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:          


        

Answer 1

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(x: T): T {
  return x;
}


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

type GenericReturnType = X extends Callable ? R : never;

type N = GenericReturnType; // number