fix: support TS resolution in JS files when allowJs is set (#535)

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

Author: nwalters512

src/cjs/index.ts

@@ -205,7 +205,10 @@ const resolveTsFilename = (
 
 	if (
 		parent?.filename
-		&& isTsFilePatten.test(parent.filename)
+		&& (
+			isTsFilePatten.test(parent.filename)
+			|| tsconfig?.config.compilerOptions?.allowJs
+		)
 		&& tsPath
 	) {
 		for (const tryTsPath of tsPath) {

src/esm/loaders.ts

@@ -17,6 +17,7 @@ import {
 	isJsonPattern,
 	getFormatFromFileUrl,
 	fileProtocol,
+	allowJs,
 	type MaybePromise,
 	type NodeError,
 } from './utils.js';
@@ -166,13 +167,11 @@ export const resolve: resolve = async (
 		}
 	}
 
-	/**
-	 * Typescript gives .ts, .cts, or .mts priority over actual .js, .cjs, or .mjs extensions
-	 */
-	if (
-		// !recursiveCall &&
-		tsExtensionsPattern.test(context.parentURL!)
-	) {
+	// Typescript gives .ts, .cts, or .mts priority over actual .js, .cjs, or .mjs extensions
+	//
+	// If `allowJs` is set in `tsconfig.json`, then we'll apply the same resolution logic
+	// to files without a TypeScript extension.
+	if (tsExtensionsPattern.test(context.parentURL!) || allowJs) {
 		const tsPaths = resolveTsPath(specifier);
 		if (tsPaths) {
 			for (const tsPath of tsPaths) {

src/esm/utils.ts

@@ -19,6 +19,7 @@ const tsconfig = (
 
 export const fileMatcher = tsconfig && createFilesMatcher(tsconfig);
 export const tsconfigPathsMatcher = tsconfig && createPathsMatcher(tsconfig);
+export const allowJs = tsconfig?.config.compilerOptions?.allowJs ?? false;
 
 export const fileProtocol = 'file://';
 

tests/specs/smoke.ts

@@ -218,6 +218,14 @@ const files = {
 
 	'file.txt': 'hello',
 
+	'import-typescript-parent.js': sourcemap.tag`
+	import './import-typescript-child.js';
+	`,
+
+	'import-typescript-child.ts': sourcemap.tag`
+	console.log('imported');
+	`,
+
 	node_modules: {
 		'pkg-commonjs': {
 			'package.json': JSON.stringify({
@@ -663,6 +671,17 @@ export default testSuite(async ({ describe }, { tsx }: NodeApis) => {
 						expect(pTsconfigAllowJs.stderr).toMatch('Error: No error thrown');
 						expect(pTsconfigAllowJs.stdout).toBe('');
 					});
+
+					test('allowJs in tsconfig.json', async ({ onTestFail }) => {
+						const pTsconfigAllowJs = await tsx(['--tsconfig', 'tsconfig/tsconfig-allowJs.json', 'import-typescript-parent.js'], fixture.path);
+						onTestFail((error) => {
+							console.error(error);
+							console.log(pTsconfigAllowJs);
+						});
+						expect(pTsconfigAllowJs.failed).toBe(false);
+						expect(pTsconfigAllowJs.stderr).toBe('');
+						expect(pTsconfigAllowJs.stdout).toBe('imported');
+					});
 				});
 			});
 		}