add support for Node.JS native ES modules

.eslintrc.yml

@@ -31,7 +31,12 @@ overrides:
       ecmaVersion: 2017
     env:
       browser: false
-
+  - files:
+      - esm-utils.js
+    parserOptions:
+      ecmaVersion: 2018
+      sourceType: module
+    parser: babel-eslint
   - files:
       - test/**/*.{js,mjs}
     env:

docs/index.md

@@ -39,6 +39,7 @@ Mocha is a feature-rich JavaScript test framework running on [Node.js][] and in
 - [mocha.opts file support](#-opts-path)
 - clickable suite titles to filter test execution
 - [node debugger support](#-inspect-inspect-brk-inspect)
+- [node native ES modules support](#nodejs-native-esm-support)
 - [detects multiple calls to `done()`](#detects-multiple-calls-to-done)
 - [use any assertion library you want](#assertions)
 - [extensible reporting, bundled with 9+ reporters](#reporters)
@@ -1538,6 +1539,38 @@ Alias: `HTML`, `html`
 
 **The HTML reporter is not intended for use on the command-line.**
 
+## Node.JS native ESM support
+
+Mocha supports writing your tests as ES modules (without needing to use the `esm` polyfill module), and not just using CommonJS. For example:
+
+```js
+// test.mjs
+import {add} from './add.mjs';
+import assert from 'assert';
+
+it('should add to numbers from an es module', () => {
+  assert.equal(add(3, 5), 8);
+});
+```
+
+To enable this you don't need to do anything special. Write your test file as an ES module. In Node.js
+this means either ending the file with a `.mjs` extension, or, if you want to use the regular `.js` extension, by
+adding `"type": "module"` to your `package.json`.
+More information can be found in the [Node.js documentation](https://nodejs.org/api/esm.html).
+
+> Mocha supports ES modules only from Node.js v12.11.0 and above. Also note that
+> to enable this in vesions smaller than 13.2.0, you need to add `--experimental-modules` when running
+> Mocha. From version 13.2.0 of Node.js, you can use ES modules without any flags.
+
+### Limitations
+
+- "Watch mode" (i.e. using `--watch` options) does not currently support ES Module test files,
+  although we intend to support it in the future
+- [Custom reporters](#third-party-reporters) and [custom interfaces](#interfaces)
+  can currently only be CommonJS files, although we intend to support it in the future
+- `mocharc.js` can only be a CommonJS file (can also be called `mocharc.cjs`),
+  although we intend to support ESM in the future
+
 ## Running Mocha in the Browser
 
 Mocha runs in the browser. Every release of Mocha will have new builds of `./mocha.js` and `./mocha.css` for use in the browser.
@@ -1609,17 +1642,17 @@ mocha.setup({
 
 ### Browser-specific Option(s)
 
-Browser Mocha supports many, but not all [cli options](#command-line-usage).  
+Browser Mocha supports many, but not all [cli options](#command-line-usage).
 To use a [cli option](#command-line-usage) that contains a "-", please convert the option to camel-case, (eg. `check-leaks` to `checkLeaks`).
 
 #### Options that differ slightly from [cli options](#command-line-usage):
 
-`reporter` _{string|constructor}_  
+`reporter` _{string|constructor}_
 You can pass a reporter's name or a custom reporter's constructor. You can find **recommended** reporters for the browser [here](#reporting). It is possible to use [built-in reporters](#reporters) as well. Their employment in browsers is neither recommended nor supported, open the console to see the test results.
 
 #### Options that _only_ function in browser context:
 
-`noHighlighting` _{boolean}_  
+`noHighlighting` _{boolean}_
 If set to `true`, do not attempt to use syntax highlighting on output test code.
 
 ### Reporting
@@ -1701,7 +1734,8 @@ tests as shown below:
 
 In addition to supporting the deprecated [`mocha.opts`](#mochaopts) run-control format, Mocha now supports configuration files, typical of modern command-line tools, in several formats:
 
-- **JavaScript**: Create a `.mocharc.js` in your project's root directory, and export an object (`module.exports = {/* ... */}`) containing your configuration.
+- **JavaScript**: Create a `.mocharc.js` (or `mocharc.cjs` when using [`"type"="module"`](#nodejs-native-esm-support) in your `package.json`)
+  in your project's root directory, and export an object (`module.exports = {/* ... */}`) containing your configuration.
 - **YAML**: Create a `.mocharc.yaml` (or `.mocharc.yml`) in your project's root directory.
 - **JSON**: Create a `.mocharc.json` (or `.mocharc.jsonc`) in your project's root directory. Comments — while not valid JSON — are allowed in this file, and will be ignored by Mocha.
 - **package.json**: Create a `mocha` property in your project's `package.json`.

karma.conf.js

@@ -35,6 +35,7 @@ module.exports = config => {
           .ignore('chokidar')
           .ignore('fs')
           .ignore('glob')
+          .ignore('./lib/esm-utils.js')
           .ignore('path')
           .ignore('supports-color')
           .on('bundled', (err, content) => {

lib/cli/cli.js

@@ -12,12 +12,12 @@
  */
 
 const debug = require('debug')('mocha:cli:cli');
-const symbols = require('log-symbols');
 const yargs = require('yargs/yargs');
 const path = require('path');
+const ansi = require('ansi-colors');
+const symbols = require('log-symbols');
 const {loadOptions, YARGS_PARSER_CONFIG} = require('./options');
 const commands = require('./commands');
-const ansi = require('ansi-colors');
 const {repository, homepage, version, gitter} = require('../../package.json');
 
