Add support for Node.JS native ES modules (#4038)

.eslintrc.yml

@@ -31,7 +31,14 @@ overrides:
       ecmaVersion: 2017
     env:
       browser: false
-
+  - files:
+      - esm-utils.js
+    parserOptions:
+      ecmaVersion: 2018
+      sourceType: module
+    parser: babel-eslint
+    env:
+      browser: false
   - 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)
@@ -70,6 +71,7 @@ Mocha is a feature-rich JavaScript test framework running on [Node.js][] and in
 - [Command-Line Usage](#command-line-usage)
 - [Interfaces](#interfaces)
 - [Reporters](#reporters)
+- [Node.JS native ESM support](#nodejs-native-esm-support)
 - [Running Mocha in the Browser](#running-mocha-in-the-browser)
 - [Desktop Notification Support](#desktop-notification-support)
 - [Configuring Mocha (Node.js)](#configuring-mocha-nodejs)
@@ -354,11 +356,11 @@ With its default "BDD"-style interface, Mocha provides the hooks `before()`, `af
 ```js
 describe('hooks', function() {
   before(function() {
-    // runs before all tests in this block
+    // runs once before the first test in this block
   });
 
   after(function() {
-    // runs after all tests in this block
+    // runs once after the last test in this block
   });
 
   beforeEach(function() {
@@ -868,7 +870,8 @@ Configuration
   --package  Path to package.json for config                            [string]
 
 File Handling
-  --extension          File extension(s) to load           [array] [default: js]
+  --extension          File extension(s) to load
+                                           [array] [default: ["js","cjs","mjs"]]
   --file               Specify file(s) to be loaded prior to root suite
                        execution                       [array] [default: (none)]
   --ignore, --exclude  Ignore file(s) or glob pattern(s)
@@ -1538,6 +1541,42 @@ Alias: `HTML`, `html`
 
 **The HTML reporter is not intended for use on the command-line.**
 
+## Node.JS native ESM support
+
+> _New in v7.1.0_
+
+Mocha supports writing your tests as ES modules, 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. To enable this in versions 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.
+
+### Current Limitations
+
+Node.JS native ESM support still has status: **Stability: 1 - Experimental**
+
+- [Watch mode](#-watch-w) does not support ES Module test files
+- [Custom reporters](#third-party-reporters) and [custom interfaces](#interfaces)
+  can only be CommonJS files
+- [Required modules](#-require-module-r-module) can only be CommonJS files
+- [Configuration file](#configuring-mocha-nodejs) can only be a CommonJS file (`mocharc.js` or `mocharc.cjs`)
+- When using module-level mocks via libs like `proxyquire`, `rewiremock` or `rewire`, hold off on using ES modules for your test files
+- Node.JS native ESM support does not work with [esm][npm-esm] module
+
 ## 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 +1648,17 @@ 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 +1740,8 @@ 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/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

@@ -15,8 +15,6 @@ const collectFiles = require('./collect-files');
 
 const cwd = (exports.cwd = process.cwd());
 
-exports.watchRun = watchRun;
-
 /**
  * Exits Mocha when tests + code under test has finished execution (default)
  * @param {number} code - Exit code; typically # of failures
@@ -92,19 +90,21 @@ exports.handleRequires = (requires = []) => {
 };
 
 /**
- * Collect test files and run mocha instance.
+ * Collect and load test files, then run mocha instance.
  * @param {Mocha} mocha - Mocha instance
  * @param {Options} [opts] - Command line options
  * @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 singleRun = async (mocha, {exit}, fileCollectParams) => {
   const files = collectFiles(fileCollectParams);
   debug('running tests with files', files);
   mocha.files = files;
+
+  await mocha.loadFilesAsync();
   return mocha.run(exit ? exitMocha : exitMochaLater);
 };
 
@@ -113,8 +113,9 @@ exports.singleRun = (mocha, {exit}, fileCollectParams) => {
  * @param {Mocha} mocha - Mocha instance
  * @param {Object} opts - Command line options
  * @private
+ * @returns {Promise}
  */
-exports.runMocha = (mocha, options) => {
+exports.runMocha = async (mocha, options) => {
   const {
     watch = false,
     extension = [],
@@ -140,7 +141,7 @@ exports.runMocha = (mocha, options) => {
   if (watch) {
     watchRun(mocha, {watchFiles, watchIgnore}, fileCollectParams);
   } else {
-    exports.singleRun(mocha, {exit}, fileCollectParams);
+    await singleRun(mocha, {exit}, fileCollectParams);
   }
 };
 

lib/cli/run.js

@@ -87,7 +87,6 @@ exports.builder = yargs =>
       },
       extension: {
         default: defaults.extension,
-        defaultDescription: 'js',
         description: 'File extension(s) to load',
         group: GROUPS.FILES,
         requiresArg: true,
@@ -299,8 +298,14 @@ 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('\n' + (err.stack || `Error: ${err.message || err}`));
+    process.exit(1);
+  }
 };

lib/esm-utils.js

@@ -0,0 +1,31 @@
+const url = require('url');
+const path = require('path');
+
+const requireOrImport = async file => {
+  file = path.resolve(file);
+
+  if (path.extname(file) === '.mjs') {
+    return import(url.pathToFileURL(file));
+  }
+  // This 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 requireOrImport(file);
+    postLoadFunc(file, result);
+  }
+};

lib/mocha.js

@@ -14,6 +14,7 @@ var utils = require('./utils');
 var mocharc = require('./mocharc.json');
 var errors = require('./errors');
 var Suite = require('./suite');
+var esmUtils = utils.supportsEsModules() ? require('./esm-utils') : undefined;
 var createStatsCollector = require('./stats-collector');
 var createInvalidReporterError = errors.createInvalidReporterError;
 var createInvalidInterfaceError = errors.createInvalidInterfaceError;
@@ -290,16 +291,18 @@ Mocha.prototype.ui = function(ui) {
 };
 
 /**
- * Loads `files` prior to execution.
