Import-first loading of test files (#4635)

First try ESM, and only if that fails, load test file using require.
This enables an ESM loader to have first dibs at transforming the file.

docs/index.md

@@ -1065,7 +1065,6 @@ Require a module before loading the user interface or test files. This is useful
 
 - Test harnesses
 - Assertion libraries that augment built-ins or global scope (such as [should.js][npm-should.js])
-- Instant ECMAScript modules via [esm][npm-esm]
 - Compilers such as Babel via [@babel/register][npm-babel-register] or TypeScript via [ts-node][npm-ts-node] (using `--require ts-node/register`). See [Babel][example-babel] or [TypeScript][example-typescript] working examples.
 
 Modules required in this manner are expected to do work synchronously; Mocha won't wait for async tasks in a required module to finish.
@@ -2034,20 +2033,15 @@ this means either ending the file with a `.mjs` extension, or, if you want to us
 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.
-> (Mocha _will_ load ESM even in Node v10, but this is not officially supported. Use at your own risk.)
-
 ### 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
 - [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
+- When using module-level mocks via libs like `proxyquire`, `rewiremock` or `rewire`,
+  hold off on using ES modules for your test files. You can switch to using `testdouble`,
+  which does support ESM.
 
 ## Running Mocha in the Browser
 
@@ -2426,7 +2420,6 @@ or the [source](https://github.com/mochajs/mocha/blob/master/lib/mocha.js).
 [npm]: https://npmjs.org/
 [npm-babel-register]: https://npm.im/@babel/register
 [npm-chai-as-promised]: https://www.npmjs.com/package/chai-as-promised
-[npm-esm]: https://npm.im/esm
 [npm-glob]: https://www.npmjs.com/package/glob
 [npm-growl]: https://npm.im/growl
 [npm-mocha-lcov-reporter]: https://npm.im/mocha-lcov-reporter

lib/esm-utils.js

@@ -30,14 +30,56 @@ const formattedImport = async file => {
   return import(file);
 };
 
-exports.requireOrImport = async file => {
+const hasStableEsmImplementation = (() => {
+  const [major, minor] = process.version.split('.');
+  // ESM is stable from v12.22.0 onward
+  // https://nodejs.org/api/esm.html#esm_modules_ecmascript_modules
+  return parseInt(major.slice(1), 10) > 12 || parseInt(minor, 10) >= 22;
+})();
+
+exports.requireOrImport = hasStableEsmImplementation
+  ? async file => {
+      if (path.extname(file) === '.mjs') {
+        return formattedImport(file);
+      }
+      try {
+        return dealWithExports(await formattedImport(file));
+      } catch (err) {
+        if (
+          err.code === 'ERR_MODULE_NOT_FOUND' ||
+          err.code === 'ERR_UNKNOWN_FILE_EXTENSION'
+        ) {
+          return require(file);
+        } else {
+          throw err;
+        }
+      }
+    }
+  : implementationOfRequireOrImportForUnstableEsm;
+
+function dealWithExports(module) {
+  if (module.default) {
+    return module.default;
+  } else {
+    return {...module, default: undefined};
+  }
+}
+
+exports.loadFilesAsync = async (files, preLoadFunc, postLoadFunc) => {
+  for (const file of files) {
+    preLoadFunc(file);
+    const result = await exports.requireOrImport(path.resolve(file));
+    postLoadFunc(file, result);
+  }
+};
+
+/* istanbul ignore next */
+async function implementationOfRequireOrImportForUnstableEsm(file) {
   if (path.extname(file) === '.mjs') {
     return formattedImport(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.
+  // This is currently the only known way of figuring out whether a file is CJS or ESM in
+  // Node.js that doesn't necessitate calling `import` first.
   try {
     return require(file);
   } catch (err) {
@@ -47,12 +89,4 @@ exports.requireOrImport = async file => {
       throw err;
     }
   }
-};
-
-exports.loadFilesAsync = async (files, preLoadFunc, postLoadFunc) => {
-  for (const file of files) {
-    preLoadFunc(file);
-    const result = await exports.requireOrImport(path.resolve(file));
-    postLoadFunc(file, result);
-  }
-};
+}

test/integration/fixtures/exit.fixture.js

@@ -4,5 +4,5 @@ var net = require('net');
 
 it('should hang when --no-exit used', function (done) {
   var server = net.createServer();
-  server.listen(55555, done);
+  server.listen(55554, done);
 });