A TypeScript function that returns a boolean value is executed as a void function

It seems that in the given example, even if a function is defined to return void, a function returning a boolean still passes through the type check. Is this a bug or is there a legitimate reason for this behavior? Are there any workarounds available?

type ReturnsVoid = () => void

type ReturnsNumber = () => number

const a: ReturnsVoid = () => { }

// Surprisingly, there is no error
const b: ReturnsVoid = () => { return false; } 

// Error - as expected
const c: ReturnsNumber = () => { return false; }

// Error - as expected
const d: void = false;

Experiment in Playground

typescript

Answer №1

The behavior described is intentional and serves a specific purpose.

Another way to understand this concept is that when a void-returning callback type is used, it means "I will not pay attention to any return value you provide".

If it suits your needs, you can consider using a type that returns undefined instead:

type ReturnsUndefined = () => undefined

type ReturnsNumber = () => number

const a: ReturnsUndefined = () => undefined

const b: ReturnsUndefined = () => { return false; } // error

const c: ReturnsNumber = () => { return false; } // error

Check out this playground link for more examples.

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