typing typescript functions

functions.ts

function add(a: number, b: number): number { return a + b };

const add = (a: number, b: number): number => a + b;

type Arithmetics = (a: number, b: number) => number;
// or
/*
interface  Arithmetics {
  (a: number, b: number): number;
}
*/

const addition: Arithmetics = add;
add(1, 2); // 3

const multiplication: Arithmetics = (a, b) => a * b;