explicit-function-return-type

Require explicit return types on functions and class methods.

💭

This rule requires type information to run.

Functions in TypeScript often don't need to be given an explicit return type annotation. Leaving off the return type is less code to read or write and allows the compiler to infer it from the contents of the function.

However, explicit return types do make it visually more clear what type is returned by a function. They can also speed up TypeScript type checking performance in large codebases with many large functions.

This rule enforces that functions do have an explicit return type annotation.

.eslintrc.cjs

module.exports = {
  "rules": {
    "@typescript-eslint/explicit-function-return-type": "error"
  }
};

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Examples

❌ Incorrect

// Should indicate that no value is returned (void)
function test() {
  return;
}

// Should indicate that a number is returned
var fn = function () {
  return 1;
};

// Should indicate that a string is returned
var arrowFn = () => 'test';

class Test {
  // Should indicate that no value is returned (void)
  method() {
    return;
  }
}

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✅ Correct

// No return value should be expected (void)
function test(): void {
  return;
}

// A return value of type number
var fn = function (): number {
  return 1;
};

// A return value of type string
var arrowFn = (): string => 'test';

class Test {
  // No return value should be expected (void)
  method(): void {
    return;
  }
}

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Options

This rule accepts the following options:

type Options = [
  {
    /** Whether to allow arrow functions that start with the `void` keyword. */
    allowConciseArrowFunctionExpressionsStartingWithVoid?: boolean;
    /** Whether to ignore arrow functions immediately returning a `as const` value. */
    allowDirectConstAssertionInArrowFunctions?: boolean;
    /** Whether to ignore function expressions (functions which are not part of a declaration). */
    allowExpressions?: boolean;
    /** Whether to ignore functions that don't have generic type parameters. */
    allowFunctionsWithoutTypeParameters?: boolean;
    /** Whether to ignore functions immediately returning another function expression. */
    allowHigherOrderFunctions?: boolean;
    /** Whether to ignore immediately invoked function expressions (IIFEs). */
    allowIIFEs?: boolean;
    /** Whether to ignore type annotations on the variable of function expressions. */
    allowTypedFunctionExpressions?: boolean;
    /** An array of function/method names that will not have their arguments or return values checked. */
    allowedNames?: string[];
  },
];

const defaultOptions: Options = [
  {
    allowExpressions: false,
    allowTypedFunctionExpressions: true,
    allowHigherOrderFunctions: true,
    allowDirectConstAssertionInArrowFunctions: true,
    allowConciseArrowFunctionExpressionsStartingWithVoid: false,
    allowFunctionsWithoutTypeParameters: false,
    allowedNames: [],
    allowIIFEs: false,
  },
];

Configuring in a mixed JS/TS codebase

If you are working on a codebase within which you lint non-TypeScript code (i.e. .js/.mjs/.cjs/.jsx), you should ensure that you should use ESLint overrides to only enable the rule on .ts/.mts/.cts/.tsx files. If you don't, then you will get unfixable lint errors reported within .js/.mjs/.cjs/.jsx files.

{
  "rules": {
    // disable the rule for all files
    "@typescript-eslint/explicit-function-return-type": "off",
  },
  "overrides": [
    {
      // enable the rule specifically for TypeScript files
      "files": ["*.ts", "*.mts", "*.cts", "*.tsx"],
      "rules": {
        "@typescript-eslint/explicit-function-return-type": "error",
      },
    },
  ],
}

allowExpressions

Examples of code for this rule with { allowExpressions: true }:

❌ Incorrect

function test() {}

const fn = () => {};

export default () => {};

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✅ Correct

node.addEventListener('click', () => {});

node.addEventListener('click', function () {});

const foo = arr.map(i => i * i);

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allowTypedFunctionExpressions

Examples of code for this rule with { allowTypedFunctionExpressions: true }:

❌ Incorrect

let arrowFn = () => 'test';

let funcExpr = function () {
  return 'test';
};

let objectProp = {
  foo: () => 1,
};

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✅ Correct

type FuncType = () => string;

let arrowFn: FuncType = () => 'test';

let funcExpr: FuncType = function () {
  return 'test';
};

let asTyped = (() => '') as () => string;
let castTyped = <() => string>(() => '');

interface ObjectType {
  foo(): number;
}
let objectProp: ObjectType = {
  foo: () => 1,
};
let objectPropAs = {
  foo: () => 1,
} as ObjectType;
let objectPropCast = <ObjectType>{
  foo: () => 1,
};

declare function functionWithArg(arg: () => number);
functionWithArg(() => 1);

declare function functionWithObjectArg(arg: { method: () => number });
functionWithObjectArg({
  method() {
    return 1;
  },
});

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allowHigherOrderFunctions

Examples of code for this rule with { allowHigherOrderFunctions: true }:

❌ Incorrect

var arrowFn = () => () => {};

function fn() {
  return function () {};
}

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✅ Correct

var arrowFn = () => (): void => {};

function fn() {
  return function (): void {};
}

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allowDirectConstAssertionInArrowFunctions

Examples of code for this rule with { allowDirectConstAssertionInArrowFunctions: true }:

❌ Incorrect

const func = (value: number) => ({ type: 'X', value }) as any;
const func = (value: number) => ({ type: 'X', value }) as Action;

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✅ Correct

const func = (value: number) => ({ foo: 'bar', value }) as const;
const func = () => x as const;

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allowConciseArrowFunctionExpressionsStartingWithVoid

Examples of code for this rule with { allowConciseArrowFunctionExpressionsStartingWithVoid: true }:

❌ Incorrect

var join = (a: string, b: string) => `${a}${b}`;

const log = (message: string) => {
  console.log(message);
};

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✅ Correct

var log = (message: string) => void console.log(message);

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allowFunctionsWithoutTypeParameters

Examples of code for this rule with { allowFunctionsWithoutTypeParameters: true }:

❌ Incorrect

function foo<T>(t: T) {
  return t;
}

const bar = <T>(t: T) => t;

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✅ Correct

function foo<T>(t: T): T {
  return t;
}

const bar = <T>(t: T): T => t;

function allowedFunction(x: string) {
  return x;
}

const allowedArrow = (x: string) => x;

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allowedNames

You may pass function/method names you would like this rule to ignore, like so:

{
  "@typescript-eslint/explicit-function-return-type": [
    "error",
    {
      "allowedNames": ["ignoredFunctionName", "ignoredMethodName"]
    }
  ]
}

allowIIFEs

Examples of code for this rule with { allowIIFEs: true }:

❌ Incorrect

var func = () => 'foo';

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✅ Correct

var foo = (() => 'foo')();

var bar = (function () {
  return 'bar';
})();

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When Not To Use It

If you don't find the added cost of explicitly writing function return types to be worth the visual clarity, or your project is not large enough for it to be a factor in type checking performance, then you will not need this rule.

Further Reading

• TypeScript Functions

Resources

• Rule source
• Test source