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// assignment target instead. See issue #20793.
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name = `${exportsName}${propertyAccess(
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typeof used === "string" ? [used] : used
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)} = `;
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)}`;
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content = `/* harmony default export */ ${name} = `;
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} else {
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content = `/* unused harmony default export */ var ${name} = `;
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}
@@ -221,7 +225,7 @@ HarmonyExportExpressionDependency.Template = class HarmonyExportDependencyTempla
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source.replace(
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dep.range[1],
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dep.rangeStatement[1] - 0.5,
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`);\nReflect.getOwnPropertyDescriptor(${name}, "name").writable || Reflect.defineProperty(${name}, "name", { value: "default", configurable: true });`
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`);\n(Object.getOwnPropertyDescriptor(${name}, "name") || {}).writable || Object.defineProperty(${name}, "name", { value: "default", configurable: true });`
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);
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} else {
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source.replace(dep.range[1], dep.rangeStatement[1] - 0.5, ");");

‎test/configCases/anonymous-default-export-name/concat-anonymous-function/index.js‎

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import inner from "./inner";
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it("should handle anonymous default function declaration under module concatenation", () => {
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// When ConcatenationScope renames `__WEBPACK_DEFAULT_EXPORT__` to a
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// module-scoped symbol, any Reflect.defineProperty InitFragment added
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// for the anonymous default fix-up must reference the renamed symbol.
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expect(inner()).toBe(42);
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expect(inner.name).toBe("default");
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});

‎test/configCases/anonymous-default-export-name/concat-anonymous-function/inner.js‎

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export default function () {
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return 42;
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}

‎test/configCases/anonymous-default-export-name/concat-anonymous-function/webpack.config.js‎

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"use strict";
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/** @type {import("../../../../").Configuration} */
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module.exports = {
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mode: "production",
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devtool: false,
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optimization: {
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concatenateModules: true,
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minimize: false
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}
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};

‎test/configCases/anonymous-default-export-name/es5-async-arrow/index.js‎

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import load from "./loader";
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it("should handle anonymous async arrow default export in ES5 env", async () => {
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// `export default async (v) => v * 2` — ArrowFunctionExpression (async),
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// matches isAnonymousDefault. Ensure the Reflect fix-up references the
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// real assignment target, not an undeclared `__WEBPACK_DEFAULT_EXPORT__`.
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const result = await load(21);
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expect(result).toBe(42);
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});

‎test/configCases/anonymous-default-export-name/es5-async-arrow/loader.js‎

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export default async (/** @type {number} */ value) => value * 2;

‎test/configCases/anonymous-default-export-name/es5-async-arrow/webpack.config.js‎

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"use strict";
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/** @type {import("../../../../").Configuration} */
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module.exports = {
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mode: "development",
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devtool: false,
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output: {
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environment: {
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module: false,
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const: false,
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arrowFunction: false,
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bigIntLiteral: false,
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destructuring: false,
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dynamicImport: false,
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forOf: false
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}
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}
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};

‎test/configCases/anonymous-default-export-name/es5-class-extends/Base.js‎

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export class Base {
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kind() {
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return "base";
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}
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}

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