Obtaining the return type of a function

Question

I have the following function:

function test(): number {
    return 42;
}

I can obtain the type of the function by using typeof

Answer 1

Edit: This is not needed with TS 2.8 any more! ReturnType gives the return type. See accepted answer.

---

A variant on some of the previous answers that I'm using, which works in strictNullChecks and hides the inference gymnastics a bit:

function getReturnType(fn: (...args: any[]) => R): R {
  return {} as R;
}

Usage:

function foo() {
  return {
    name: "",
    bar(s: string) { // doesn't have to be shorthand, could be `bar: barFn` 
      return 123;
    }
  }
}

const _fooReturnType = getReturnType(foo);
export type Foo = typeof _fooReturnType; // type Foo = { name: string; bar(s: string): number; }

It does call the getReturnType function, but it does not call the original function. You could prevent the getReturnType call using (false as true) && getReturnType(foo) but IMO this just makes it more confusing.

I just used this method with some regexp find/replace to migrate an old Angular 1.x project that had ~1500 factory functions written like this, originally in JS, and added the Foo etc types to all uses... amazing the broken code one will find. :)