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Add consistent-function-style rule (#3137)
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‎docs/rules/consistent-function-style.md‎

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# consistent-function-style
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📝 Enforce function syntax by role.
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🚫 This rule is _disabled_ in the following [configs](https://github.com/sindresorhus/eslint-plugin-unicorn#recommended-config): ✅ `recommended`, ☑️ `unopinionated`.
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💡 This rule is manually fixable by [editor suggestions](https://eslint.org/docs/latest/use/core-concepts#rule-suggestions).
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<!-- end auto-generated rule header -->
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<!-- Do not manually modify this header. Run: `npm run fix:eslint-docs` -->
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Enforce function syntax by role.
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By default, this rule reports nothing. Configure the roles you want to enforce.
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This rule does not autofix. Function declarations, function expressions, arrow functions, and object methods are not always semantic equivalents, so `--fix` would be too risky for a style rule.
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The rule offers editor suggestions only for simple callback conversions between anonymous function expressions and arrow functions. Suggestions are skipped when the callback contains comments, `this`, `arguments`, `super`, `new.target`, direct `eval()`, generators, named function expressions, or TypeScript type parameters.
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## Examples
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```js
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// eslint unicorn/consistent-function-style: ["error", {namedFunctions: "declaration"}]
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// ❌
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const parse = value => value;
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// ✅
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function parse(value) {
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return value;
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}
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```
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```js
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// ✅ Reassigned variables are ignored by default.
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let transform = value => value;
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if (debug) {
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transform = wrap(transform);
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}
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```
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```js
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// ✅ Callbacks are ignored by default.
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items.map(item => item.id);
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```
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## Options
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Type: `object`
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Default:
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```js
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{
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default: 'ignore',
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namedFunctions: 'ignore',
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namedExports: 'ignore',
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callbacks: 'ignore',
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objectProperties: 'ignore',
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reassignedVariables: 'ignore',
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typedVariables: 'ignore',
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}
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```
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Each option chooses the expected style for a role.
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When multiple roles apply, the most specific role wins. Roles are checked in this order:
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```js
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typedVariables
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reassignedVariables
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namedExports
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callbacks
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objectProperties
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namedFunctions
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default
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```
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### `default`
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Allowed values: `'declaration'`, `'function-expression'`, `'arrow-function'`, `'ignore'`
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Fallback style for functions that do not match a more specific role.
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### `namedFunctions`
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Allowed values: `'declaration'`, `'function-expression'`, `'arrow-function'`, `'ignore'`
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Style for function declarations and named variable functions.
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### `namedExports`
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Allowed values: `'declaration'`, `'function-expression'`, `'arrow-function'`, `'ignore'`
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Style for inline named function exports, such as `export function foo() {}` and `export const foo = () => {}`. Export specifier lists like `export {foo}` are ignored.
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### `callbacks`
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Allowed values: `'function-expression'`, `'arrow-function'`, `'ignore'`
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Style for inline functions passed as call or constructor arguments.
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### `objectProperties`
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Allowed values: `'method'`, `'function-expression'`, `'arrow-function'`, `'ignore'`
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Style for object literal function properties.
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### `reassignedVariables`
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Allowed values: `'function-expression'`, `'arrow-function'`, `'ignore'`
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Style for variable functions whose binding is reassigned after initialization.
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### `typedVariables`
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Allowed values: `'function-expression'`, `'arrow-function'`, `'ignore'`
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Style for TypeScript variable functions with an explicit type annotation.
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## Limitations
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The rule ignores anonymous default exports, export specifier lists, IIFEs, accessors, class methods, class fields, destructuring defaults, TypeScript overload declarations, and generators when the expected style is `'arrow-function'`. Most reports do not have suggestions, because the safe rewrite surface is intentionally narrow.

‎docs/rules/no-subtraction-comparison.md‎

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## Caveats
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The rewrite is only behavior-preserving for numbers, so it is auto-fixed only when both operands are known to be numbers. For example, `"10" - "5" > 0` is `true`, but `"10" > "5"` is `false`, since strings compare lexicographically. When the operands are not provably numbers, the rule offers a suggestion instead of an auto-fix.
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The rewrite is only behavior-preserving for numbers. For example, `"10" - "5" > 0` is `true`, but `"10" > "5"` is `false`, since strings compare lexicographically. The rule auto-fixes strict ordering comparisons (`>` and `<`) when both operands are known to be numbers. For non-strict ordering and equality comparisons, it auto-fixes only when both operands are statically known finite numbers. Otherwise, the rule offers a suggestion instead of an auto-fix.
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There is also a numeric edge case the rule does not special-case: when both operands are the same infinity, `Infinity - Infinity` is `NaN`, so `Infinity - Infinity >= 0` is `false` while `Infinity >= Infinity` is `true`.
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The extra restriction for non-strict ordering and equality comparisons avoids the infinity edge case: `Infinity - Infinity` is `NaN`, so `Infinity - Infinity >= 0` is `false` while `Infinity >= Infinity` is `true`.

‎readme.md‎

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| [consistent-empty-array-spread](docs/rules/consistent-empty-array-spread.md) | Prefer consistent types when spreading a ternary in an array literal. | ✅ | 🔧 | |
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| [consistent-existence-index-check](docs/rules/consistent-existence-index-check.md) | Enforce consistent style for element existence checks with `indexOf()`, `lastIndexOf()`, `findIndex()`, and `findLastIndex()`. | ✅ ☑️ | 🔧 | |
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| [consistent-function-scoping](docs/rules/consistent-function-scoping.md) | Move function definitions to the highest possible scope. | ✅ | | |
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| [consistent-function-style](docs/rules/consistent-function-style.md) | Enforce function syntax by role. | | | 💡 |
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| [consistent-json-file-read](docs/rules/consistent-json-file-read.md) | Enforce consistent JSON file reads before `JSON.parse()`. | ✅ | 🔧 | |
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| [consistent-template-literal-escape](docs/rules/consistent-template-literal-escape.md) | Enforce consistent style for escaping `${` in template literals. | ✅ | 🔧 | |
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| [custom-error-definition](docs/rules/custom-error-definition.md) | Enforce correct `Error` subclassing. | | 🔧 | |

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