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eslint/func-style Style

🚧 An auto-fix is planned for this rule, but not implemented at this time.

What it does

Enforce the consistent use of either function declarations or expressions assigned to variables.

Why is this bad?

This rule enforces a particular type of function style, either function declarations or expressions assigned to variables. You can specify which you prefer in the configuration.

Examples

// function declaration
function doSomething() {
  // ...
}

// arrow function expression assigned to a variable
const doSomethingElse = () => {
  // ...
};

// function expression assigned to a variable
const doSomethingAgain = function () {
  // ...
};

Examples of incorrect code for this rule with the default "expression" option:

/* func-style: ["error", "expression"] */

function foo() {
  // ...
}

Examples of incorrect code for this rule with the "declaration" option:

/* func-style: ["error", "declaration"] */
var foo = function () {
  // ...
};

var foo = () => {};

Examples of incorrect code for this rule with the "declaration" and {"overrides": { "namedExports": "expression" }} option:

/* func-style: ["error", "declaration", { "overrides": { "namedExports": "expression" } }] */
export function foo() {
  // ...
}

Examples of incorrect code for this rule with the "expression" and {"overrides": { "namedExports": "declaration" }} option:

/* func-style: ["error", "expression", { "overrides": { "namedExports": "declaration" } }] */
export var foo = function () {
  // ...
};

export var bar = () => {};

Examples of correct code for this rule with the default "expression" option:

/* func-style: ["error", "expression"] */
var foo = function () {
  // ...
};

Examples of correct code for this rule with the "declaration" option:

/* func-style: ["error", "declaration"] */
function foo() {
  // ...
}
// Methods (functions assigned to objects) are not checked by this rule
SomeObject.foo = function () {
  // ...
};

Examples of additional correct code for this rule with the "declaration", { "allowArrowFunctions": true } options:

/* func-style: ["error", "declaration", { "allowArrowFunctions": true }] */
var foo = () => {};

Examples of correct code for this rule with the "declaration" and {"overrides": { "namedExports": "expression" }} option:

/* func-style: ["error", "declaration", { "overrides": { "namedExports": "expression" } }] */
export var foo = function () {
  // ...
};
export var bar = () => {};

Examples of correct code for this rule with the "expression" and {"overrides": { "namedExports": "declaration" }} option:

/* func-style: ["error", "expression", { "overrides": { "namedExports": "declaration" } }] */
export function foo() {
  // ...
}

Examples of correct code for this rule with the {"overrides": { "namedExports": "ignore" }} option:

/* func-style: ["error", "expression", { "overrides": { "namedExports": "ignore" } }] */
export var foo = function () {
  // ...
};

export var bar = () => {};
export function baz() {
  // ...
}

Configuration

This rule accepts a configuration object with the following properties:

allowArrowFunctions

type: boolean

default: false

When true, arrow functions are allowed regardless of the style setting.

allowTypeAnnotation

type: boolean

default: false

When true, functions with type annotations are allowed regardless of the style setting.

namedExports

type: "ignore" | "expression" | "declaration"

default: null

Override the style specifically for named exports. Can be "expression", "declaration", or "ignore" (default).

style

type: "expression" | "declaration"

default: "expression"

The style to enforce. Either "expression" (default) or "declaration".

How to use

To enable this rule using the config file or in the CLI, you can use:

Config (.oxlintrc.json)

{
  "rules": {
    "func-style": "error"
  }
}

CLI

oxlint --deny func-style

References

• Rule Source