[SOLVED] Typescript ReturnType of generic function

Issue

The new ReturnType in TypeScript 2.8 is a really useful feature that lets you extract the return type of a particular function.

function foo(e: number): number {
    return e;
}

type fooReturn = ReturnType<typeof foo>; // number

However, I’m having trouble using it in the context of generic functions.

function foo<T>(e: T): T {
    return e;
}

type fooReturn = ReturnType<typeof foo>; // type fooReturn = {}

type fooReturn = ReturnType<typeof foo<number>>; // syntax error

type fooReturn = ReturnType<(typeof foo)<number>>; // syntax error

Is there a way extract the return type that a generic function would have given particular type parameters?

Solution

TypeScript compiler does not see typeof foo as generic type. I’d say it’s a bug in the compiler.

However, TypeScript has callable interfaces which can be generic without any problems, so if you introduce a callable interface compatible with the signature of your function, you can implement your own equivalent of ReturnType like this:

function foo<T>(x: T): T {
  return x;
}


interface Callable<R> {
  (...args: any[]): R;
}

type GenericReturnType<R, X> = X extends Callable<R> ? R : never;

type N = GenericReturnType<number, typeof foo>; // number

Answered By – artem

Answer Checked By – Mary Flores (BugsFixing Volunteer)