How to have functions pass arguments with the same overloads?

Issue

I am trying to create a function passes its arguments to another function. Both of these functions need to have the same overloads.

function original (a: number): boolean;
function original (a: string, b: string): boolean;
function original (a: number | string, b?: string): boolean {
  return true;
}

function pass (a: number): boolean;
function pass (a: string, b: string): boolean;
function pass (a: number | string, b?: string): boolean {
  return original(a, b);
}

This does not work.

Argument of type 'string | number' is not assignable to parameter of type 'string'.

Type 'number' is not assignable to type 'string'.(2345) input.tsx(4, 10): The call would have succeeded against this implementation, but implementation signatures of overloads are not externally visible.

Playground

Solution

You can just declare your pass function as being of the type of the original function using the typeof operator, e.g.

function original (a: number): boolean;
function original (a: string, b: string): boolean;
function original (): boolean {
  return true;
}

let pass: typeof original =  () => {
  return true;
};

pass(5);         // all good
pass('a', 'b');  // all good
pass(45, 'b');   // error here

Playground here

Answered By - dezfowler