prefer-module.md

Prefer JavaScript modules (ESM) over CommonJS

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Prefer using the JavaScript module format over the legacy CommonJS module format. Together with other changes, this helps the ecosystem migrate to a single, native module format.

1. Disallows 'use strict' directive.

   JavaScript modules use “Strict Mode” by default.

2. Disallows “Global Return”.

   This is a CommonJS-only feature.

3. Disallows the global variables __dirname and __filename.

   They are not available in JavaScript modules.

   Starting with Node.js 20.11, import.meta.dirname and import.meta.filename have been introduced in ES modules, providing identical functionality to __dirname and __filename in CommonJS (CJS).

   For most cases in Node.js 20.11 and later:

   const __dirname = import.meta.dirname;
   const __filename = import.meta.filename;

   Replacements for older versions:

   import {fileURLToPath} from 'node:url';
   import path from 'node:path';
   
   const __filename = fileURLToPath(import.meta.url);
   const __dirname = path.dirname(fileURLToPath(import.meta.url));

   However, in most cases, this is better:

   import {fileURLToPath} from 'node:url';
   
   const foo = fileURLToPath(new URL('foo.js', import.meta.url));

   And many Node.js APIs accept URL directly, so you can just do this:

   const foo = new URL('foo.js', import.meta.url);

4. Disallows require(…).

   require(…) can be replaced by import … or import(…).

5. Disallows exports and module.exports.

   export … should be used in JavaScript modules.

.cjs files are ignored.

Fail

'use strict';

// …

if (foo) {
	return;
}

// …

const file = path.join(__dirname, 'foo.js');

const content = fs.readFileSync(__filename, 'utf8');

const {fromPairs} = require('lodash');

module.exports = foo;

exports.foo = foo;

Pass

function run() {
	if (foo) {
		return;
	}

	// …
}

run();

const file = fileURLToPath(new URL('foo.js', import.meta.url));

import {fromPairs} from 'lodash-es';

export default foo;

export {foo};

Resources

• Get Ready For ESM by @sindresorhus