# Prefer JavaScript modules (ESM) over CommonJS

💼 This rule is enabled in the ✅ `recommended` [config](https://github.com/sindresorhus/eslint-plugin-unicorn#preset-configs).

🔧💡 This rule is automatically fixable by the [`--fix` CLI option](https://eslint.org/docs/latest/user-guide/command-line-interface#--fix) and manually fixable by [editor suggestions](https://eslint.org/docs/latest/use/core-concepts#rule-suggestions).

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Prefer using the [JavaScript module](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Modules) 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.

1. Disallows “Global Return”.

	This is a CommonJS-only feature.

1. Disallows the global variables `__dirname` and `__filename`.

	They are [not available in JavaScript modules](https://nodejs.org/api/esm.html#esm_no_filename_or_dirname).

	Starting with Node.js 20.11, [`import.meta.dirname`](https://nodejs.org/api/esm.html#importmetadirname) and [`import.meta.filename`](https://nodejs.org/api/esm.html#importmetafilename) 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:

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

	Replacements for older versions:

	```js
	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:

	```js
	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:

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

1. Disallows `require(…)`.

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

1. Disallows `exports` and `module.exports`.

	`export …` should be used in JavaScript modules.

*`.cjs` files are ignored.*

## Fail

```js
'use strict';

// …
```

```js
if (foo) {
	return;
}

// …
```

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

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

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

```js
module.exports = foo;
```

```js
exports.foo = foo;
```

## Pass

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

	// …
}

run();
```

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

```js
import {fromPairs} from 'lodash-es';
```

```js
export default foo;
```

```js
export {foo};
```

## Resources

- [Get Ready For ESM](https://gist.github.com/sindresorhus/a39789f98801d908bbc7ff3ecc99d99c) by @sindresorhus