explicit-function-return-type.md

explicit-function-return-type

Require explicit return types on functions and class methods.

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.

❌ 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;
  }
}

✅ 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;
  }
}

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;
  /**
   * An array of function/method names that will not have their arguments or their return values checked.
   */
  allowedNames?: string[];
};

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

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 () => {};

✅ Correct

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

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

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

allowTypedFunctionExpressions

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

❌ Incorrect

let arrowFn = () => 'test';

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

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

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

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

allowHigherOrderFunctions

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

❌ Incorrect

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

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

✅ Correct

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

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

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);

✅ Correct

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

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);
};

✅ Correct

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

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"]
    }
  ]
}

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

Attributes

• Configs:
  • ✅ Recommended
  • 🔒 Strict
• 🔧 Fixable
• 💭 Requires type information