TypeScript gives you the ability to provide more than one type signature to the same function.

type callback = (result: number) => void;

function asyncAdd(a: number, b: number): Promise<number>;
function asyncAdd(a: number, b: number, fn: callback): void;
function asyncAdd(a: number, b: number, fn?: callback) {
  const result = a + b;
  if (fn) return fn(result);
  else return Promise.resolve(result);
}

Your Mission

We're going to to make an add function that works with partial application. You can use this playground.

add(2, 2); // returns 4
add(2); // returns a function that accepts the second number

const addTwo = add(2);
addTwo(3); // returns 5

In case this is somewhat unfamiliar to you, this is what it might look like in JavaScript.

function add(a, b) {
  if (b === undefined) return (b) => a + b;
  return a + b;
}

(You can take peek at the solution here.)