enable ES module mode automatically

by prepending --experimental-modules flag https://nodejs.org/api/esm.html#esm_enabling

Author: shinnn

.gitignore

@@ -1,2 +1,3 @@
 coverage
 node_modules
+tmp

README.md

@@ -27,9 +27,10 @@ All files |    95.15 |    79.52 |      100 |    95.09 |                   |
 ----------|----------|----------|----------|----------|-------------------|
 ```
 
-This is a wrapper of [c8](https://github.com/bcoe/c8), different in the following points:
+This is an opinionated wrapper of [c8](https://github.com/bcoe/c8), different in the following points:
 
 * Runs both [`html` and `text` reporter](https://github.com/istanbuljs/nyc#running-reports) by default
+* Automatically enables [ECMAScript module](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/import) support for `.mjs` files
 * The first argument can be a JavaScript file path instead of a command
 * Built-in [Codecov](https://codecov.io) support
 
@@ -49,13 +50,19 @@ Once this package is installed to the project directory, users can execute `cove
 
 <img alt="An example of the HTML report" src="screenshot.png" width="330px" align="right">
 
-Execute a JavaScript file with Node.js or run a command, print code coverage to the stdout and write [HTML reports](https://istanbul.js.org/docs/advanced/alternative-reporters/#html) under the `./coverage` directory.
+Execute the command, print code coverage to the stdout and write [HTML reports](https://istanbul.js.org/docs/advanced/alternative-reporters/#html) under the `./coverage` directory.
+
+`<file|command>` can be either a JavaScript file path or a command. If a path is provided, the file is run with the `node` command.
 
 ```console
-$ coverage /path/to/entry-point.js # ↑ is the same as ↓
+$ coverage /path/to/entry-point.js # is the same as ↓
 $ coverage node /path/to/entry-point.js
 ```
 
+If the provided JavaScript path ends with `.mjs`, [ECMAScript module mode](https://nodejs.org/api/esm.html) is automatically [enabled](https://nodejs.org/api/esm.html#esm_enabling) and [`--es-module-specifier-resolution`](https://nodejs.org/api/esm.html#esm_customizing_esm_specifier_resolution_algorithm) is set to `node`.
+
+#### Reporters
+
 Users can override the default format of reports with `--reporter` option.
 
 ```console
@@ -79,6 +86,8 @@ Lines        : 100% ( 1/1 )
 $ coverage --reporter=none example.js # No reports
 ```
 
+#### Codecov integration
+
 When the execution exits with code `0` on a CI service or [GitHub Actions](https://github.com/features/actions), it automatically uploads the generated coverage to [Codecov](https://docs.codecov.io/docs). Whether `CODECOV_TOKEN` environment variable is required or not varies between [services](https://github.com/codecov/codecov-bash#ci-providers).
 
 ```console

index.js

@@ -20,7 +20,7 @@ const c8PackageData = require(c8PackageJsonPath);
 normalizePackageData(c8PackageData);
 
 const c8BinPath = join(dirname(c8PackageJsonPath), c8PackageData.bin.c8);
-const [nodePath] = process.argv;
+const nodePath = process.execPath;
 const optionArgs = process.argv.slice(2);
 const {_: [command], reporter} = yargsParser(optionArgs);
 const promisifiedWhich = promisify(which);
@@ -65,7 +65,17 @@ const codecovBashPath = process.platform === 'win32' ? join(cwd, 'coverage', uui
 				process.exit(127);
 			}
 
-			optionArgs.splice(optionArgs.indexOf(command), 1, nodePath, entryPath);
+			optionArgs.splice(
+				optionArgs.indexOf(command),
+				1,
+				nodePath,
+				...ext === 'mjs' ? [
+					'--experimental-modules',
+					'--es-module-specifier-resolution=node',
+					'--no-warnings'
+				] : [],
+				entryPath
+			);
 		}
 
 		return [

package-lock.json

@@ -44,6 +44,7 @@
 		"@shinnn/eslint-config": "^6.10.2",
 		"eslint": "^5.16.0",
 		"lodash": "^4.17.11",
+		"rmfr": "^2.0.0",
 		"tape": "^4.10.1"
 	},
 	"eslintConfig": {

package.json

@@ -1,14 +1,16 @@
 'use strict';
 
+const {dirname, join} = require('path');
 const {execFile} = require('child_process');
-const {join} = require('path');
+const {mkdir, writeFile} = require('fs').promises;
 const {promisify} = require('util');
 
 const attempt = require('lodash/attempt');
+const rmfr = require('rmfr');
 const test = require('tape');
 
 const coverage = require.resolve('.');
-const execNode = promisify(execFile).bind(null, process.argv[0]);
+const execNode = promisify(execFile).bind(null, process.execPath);
 const timeout = 1000000;
 
 test('A `coverage` command with a command', async t => {
@@ -54,14 +56,14 @@ test('A `coverage` command with a file path', async t => {
 	t.end();
 });
 
-test('A `coverage` command with nyc flags', async t => {
+test('A `coverage` command with c8 flags', async t => {
 	try {
 		await execNode([coverage, '--reporter=unknown', 'node', '--version'], {timeout});
 		t.fail('Unexpectedly succeeded.');
 	} catch ({stderr}) {
 		t.ok(
 			stderr.includes('Cannot find module \'unknown\''),
-			'should pass flags to the underlying `nyc` command.'
+			'should pass flags to the underlying `c8` command.'
 		);
 	}
 
@@ -158,3 +160,22 @@ test('A `coverage report` command', async t => {
 
 	t.end();
 });
+
+if (Number(process.versions.node.split('.')[0]) >= 12) {
+	test('A `coverage` command with a .mjs file path', async t => {
+		const tmpFile = join(__dirname, 'tmp', 'tmp.mjs');
+
+		await rmfr(dirname(tmpFile));
+		await mkdir(dirname(tmpFile));
+		await writeFile(tmpFile, 'import {open} from "fs"');
+		await execNode([
+			coverage,
+			'--exclude=tmp.mjs',
+			`--temp-directory=${join(dirname(tmpFile), '_')}`,
+			tmpFile
+		], {timeout});
+		t.pass('should enable ECMAScript modules for .mjs files automatically.');
+
+		t.end();
+	});
+}