# eslint-plugin-prefer-arrow-functions

> An ESLint Plugin to Lint and auto-fix plain Functions into Arrow Functions, in all cases where conversion would result
> in the same behaviour (Arrow Functions do not support `this`, `arguments`, or `new.target` for example).

## Installation

```bash
npm install --save-dev eslint eslint-plugin-prefer-arrow-functions
```

## Configuration

### Flat

For ESLint 9 and above, use the shared config `all` in `eslint.config.mjs`:

```js
import pluginJs from '@eslint/js';
import preferArrowFunctions from 'eslint-plugin-prefer-arrow-functions';

export default [
  pluginJs.configs.all,
  preferArrowFunctions.configs.all,
];
```

Or configure the rule(s) on your own:

```js
import pluginJs from '@eslint/js';
import preferArrowFunctions from 'eslint-plugin-prefer-arrow-functions';

export default [
  pluginJs.configs.all,
  {
    plugins: {
      'prefer-arrow-functions': preferArrowFunctions
    },
    rules: {
      'prefer-arrow-functions/prefer-arrow-functions': [
        'warn',
        {
          allowedNames: [],
          allowNamedFunctions: false,
          allowObjectProperties: false,
          classPropertiesAllowed: false,
          disallowPrototype: false,
          returnStyle: 'unchanged',
          singleReturnOnly: false,
        },
      ],
    },
  },
];
```

### Legacy

For ESLint 8 and below.

Add the plugin to the `plugins` section and the rule to the `rules` section in your `.eslintrc`. The default values for
options are listed in this example.

```json
{
  "plugins": ["prefer-arrow-functions"],
  "rules": {
    "prefer-arrow-functions/prefer-arrow-functions": [
      "warn",
      {
        "allowedNames": [],
        "allowNamedFunctions": false,
        "allowObjectProperties": false,
        "classPropertiesAllowed": false,
        "disallowPrototype": false,
        "returnStyle": "unchanged",
        "singleReturnOnly": false
      }
    ]
  }
}
```

## Options

### `allowedNames`

An optional array of function names to ignore. When set, the rule won't report named functions such as
`function foo() {}` whose name is identical to a member of this array.

### `allowNamedFunctions`

Controls how named functions are handled:

- When `true`, the rule won't report any named functions such as `function foo() {}`. Anonymous functions such as
  `const foo = function() {}` will still be reported.
- When `"only-expressions"`, the rule will allow named function expressions (like `callback(function namedFn() {})`) but
  will still transform named function declarations (like `function foo() {}`).
- When `false` (default), all functions will be transformed to arrow functions when safe to do so.

### `allowObjectProperties`

If set to true, the rule won't report named methods such as

```js
const myObj = {
  hello() {},
};
```

### `classPropertiesAllowed`

When `true`, functions defined as
[class instance fields](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Classes#Field_declarations)
will be converted to arrow functions when doing so would not alter or break their behaviour.

### `disallowPrototype`

When `true`, functions assigned to a `prototype` will be converted to arrow functions when doing so would not alter or
break their behaviour.

### `returnStyle`

- When `"implicit"`, arrow functions such as `x => { return x; }` will be converted to `x => x`.
- When `"explicit"`, arrow functions such as `x => x` will be converted to `x => { return x; }`.
- When `"unchanged"` or not set, arrow functions will be left as they were.

### `singleReturnOnly`

When `true`, only `function` declarations which _only_ contain a return statement will be converted. Functions
containing block statements will be ignored.

> This option works well in conjunction with ESLint's built-in
> [arrow-body-style](http://eslint.org/docs/rules/arrow-body-style) set to `as-needed`.

## Credits

This project is a fork of <https://github.com/TristonJ/eslint-plugin-prefer-arrow> by
[Triston Jones](https://github.com/TristonJ).

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