Typescript void negation: requiring functions to not return void

Question

Typescript void negation: requiring functions to not return void

How can I ensure a function always returns a value in TypeScript?

Due to the fact that void is a subtype of any, I haven't been able to find any generics that successfully exclude void from any.

My current workaround looks like this:

type NotVoid = { [key: string]: NotVoid } | object | string | boolean | symbol | number | null | undefined

The solution above seems quite lengthy. Is there a more concise way to achieve this?

I am aware of a proposal for negation, but I need a solution today using TypeScript specifically, not just a linting rule. Thank you!

typescript

Answer 1

Answer №1

Consider using the following approach:

type NotVoid<T extends Function> = (() => undefined) extends T ? never : T;
const func = <T extends Function>(fn: NotVoid<T>) => fn(); 

func(() => 0)  // works fine
func(() => {}) // Error: Argument of type '() => void' is not compatible with parameter of type 'never'.

It's worth noting that this method may still result in errors when dealing with func(() => undefined) and func(() => null).

Answer 2

Consider using the following approach:

type NotVoid<T extends Function> = (() => undefined) extends T ? never : T;
const func = <T extends Function>(fn: NotVoid<T>) => fn(); 

func(() => 0)  // works fine
func(() => {}) // Error: Argument of type '() => void' is not compatible with parameter of type 'never'.

It's worth noting that this method may still result in errors when dealing with func(() => undefined) and func(() => null).

Typescript void negation: requiring functions to not return void

Answer №1

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