fix(cjs): esm interop in .mjs files (#32)

fixes #334

src/utils/transform/get-esbuild-options.ts

@@ -54,6 +54,8 @@ export const patchOptions = (
 			// https://github.com/evanw/esbuild/issues/1932
 			if (extension === '.cts' || extension === '.mts') {
 				options.sourcefile = `${originalSourcefile.slice(0, -3)}ts`;
+			} else if (extension === '.mjs') { // only used by CJS loader
+				options.sourcefile = `${originalSourcefile.slice(0, -3)}js`;
 			}
 		} else {
 			// esbuild errors to detect loader when a file doesn't have an extension

tests/fixtures.ts

@@ -149,14 +149,20 @@ export const files = {
 	exports.named = 'named';
 	`,
 
-	'mjs/index.mjs': outdent`
-	import assert from 'assert';
-	export const mjsHasCjsContext = ${cjsContextCheck};
-
-	import ('pkg-commonjs').then((m) => assert(
-		!(typeof m.default === 'object' && ('default' in m.default)),
-	));
-	`,
+	mjs: {
+		'index.mjs': outdent`
+		import assert from 'assert';
+		import value from './value.mjs';
+		export const mjsHasCjsContext = ${cjsContextCheck};
+
+		assert(value === 1, 'wrong default export');
+
+		import ('pkg-commonjs').then((m) => assert(
+			!(typeof m.default === 'object' && ('default' in m.default)),
+		));
+		`,
+		'value.mjs': 'export default 1',
+	},
 
 	'ts/index.ts': sourcemap.tag`
 	import assert from 'assert';