+ * Loads `files` prior to execution. Does not support ES Modules.
  *
  * @description
  * The implementation relies on Node's `require` to execute
  * the test interface functions and will be subject to its cache.
+ * Supports only CommonJS modules. To load ES modules, use Mocha#loadFilesAsync.
  *
  * @private
  * @see {@link Mocha#addFile}
  * @see {@link Mocha#run}
  * @see {@link Mocha#unloadFiles}
+ * @see {@link Mocha#loadFilesAsync}
  * @param {Function} [fn] - Callback invoked upon completion.
  */
 Mocha.prototype.loadFiles = function(fn) {
@@ -314,6 +317,49 @@ Mocha.prototype.loadFiles = function(fn) {
   fn && fn();
 };
 
+/**
+ * Loads `files` prior to execution. Supports Node ES Modules.
+ *
+ * @description
+ * The implementation relies on Node's `require` and `import` to execute
+ * the test interface functions and will be subject to its cache.
+ * Supports both CJS and ESM modules.
+ *
+ * @public
+ * @see {@link Mocha#addFile}
+ * @see {@link Mocha#run}
+ * @see {@link Mocha#unloadFiles}
+ * @returns {Promise}
+ * @example
+ *
+ * // loads ESM (and CJS) test files asynchronously, then runs root suite
+ * mocha.loadFilesAsync()
+ *   .then(() => mocha.run(failures => process.exitCode = failures ? 1 : 0))
+ *   .catch(() => process.exitCode = 1);
+ */
+Mocha.prototype.loadFilesAsync = function() {
+  var self = this;
+  var suite = this.suite;
+  this.loadAsync = true;
+
+  if (!esmUtils) {
+    return new Promise(function(resolve) {
+      self.loadFiles(resolve);
+    });
+  }
+
+  return esmUtils.loadFilesAsync(
+    this.files,
+    function(file) {
+      suite.emit(EVENT_FILE_PRE_REQUIRE, global, file, self);
+    },
+    function(file, resultModule) {
+      suite.emit(EVENT_FILE_REQUIRE, resultModule, file, self);
+      suite.emit(EVENT_FILE_POST_REQUIRE, global, file, self);
+    }
+  );
+};
+
 /**
  * Removes a previously loaded file from Node's `require` cache.
  *
@@ -330,8 +376,9 @@ Mocha.unloadFile = function(file) {
  * Unloads `files` from Node's `require` cache.
  *
  * @description
- * This allows files to be "freshly" reloaded, providing the ability
+ * This allows required files to be "freshly" reloaded, providing the ability
  * to reuse a Mocha instance programmatically.
+ * Note: does not clear ESM module files from the cache
  *
  * <strong>Intended for consumers &mdash; not used internally</strong>
  *
@@ -842,10 +889,14 @@ Object.defineProperty(Mocha.prototype, 'version', {
  * @see {@link Mocha#unloadFiles}
  * @see {@link Runner#run}
  * @param {DoneCB} [fn] - Callback invoked when test execution completed.
- * @return {Runner} runner instance
+ * @returns {Runner} runner instance
+ * @example
+ *
+ * // exit with non-zero status if there were test failures
+ * mocha.run(failures => process.exitCode = failures ? 1 : 0);
  */
 Mocha.prototype.run = function(fn) {
-  if (this.files.length) {
+  if (this.files.length && !this.loadAsync) {
     this.loadFiles();
   }
   var suite = this.suite;

lib/mocharc.json

@@ -1,6 +1,6 @@
 {
   "diff": true,
-  "extension": ["js"],
+  "extension": ["js", "cjs", "mjs"],
   "opts": "./test/mocha.opts",
   "package": "./package.json",
   "reporter": "spec",