explicit-function-return-type.md

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:

• allowExpressions if true, only functions which are part of a declaration will be checked

By default, allowExpressions: false is used, meaning all declarations and expressions must have a return type.

allowExpressions

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

function test() {}

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

type FuncType = () => string;

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

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

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