 /**
@@ -48,7 +48,9 @@ exports.main = (argv = process.argv.slice(2)) => {
     .fail((msg, err, yargs) => {
       debug(err);
       yargs.showHelp();
-      console.error(`\n${symbols.error} ${ansi.red('ERROR:')} ${msg}`);
+      console.error(
+        `\n${symbols.error} ${ansi.red('ERROR:')} ${err ? err.message : msg}`
+      );
       process.exit(1);
     })
     .help('help', 'Show usage information & exit')

lib/cli/config.js

@@ -21,6 +21,7 @@ const findUp = require('find-up');
  * @private
  */
 exports.CONFIG_FILES = [
+  '.mocharc.cjs',
   '.mocharc.js',
   '.mocharc.yaml',
   '.mocharc.yml',
@@ -75,7 +76,7 @@ exports.loadConfig = filepath => {
   try {
     if (ext === '.yml' || ext === '.yaml') {
       config = parsers.yaml(filepath);
-    } else if (ext === '.js') {
+    } else if (ext === '.js' || ext === '.cjs') {
       config = parsers.js(filepath);
     } else {
       config = parsers.json(filepath);

lib/cli/options.js

@@ -265,7 +265,7 @@ module.exports.loadPkgRc = loadPkgRc;
  * Priority list:
  *
  * 1. Command-line args
- * 2. RC file (`.mocharc.js`, `.mocharc.ya?ml`, `mocharc.json`)
+ * 2. RC file (`.mocharc.c?js`, `.mocharc.ya?ml`, `mocharc.json`)
  * 3. `mocha` prop of `package.json`
  * 4. `mocha.opts`
  * 5. default configuration (`lib/mocharc.json`)

lib/cli/run-helpers.js

@@ -98,21 +98,29 @@ exports.handleRequires = (requires = []) => {
  * @param {boolean} [opts.exit] - Whether or not to force-exit after tests are complete
  * @param {Object} fileCollectParams - Parameters that control test
  *   file collection. See `lib/cli/collect-files.js`.
- * @returns {Runner}
+ * @returns {Promise<Runner>}
  * @private
  */
 exports.singleRun = (mocha, {exit}, fileCollectParams) => {
   const files = collectFiles(fileCollectParams);
   debug('running tests with files', files);
   mocha.files = files;
-  return mocha.run(exit ? exitMocha : exitMochaLater);
+
+  return mocha.runAsync().then(exitCode => {
+    if (exit) {
+      exitMocha(exitCode);
+    } else {
+      exitMochaLater(exitCode);
+    }
+  });
 };
 
 /**
  * Actually run tests
  * @param {Mocha} mocha - Mocha instance
  * @param {Object} opts - Command line options
  * @private
+ * @returns {Promise}
  */
 exports.runMocha = (mocha, options) => {
   const {
@@ -140,7 +148,7 @@ exports.runMocha = (mocha, options) => {
   if (watch) {
     watchRun(mocha, {watchFiles, watchIgnore}, fileCollectParams);
   } else {
-    exports.singleRun(mocha, {exit}, fileCollectParams);
+    return exports.singleRun(mocha, {exit}, fileCollectParams);
   }
 };
 

lib/cli/run.js

@@ -87,7 +87,7 @@ exports.builder = yargs =>
       },
       extension: {
         default: defaults.extension,
-        defaultDescription: 'js',
+        defaultDescription: 'js,cjs,mjs',
         description: 'File extension(s) to load',
         group: GROUPS.FILES,
         requiresArg: true,
@@ -299,8 +299,14 @@ exports.builder = yargs =>
     .number(types.number)
     .alias(aliases);
 
-exports.handler = argv => {
+exports.handler = async function(argv) {
   debug('post-yargs config', argv);
   const mocha = new Mocha(argv);
-  runMocha(mocha, argv);
+
+  try {
+    await runMocha(mocha, argv);
+  } catch (err) {
+    console.error(err.stack || `Error: ${err.message || err}`);
+    process.exit(1);
+  }
 };

lib/esm-utils.js

@@ -0,0 +1,35 @@
+// This file is allowed to use async/await because it is not exposed to browsers (see the `eslintrc`),
+// and Node supports async/await in all its non-dead version.
+
+const url = require('url');
+const path = require('path');
+
+exports.requireOrImport = async file => {
+  file = path.resolve(file);
+
+  if (path.extname(file) === '.mjs') {
+    return import(url.pathToFileURL(file));
+  }
+  // This way of figuring out whether a test file is CJS or ESM is currently the only known
+  // way of figuring out whether a file is CJS or ESM.
+  // If Node.js or the community establish a better procedure for that, we can fix this code.
+  // Another option here would be to always use `import()`, as this also supports CJS, but I would be
+  // wary of using it for _all_ existing test files, till ESM is fully stable.
+  try {
+    return require(file);
+  } catch (err) {
+    if (err.code === 'ERR_REQUIRE_ESM') {
+      return import(url.pathToFileURL(file));
+    } else {
+      throw err;
+    }
+  }
+};
+
+exports.loadFilesAsync = async (files, preLoadFunc, postLoadFunc) => {
+  for (const file of files) {
+    preLoadFunc(file);
+    const result = await exports.requireOrImport(file);
+    postLoadFunc(file, result);
+  }
+};