Support ESM files

Fixes #5.

Author: novemberborn

Diff for: README.md

@@ -39,6 +39,10 @@ Output files are expected to have the `.js` extension.
 
 AVA searches your entire project for `*.js`, `*.cjs`, `*.mjs` and `*.ts` files (or other extensions you've configured). It will ignore such files found in the `rewritePaths` targets (e.g. `build/`). If you use more specific paths, for instance `build/main/`, you may need to change AVA's `files` configuration to ignore other directories.
 
+## ES Modules
+
+When used with AVA 4, if your `package.json` has configured `"type": "module"`, or you've configured AVA to treat the `js` extension as `module`, then `@ava/typescript` will import the output file as an ES module. Note that this is based on the *output file*, not the `ts` extension.
+
 ## Add additional extensions
 
 You can configure AVA to recognize additional file extensions. To add (partial†) JSX support:

Diff for: index.js

@@ -1,5 +1,6 @@
 import fs from 'node:fs';
 import path from 'node:path';
+import {pathToFileURL} from 'node:url';
 import escapeStringRegexp from 'escape-string-regexp';
 import execa from 'execa';
 
@@ -149,6 +150,7 @@ export default function typescriptProvider({negotiateProtocol}) {
 		},
 
 		worker({extensionsToLoadAsModules, state: {extensions, rewritePaths}}) {
+			const useImport = extensionsToLoadAsModules.includes('js');
 			const testFileExtension = new RegExp(`\\.(${extensions.map(ext => escapeStringRegexp(ext)).join('|')})$`);
 
 			return {
@@ -157,16 +159,10 @@ export default function typescriptProvider({negotiateProtocol}) {
 				},
 
 				async load(ref, {requireFn}) {
-					for (const extension of extensionsToLoadAsModules) {
-						if (ref.endsWith(`.${extension}`)) {
-							throw new Error('@ava/typescript cannot yet load ESM files');
-						}
-					}
-
 					const [from, to] = rewritePaths.find(([from]) => ref.startsWith(from));
 					// TODO: Support JSX preserve mode — https://www.typescriptlang.org/docs/handbook/jsx.html
 					const rewritten = `${to}${ref.slice(from.length)}`.replace(testFileExtension, '.js');
-					return requireFn(rewritten);
+					return useImport ? import(pathToFileURL(rewritten)) : requireFn(rewritten); // eslint-disable-line node/no-unsupported-features/es-syntax
 				},
 			};
 		},

Diff for: package.json

@@ -44,6 +44,7 @@
     "files": [
       "!test/broken-fixtures/**"
     ],
+    "ignoredByWatcher": ["test/fixtures/**", "test/broken-fixtures/**"],
     "timeout": "60s"
   },
   "xo": {

Diff for: test/compilation.js

@@ -30,7 +30,7 @@ test('worker(): load rewritten paths files', withProvider, async (t, provider) =
 	const {state} = await compile(provider);
 	const {stdout, stderr} = await execa.node(
 		path.join(__dirname, 'fixtures/install-and-load'),
-		[JSON.stringify(state), path.join(__dirname, 'fixtures/ts', 'file.ts')],
+		[JSON.stringify({state}), path.join(__dirname, 'fixtures/ts', 'file.ts')],
 		{cwd: path.join(__dirname, 'fixtures')},
 	);
 	if (stderr.length > 0) {
@@ -44,7 +44,7 @@ test('worker(): runs compiled files', withProvider, async (t, provider) => {
 	const {state} = await compile(provider);
 	const {stdout, stderr} = await execa.node(
 		path.join(__dirname, 'fixtures/install-and-load'),
-		[JSON.stringify(state), path.join(__dirname, 'fixtures/compiled', 'index.ts')],
+		[JSON.stringify({state}), path.join(__dirname, 'fixtures/compiled', 'index.ts')],
 		{cwd: path.join(__dirname, 'fixtures')},
 	);
 	if (stderr.length > 0) {

Diff for: test/esm.js

@@ -0,0 +1,33 @@
+import path from 'node:path';
+import {fileURLToPath} from 'node:url';
+import test from 'ava';
+import execa from 'execa';
+import createProviderMacro from './_with-provider.js';
+
+const __dirname = path.dirname(fileURLToPath(import.meta.url));
+const withProvider = createProviderMacro('ava-3.2', '3.2.0', path.join(__dirname, 'fixtures'));
+
+const setup = async provider => ({
+	state: await provider.main({
+		config: {
+			rewritePaths: {
+				'esm/': 'esm/',
+			},
+			compile: false,
+		},
+	}).compile(),
+});
+
+test('worker(): import ESM', withProvider, async (t, provider) => {
+	const {state} = await setup(provider);
+	const {stdout, stderr} = await execa.node(
+		path.join(__dirname, 'fixtures/install-and-load'),
+		[JSON.stringify({extensionsToLoadAsModules: ['js'], state}), path.join(__dirname, 'fixtures/esm', 'index.ts')],
+		{cwd: path.join(__dirname, 'fixtures')},
+	);
+	if (stderr.length > 0) {
+		t.log(stderr);
+	}
+
+	t.snapshot(stdout);
+});

Diff for: test/fixtures/esm/index.js

@@ -0,0 +1 @@
+console.log('logged in fixtures/esm/index.js');

Diff for: test/fixtures/esm/index.ts

@@ -0,0 +1 @@
+console.log('logged in fixtures/esm/index.ts');

Diff for: test/fixtures/install-and-load.js

@@ -14,7 +14,8 @@ const provider = makeProvider({
 
 const worker = provider.worker({
 	extensionsToLoadAsModules: [],
-	state: JSON.parse(process.argv[2]),
+	state: {},
+	...JSON.parse(process.argv[2]),
 });
 
 const ref = path.resolve(process.argv[3]);

Diff for: test/snapshots/esm.js.md

@@ -0,0 +1,11 @@
+# Snapshot report for `test/esm.js`
+
+The actual snapshot is saved in `esm.js.snap`.
+
+Generated by [AVA](https://avajs.dev).
+
+## worker(): import ESM
+
+> Snapshot 1
+
+    'logged in fixtures/esm/index.js'