Support --require of ESM; closes #4281

Allow files/modules specified in `--require` to be ESM.

CommonJS loading is still supported and the default.

lib/cli/run-helpers.js

@@ -15,6 +15,7 @@ const collectFiles = require('./collect-files');
 const {type} = require('../utils');
 const {format} = require('util');
 const {createInvalidPluginError, createUnsupportedError} = require('../errors');
+const {requireOrImport} = require('../esm-utils');
 
 /**
  * Exits Mocha when tests + code under test has finished execution (default)
@@ -81,15 +82,16 @@ exports.list = str =>
  * @returns {Promise<MochaRootHookObject|MochaRootHookFunction>} Any root hooks
  * @private
  */
-exports.handleRequires = async (requires = []) =>
-  requires.reduce((acc, mod) => {
+exports.handleRequires = async (requires = []) => {
+  const acc = [];
+  for (const mod of requires) {
     let modpath = mod;
     // this is relative to cwd
     if (fs.existsSync(mod) || fs.existsSync(`${mod}.js`)) {
       modpath = path.resolve(mod);
       debug('resolved required file %s to %s', mod, modpath);
     }
-    const requiredModule = require(modpath);
+    const requiredModule = await requireOrImport(modpath);
     if (type(requiredModule) === 'object' && requiredModule.mochaHooks) {
       const mochaHooksType = type(requiredModule.mochaHooks);
       if (/function$/.test(mochaHooksType) || mochaHooksType === 'object') {
@@ -102,8 +104,9 @@ exports.handleRequires = async (requires = []) =>
       }
     }
     debug('loaded required module "%s"', mod);
-    return acc;
-  }, []);
+  }
+  return acc;
+};
 
 /**
  * Loads root hooks as exported via `mochaHooks` from required files.

lib/esm-utils.js

@@ -1,11 +1,8 @@
-const url = require('url');
 const path = require('path');
 
-const requireOrImport = async file => {
-  file = path.resolve(file);
-
+exports.requireOrImport = async file => {
   if (path.extname(file) === '.mjs') {
-    return import(url.pathToFileURL(file));
+    return import(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.
@@ -15,7 +12,7 @@ const requireOrImport = async file => {
     return require(file);
   } catch (err) {
     if (err.code === 'ERR_REQUIRE_ESM') {
-      return import(url.pathToFileURL(file));
+      return import(file);
     } else {
       throw err;
     }
@@ -25,7 +22,7 @@ const requireOrImport = async file => {
 exports.loadFilesAsync = async (files, preLoadFunc, postLoadFunc) => {
   for (const file of files) {
     preLoadFunc(file);
-    const result = await requireOrImport(file);
+    const result = await exports.requireOrImport(path.resolve(file));
     postLoadFunc(file, result);
   }
 };