Merge pull request #554 from webpack/claude/fix-extension-alias-alignment-xC6sZ

Author: alexander-akait

.changeset/extension-alias-for-exports.md

@@ -0,0 +1,5 @@
+---
+"enhanced-resolve": minor
+---
+
+Add `extensionAliasForExports` option. When `true`, `extensionAlias` also applies to paths resolved through the `package.json` `exports` field. Off by default to match Node.js; opt in for full TypeScript-resolver parity with packages that ship `.ts` sources alongside the compiled `.js` they declare in `exports`.

README.md

@@ -67,6 +67,7 @@ const { PathType, getType } = require("./util/path");
  * @property {(AliasOptions | AliasOptionEntry[])=} alias A list of module alias configurations or an object which maps key to value
  * @property {(AliasOptions | AliasOptionEntry[])=} fallback A list of module alias configurations or an object which maps key to value, applied only after modules option
  * @property {ExtensionAliasOptions=} extensionAlias An object which maps extension to extension aliases
+ * @property {boolean=} extensionAliasForExports Also apply `extensionAlias` to paths resolved through the package.json `exports` field. Off by default (Node.js-aligned); when enabled, matches TypeScript's behavior for packages that ship TS sources alongside compiled JS.
  * @property {(string | string[])[]=} aliasFields A list of alias fields in description files
  * @property {((predicate: ResolveRequest) => boolean)=} cachePredicate A function which decides whether a request should be cached or not. An object is passed with at least `path` and `request` properties.
  * @property {boolean=} cacheWithContext Whether or not the unsafeCache should include request context as part of the cache key.
@@ -101,6 +102,7 @@ const { PathType, getType } = require("./util/path");
  * @property {AliasOptionEntry[]} fallback fallback
  * @property {Set<string | string[]>} aliasFields alias fields
  * @property {ExtensionAliasOption[]} extensionAlias extension alias
+ * @property {boolean} extensionAliasForExports apply extension alias to exports field targets
  * @property {(predicate: ResolveRequest) => boolean} cachePredicate cache predicate
  * @property {boolean} cacheWithContext cache with context
  * @property {Set<string>} conditionNames A list of exports field condition names.
@@ -270,6 +272,7 @@ function createOptions(options) {
 					],
 				}))
 			: [],
+		extensionAliasForExports: options.extensionAliasForExports || false,
 		fileSystem: options.useSyncFileSystemCalls
 			? new SyncAsyncFileSystemDecorator(
 					/** @type {SyncFileSystem} */ (
@@ -320,6 +323,7 @@ module.exports.createResolver = function createResolver(options) {
 		alias,
 		fallback,
 		aliasFields,
+		extensionAliasForExports,
 		cachePredicate,
 		cacheWithContext,
 		conditionNames,
@@ -371,6 +375,9 @@ module.exports.createResolver = function createResolver(options) {
 	resolver.ensureHook("resolveInPackage");
 	resolver.ensureHook("resolveInExistingDirectory");
 	resolver.ensureHook("importsFieldRelative");
+	if (extensionAliasForExports) {
+		resolver.ensureHook("exportsFieldRelative");
+	}
 	resolver.ensureHook("relative");
 	resolver.ensureHook("describedRelative");
 	resolver.ensureHook("directory");
@@ -595,20 +602,37 @@ module.exports.createResolver = function createResolver(options) {
 	);
 
 	// resolve-in-package
+	const exportsFieldTarget = extensionAliasForExports
+		? "exports-field-relative"
+		: "relative";
 	for (const exportsField of exportsFields) {
 		plugins.push(
 			new ExportsFieldPlugin(
 				"resolve-in-package",
 				conditionNames,
 				exportsField,
-				"relative",
+				exportsFieldTarget,
 			),
 		);
 	}
 	plugins.push(
 		new NextPlugin("resolve-in-package", "resolve-in-existing-directory"),
 	);
 
+	// exports-field-relative (opt-in via `extensionAliasForExports`):
+	// apply `extensionAlias` to paths produced by the exports field. This is
+	// off by default to match Node.js (which does not substitute extensions on
+	// bare-module targets), and on opt-in aligns with TypeScript for packages
+	// that ship TS sources alongside the compiled JS they list in `exports`.
+	if (extensionAliasForExports) {
+		for (const item of extensionAlias) {
+			plugins.push(
+				new ExtensionAliasPlugin("exports-field-relative", item, "relative"),
+			);
+		}
+		plugins.push(new NextPlugin("exports-field-relative", "relative"));
+	}
+
 	// resolve-in-existing-directory
 	plugins.push(
 		new JoinRequestPlugin("resolve-in-existing-directory", "relative"),

lib/ResolverFactory.js

@@ -131,6 +131,61 @@ describe("extension-alias", () => {
 		});
 	});
 
+	describe("exports field (extensionAliasForExports)", () => {
+		const exportsFixture = path.resolve(
+			__dirname,
+			"fixtures",
+			"exports-field-extension-alias-opt-in",
+		);
+
+		it("should not apply extension alias to exports-field targets by default (Node.js-aligned)", (done) => {
+			const defaultResolver = ResolverFactory.createResolver({
+				extensions: [".js"],
+				extensionAlias: { ".js": [".ts", ".js"] },
+				fileSystem: nodeFileSystem,
+				fullySpecified: true,
+				conditionNames: ["default"],
+			});
+			defaultResolver.resolve(
+				{},
+				exportsFixture,
+				"pkg/string.js",
+				{},
+				(err, result) => {
+					if (err) return done(err);
+					expect(result).toEqual(
+						path.resolve(exportsFixture, "./node_modules/pkg/dist/string.js"),
+					);
+					done();
+				},
+			);
+		});
+
+		it("should prefer the TS source over the exports-declared JS target when the option is enabled", (done) => {
+			const tsResolver = ResolverFactory.createResolver({
+				extensions: [".js"],
+				extensionAlias: { ".js": [".ts", ".js"] },
+				extensionAliasForExports: true,
+				fileSystem: nodeFileSystem,
+				fullySpecified: true,
+				conditionNames: ["default"],
+			});
+			tsResolver.resolve(
+				{},
+				exportsFixture,
+				"pkg/string.js",
+				{},
+				(err, result) => {
+					if (err) return done(err);
+					expect(result).toEqual(
+						path.resolve(exportsFixture, "./node_modules/pkg/dist/string.ts"),
+					);
+					done();
+				},
+			);
+		});
+	});
+
 	describe("should not apply extension alias to extensions or mainFiles field", () => {
 		const resolver = ResolverFactory.createResolver({
 			extensions: [".js"],

test/extension-alias.test.js

@@ -1281,6 +1281,11 @@ declare interface ResolveOptionsResolverFactoryObject_1 {
 	 */
 	extensionAlias: ExtensionAliasOption[];
 
+	/**
+	 * apply extension alias to exports field targets
+	 */
+	extensionAliasForExports: boolean;
+
 	/**
 	 * cache predicate
 	 */
@@ -1417,6 +1422,11 @@ declare interface ResolveOptionsResolverFactoryObject_2 {
 	 */
 	extensionAlias?: ExtensionAliasOptions;
 
+	/**
+	 * Also apply `extensionAlias` to paths resolved through the package.json `exports` field. Off by default (Node.js-aligned); when enabled, matches TypeScript's behavior for packages that ship TS sources alongside compiled JS.
+	 */
+	extensionAliasForExports?: boolean;
+
 	/**
 	 * A list of alias fields in description files
 	 */