fix: Prepare for ESM loader hooks moving to worker threads. (#3)

index.js

@@ -15,9 +15,9 @@ let nycConfig;
 let testExclude;
 let babelConfig;
 
-export async function transformSource(source, context) {
-	if (loader.isLoading()) {
-		return {source};
+export async function transformSource(source, context, defaultTransformSource) {
+	if (context.format !== 'module' || loader.isLoading()) {
+		return defaultTransformSource(source, context, defaultTransformSource);
 	}
 
 	if (!nycConfig) {
@@ -41,16 +41,14 @@ export async function transformSource(source, context) {
 		};
 	}
 
-	if (Buffer.isBuffer(source)) {
-		source = source.toString();
-	} else if (typeof source !== 'string') {
-		return {source};
-	}
-
 	const filename = fileURLToPath(context.url);
 	/* babel-plugin-istanbul does this but the early check optimizes */
 	if (!testExclude.shouldInstrument(filename)) {
-		return {source};
+		return defaultTransformSource(source, context, defaultTransformSource);
+	}
+
+	if (typeof source !== 'string') {
+		source = new TextDecoder().decode(source);
 	}
 
 	/* Can/should this handle inputSourceMap? */
@@ -71,5 +69,5 @@ export async function transformSource(source, context) {
 		]
 	});
 
-	return {source: code};
+	return defaultTransformSource(code, context, defaultTransformSource);
 }

test/index.js

@@ -1,7 +1,6 @@
 import {fileURLToPath} from 'url';
 
 import {test} from 'libtap';
-// eslint-disable-next-line import/no-unresolved
 import * as loaderHook from '@istanbuljs/esm-loader-hook';
 
 test('exports', async t => {
@@ -13,22 +12,17 @@ test('transform buffer', async t => {
 	const {source} = await loaderHook.transformSource(
 		Buffer.from('export default true;'),
 		{
+			format: 'module',
 			url: new URL('../fixtures/buffer.js', import.meta.url)
-		}
+		},
+		source => ({source})
 	);
 
 	t.match(source, /function\s+cov_/u);
 	t.match(source, /export\s+default\s+true/u);
 	t.match(source, /\/\/# sourceMappingURL=/u);
 });
 
-test('transform not string', async t => {
-	const notString = {};
-	const {source} = await loaderHook.transformSource(notString);
-
-	t.equal(source, notString);
-});
-
 test('transform hook', async t => {
 	await t.spawn(
 		process.execPath,

test/no-source-maps.js

@@ -1,12 +1,12 @@
 import {test} from 'libtap';
-// eslint-disable-next-line import/no-unresolved
 import {transformSource} from '@istanbuljs/esm-loader-hook';
 
 test('check transform', async t => {
 	const context = {
+		format: 'module',
 		url: new URL('../fixtures/no-source-map.js', import.meta.url)
 	};
-	const {source} = await transformSource('export default true;', context);
+	const {source} = await transformSource('export default true;', context, source => ({source}));
 
 	t.match(source, /function\s+cov_/u);
 	t.match(source, /export\s+default\s+true/u);