CLI: Add native support for ESM .mjs files on Node 12+

.eslintrc.json

@@ -52,7 +52,7 @@
             ],
             "parserOptions": {
                 "sourceType": "script",
-                "ecmaVersion": 2018
+                "ecmaVersion": 2020
             },
             "env": {
                 "node": true

Gruntfile.js

@@ -190,8 +190,8 @@ module.exports = function( grunt ) {
 				// "test/module-only.js",
 				// "test/module-skip.js",
 				// "test/module-todo.js",
-				"test/es2017/async-functions.js",
-				"test/es2017/throws.js"
+				"test/es2018/async-functions.js",
+				"test/es2018/throws.js"
 			]
 		},
 		"watch-repeatable": {

src/cli/run.js

@@ -12,7 +12,7 @@ const changedPendingPurge = [];
 
 let QUnit;
 
-function run( args, options ) {
+async function run( args, options ) {
 
 	// Default to non-zero exit code to avoid false positives
 	process.exitCode = 1;
@@ -48,8 +48,42 @@ function run( args, options ) {
 		const filePath = path.resolve( process.cwd(), files[ i ] );
 		delete require.cache[ filePath ];
 
+		// Node.js 12.0.0 has node_module_version=72
+		// https://nodejs.org/en/download/releases/
+		const nodeVint = process.config.variables.node_module_version;
+
 		try {
-			require( filePath );
+
+			// QUnit supports passing ESM files to the 'qunit' command when used on
+			// Node.js 12 or later. The dynamic import() keyword supports both CommonJS files
+			// (.js, .cjs) and ESM files (.mjs), so we could simply use that unconditionally on
+			// newer Node versions, regardless of the given file path.
+			//
+			// But:
+			// - Node.js 12 emits a confusing "ExperimentalWarning" when using import(),
+			//   even if just to load a non-ESM file. So we should try to avoid it on non-ESM.
+			// - This Node.js feature is still considered experimental so to avoid unexpected
+			//   breakage we should continue using require(). Consider flipping once stable and/or
+			//   as part of QUnit 3.0.
+			// - Plugins and CLI bootstrap scripts may be hooking into require.extensions to modify
+			//   or transform code as it gets loaded. For compatibility with that, we should
+			//   support that until at least QUnit 3.0.
+			// - File extensions are not sufficient to differentiate between CJS and ESM.
+			//   Use of ".mjs" is optional, as a package may configure Node to default to ESM
+			//   and optionally use ".cjs" for CJS files.
+			//
+			// https://nodejs.org/docs/v12.7.0/api/modules.html#modules_addenda_the_mjs_extension
+			// https://nodejs.org/docs/v12.7.0/api/esm.html#esm_code_import_code_expressions
+			// https://github.com/qunitjs/qunit/issues/1465
+			try {
+				require( filePath );
+			} catch ( e ) {
+				if ( e.code === "ERR_REQUIRE_ESM" && ( !nodeVint || nodeVint >= 72 ) ) {
+					await import( filePath );
+				} else {
+					throw e;
+				}
+			}
 		} catch ( e ) {
 
 			// eslint-disable-next-line no-loop-func

src/cli/utils.js

@@ -27,6 +27,8 @@ function getFilesFromArgs( args ) {
 
 	// Default to files in the test directory
 	if ( !patterns.length ) {
+
+		// TODO: In QUnit 3.0, change the default to {js,mjs}
 		patterns.push( "test/**/*.js" );
 	}
 
@@ -45,6 +47,8 @@ function getFilesFromArgs( args ) {
 			files.add( pattern );
 		} else {
 			if ( stat && stat.isDirectory() ) {
+
+				// TODO: In QUnit 3.0, change the default to {js,mjs}
 				pattern = `${pattern}/**/*.js`;
 			}
 			const results = glob( pattern, {

test/cli/fixtures/expected/tap-outputs.js

@@ -203,6 +203,15 @@ not ok 2 Example > bad
 # fail 1
 `,
 
+	"qunit ../../es2018/esm.mjs":
+`TAP version 13
+ok 1 ESM test suite > sum\\(\\)
+1..1
+# pass 1
+# skip 0
+# todo 0
+# fail 0`,
+
 	"qunit timeout":
 `TAP version 13
 not ok 1 timeout > first

test/cli/main.js

@@ -135,6 +135,28 @@ QUnit.module( "CLI Main", function() {
 		}
 	} );
 
+	if ( semver.gte( process.versions.node, "12.0.0" ) ) {
+		QUnit.test( "run ESM test suite with import statement", async function( assert ) {
+			const command = "qunit ../../es2018/esm.mjs";
+			const execution = await execute( command );
+
+			assert.equal( execution.code, 0 );
+
+			// Node 12 enabled ESM by default, without experimental flag,
+			// but left the warning in stderr. The warning was removed in Node 14.
+			// Don't bother checking stderr
+			if ( semver.gte( process.versions.node, "14.0.0" ) ) {
+				assert.equal( execution.stderr, "" );
+			}
+
+			const re = new RegExp( expectedOutput[ command ] );
+			assert.equal( re.test( execution.stdout ), true );
+			if ( !re.test( execution.stdout ) ) {
+				assert.equal( execution.stdout, expectedOutput[ command ] );
+			}
+		} );
+	}
+
 	// https://nodejs.org/dist/v12.12.0/docs/api/cli.html#cli_enable_source_maps
 	if ( semver.gte( process.versions.node, "14.0.0" ) ) {
 

test/es2018/.eslintrc.json

@@ -0,0 +1,6 @@
+{
+		"parserOptions": {
+				"ecmaVersion": 2018,
+				"sourceType": "module"
+		}
+}

test/es2018/esm.mjs

@@ -0,0 +1,9 @@
+import sum from "./sum.mjs";
+
+QUnit.module( "ESM test suite", () => {
+
+	QUnit.test( "sum()", assert => {
+		assert.equal( 5, sum( 2, 3 ) );
+	} );
+
+} );

test/es2018/sum.mjs

@@ -0,0 +1,5 @@
+function sum( a, b ) {
+	return a + b;
+}
+
+export default sum;