Require explicit return types on functions and class methods (explicit-function-return-type)

Explicit types for function return values makes it clear to any calling code what type is returned. This ensures that the return value is assigned to a variable of the correct type; or in the case where there is no return value, that the calling code doesn‘t try to use the undefined value when it shouldn’t.

Rule Details

This rule aims to ensure that the values returned from functions are of the expected type.

The following patterns are considered warnings:

// 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;
  }
}

The following patterns are not warnings:

// 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;
  }
}

Options

The rule accepts an options object with the following properties:

type Options = {
  // if true, only functions which are part of a declaration will be checked
  allowExpressions?: boolean;
  // if true, type annotations are also allowed on the variable of a function expression rather than on the function directly
  allowTypedFunctionExpressions?: boolean;
  // if true, functions immediately returning another function expression will not be checked
  allowHigherOrderFunctions?: boolean;
  // if true, arrow functions immediately returning a `as const` value will not be checked
  allowDirectConstAssertionInArrowFunctions?: boolean;
  // if true, concise arrow functions that start with the void keyword will not be checked
  allowConciseArrowFunctionExpressionsStartingWithVoid?: boolean;
};

const defaults = {
  allowExpressions: false,
  allowTypedFunctionExpressions: true,
  allowHigherOrderFunctions: true,
  allowDirectConstAssertionInArrowFunctions: true,
  allowConciseArrowFunctionExpressionsStartingWithVoid: true,
};

Configuring in a mixed JS/TS codebase

If you are working on a codebase within which you lint non-TypeScript code (i.e. .js/.jsx), you should ensure that you should use ESLint overrides to only enable the rule on .ts/.tsx files. If you don't, then you will get unfixable lint errors reported within .js/.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", "*.tsx"],
      "rules": {
        "@typescript-eslint/explicit-function-return-type": ["error"]
      }
    }
  ]
}

allowExpressions

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

function test() {}

const fn = () => {};

export default () => {};

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

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

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

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

allowTypedFunctionExpressions

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

let arrowFn = () => 'test';

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

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

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

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

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

allowHigherOrderFunctions

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

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

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

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

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

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

allowDirectConstAssertionInArrowFunctions

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

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

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

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

allowConciseArrowFunctionExpressionsStartingWithVoid

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

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

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

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

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

When Not To Use It

If you don't wish to prevent calling code from using function return values in unexpected ways, then you will not need this rule.

Further Reading

• TypeScript Functions