Add .mjs and .cjs as default extensions

lib/constants.js

@@ -29,6 +29,8 @@ const TYPESCRIPT_EXTENSION = [
 const DEFAULT_EXTENSION = [
 	'js',
 	'jsx',
+	'mjs',
+	'cjs',
 	...TYPESCRIPT_EXTENSION
 ];
 

readme.md

@@ -247,7 +247,7 @@ Use one or more [shareable configs](https://eslint.org/docs/developer-guide/shar
 
 Type: `string[]`
 
-Allow more extensions to be linted besides `.js` and `.jsx`. Make sure they're supported by ESLint or an ESLint plugin.
+Allow more extensions to be linted besides `.js`, `.jsx`, `.mjs`, and `.cjs`. Make sure they're supported by ESLint or an ESLint plugin.
 
 ### settings
 

test/cli-main.js

@@ -161,9 +161,9 @@ test('space option with boolean strings', async t => {
 
 test('extension option', async t => {
 	const cwd = path.join(__dirname, 'fixtures/custom-extension');
-	const {stdout} = await t.throwsAsync(main(['--reporter=json', '--extension=mjs'], {cwd}));
+	const {stdout} = await t.throwsAsync(main(['--reporter=json', '--extension=unknown'], {cwd}));
 	const reports = JSON.parse(stdout);
 
 	t.is(reports.length, 1);
-	t.true(reports[0].filePath.endsWith('.mjs'));
+	t.true(reports[0].filePath.endsWith('.unknown'));
 });