Typescript ReturnType of generic function

Question

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?

Answer 1

If you want to get some special generic type, You can use a fake function to wrap it.

const wrapperFoo = () => foo<number>()
type Return = ReturnType<typeof wrapperFoo>

More complex demo

function getList<T>(): {
  list: T[],
  add: (v: T) => void,
  remove: (v: T) => void,
  // ...blahblah
}
const wrapperGetList = () => getList<number>()
type List = ReturnType<typeof wrapperGetList>
// List = {list: number[], add: (v: number) => void, remove: (v: number) => void, ...blahblah}