fix: resolve .js to .ts in aliased paths (#18)

Co-authored-by: Hiroki Osame <[email protected]>
Co-authored-by: Marco Schumacher <[email protected]>

Author: privatenumber

src/index.ts

@@ -139,7 +139,13 @@ Module._resolveFilename = function (request, parent, isMain, options) {
 		&& !parent?.filename.includes(nodeModulesPath)
 	) {
 		const possiblePaths = tsconfigPathsMatcher(request);
+
 		for (const possiblePath of possiblePaths) {
+			const tsFilename = resolveTsFilename.call(this, possiblePath, parent, isMain, options);
+			if (tsFilename) {
+				return tsFilename;
+			}
+
 			try {
 				return resolveFilename.call(
 					this,
@@ -152,32 +158,43 @@ Module._resolveFilename = function (request, parent, isMain, options) {
 		}
 	}
 
-	/**
-	 * Typescript gives .ts, .cts, or .mts priority over actual .js, .cjs, or .mjs extensions
-	 */
-	if (parent && isTsFilePatten.test(parent.filename)) {
-		const tsPath = resolveTsPath(request);
-
-		if (tsPath) {
-			try {
-				return resolveFilename.call(
-					this,
-					tsPath,
-					parent,
-					isMain,
-					options,
-				);
-			} catch (error) {
-				const { code } = error as any;
-				if (
-					code !== 'MODULE_NOT_FOUND'
-					&& code !== 'ERR_PACKAGE_PATH_NOT_EXPORTED'
-				) {
-					throw error;
-				}
-			}
-		}
+	const tsFilename = resolveTsFilename.call(this, request, parent, isMain, options);
+	if (tsFilename) {
+		return tsFilename;
 	}
 
 	return resolveFilename.call(this, request, parent, isMain, options);
 };
+
+/**
+ * Typescript gives .ts, .cts, or .mts priority over actual .js, .cjs, or .mjs extensions
+ */
+function resolveTsFilename(
+	this: ThisType<typeof resolveFilename>,
+	request: string,
+	parent: any,
+	isMain: boolean,
+	options?: any,
+) {
+	const tsPath = resolveTsPath(request);
+
+	if (parent && isTsFilePatten.test(parent.filename) && tsPath) {
+		try {
+			return resolveFilename.call(
+				this,
+				tsPath,
+				parent,
+				isMain,
+				options,
+			);
+		} catch (error) {
+			const { code } = error as any;
+			if (
+				code !== 'MODULE_NOT_FOUND'
+				&& code !== 'ERR_PACKAGE_PATH_NOT_EXPORTED'
+			) {
+				throw error;
+			}
+		}
+	}
+}

tests/fixtures/tsconfig/src/paths-prefix-match-js.ts

@@ -0,0 +1,3 @@
+import value from 'p/nested-resolve-target.js';
+
+console.log(value);

tests/specs/typescript/tsconfig.ts

@@ -50,6 +50,13 @@ export default testSuite(async ({ describe }, node: NodeApis) => {
 				expect(nodeProcess.stdout).toBe('nested-resolve-target');
 			});
 
+			test('resolves paths via .js', async () => {
+				const nodeProcess = await node.load('./src/paths-prefix-match-js.ts', {
+					cwd: './tsconfig',
+				});
+				expect(nodeProcess.stdout).toBe('nested-resolve-target');
+			});
+
 			describe('dependency', ({ test }) => {
 				test('resolve current directory', async () => {
 					const nodeProcess = await node.load('./dependency-resolve-current-directory', {