add.d.ts

export declare function add(a: number, b: number): number;
export declare function add(a: number, b: number, c: number): number;
export declare function add(a: number, b: number, c: number, d: number): number;

add.js

function add() {
	switch (arguments.length) {
		case 2:
			return arguments[0] + arguments[1];
		case 3:
			return arguments[0] + arguments[1] + arguments[2];
		case 4:
			return arguments[0] + arguments[1] + arguments[2] + arguments[3];
		default:
			throw new Error("Too few or too many arguments. Number of arguments: " + arguments.length);
	}
}
module.exports = { add };

adding.ts

import { add } from "./add";

console.log("4 + 4 =", add(4,4));
console.log("4 + 4 + 4 =", add(4,4,4));
console.log("4 + 4 + 4 + 4 =", add(4,4,4,4));