fix(commonjs): add .cjs to default file extensions. (#524)

* Added .cjs and .mjs to default file extensions.

* Removed accidental yarn.lock.

* Added test.

* Added snapshots.

* Removed unneeded extension.

* Moved extension check, refactored Array.indexOf() to Array.includes()

packages/commonjs/src/index.js

@@ -188,8 +188,9 @@ export default function commonjs(options = {}) {
  },
 
  transform(code, id) {
- if (id !== DYNAMIC_PACKAGES_ID && !id.startsWith(DYNAMIC_JSON_PREFIX)) {
- if (!filter(id) || extensions.indexOf(extname(id)) === -1) {
+ const extName = extname(id);
+ if (extName !== '.cjs' && id !== DYNAMIC_PACKAGES_ID && !id.startsWith(DYNAMIC_JSON_PREFIX)) {
+ if (!filter(id) || !extensions.includes(extName)) {
  setIsCjsPromise(id, null);
  return null;
  }

packages/commonjs/test/fixtures/samples/cjs-extension/export.cjs

@@ -0,0 +1,3 @@
+module.exports = {
+ test: 42
+};

packages/commonjs/test/fixtures/samples/cjs-extension/main.js

@@ -0,0 +1,3 @@
+const { test } = require('./export.cjs');
+
+console.log(test);

packages/commonjs/test/snapshots/test.js.md

@@ -21,3 +21,24 @@ Generated by [AVA](https://avajs.dev).
  ␊
  module.exports = main;␊
  `
+
+## imports .cjs file extension by default
+
+> Snapshot 1
+
+ `'use strict';␊
+ ␊
+ var _export = {␊
+  test: 42␊
+ };␊
+ ␊
+ const { test } = _export;␊
+ ␊
+ console.log(test);␊
+ ␊
+ var main = {␊
+ ␊
+ };␊
+ ␊
+ module.exports = main;␊
+ `

packages/commonjs/test/test.js

@@ -639,3 +639,12 @@ test('logs a warning when the deprecated namedExports option is used', async (t)
  'The namedExports option from "@rollup/plugin-commonjs" is deprecated. Named exports are now handled automatically.'
  );
 });
+
+test('imports .cjs file extension by default', async (t) => {
+ const bundle = await rollup({
+ input: 'fixtures/samples/cjs-extension/main.js',
+ plugins: [commonjs()]
+ });
+ const code = await getCodeFromBundle(bundle);
+ t.snapshot(code);
+});