Function Overloads in TypeScript

sum.ts

function sum(a: number, b: number): number;
function sum(a: string, b: string): string;
function sum(a: number | string, b: number | string): number | string {
  if (typeof a === "number" && typeof b === "number") {
    return a + b;
  } else {
    return `${parseInt(`${a}`, 10) + parseInt(`${b}`, 10)}`;
  }
}

const sumAsNumber = sum(1000, 337);
const sumAsString = sum("1000", "337");

console.log(sumAsNumber, typeof sumAsNumber);
console.log(sumAsString, typeof sumAsString);