add.js

function add(n) {
  sum = n;
  const proxy = new Proxy(function a () {}, {
    get () {
      return () => sum;
    },
    apply (receiver, ...args) {
      sum += args[1][0];
      return proxy;
    }
  });
  return proxy
}

Allows you to add numbers in a functional format, i.e. add(1)(2)(3). Can take unlimited numbers unlike a lot of hard-coded solutions. It also does not need the final empty invocation like many recursive solutions.

console.log(add(1)(2)(3)); // 6
console.log(add(1)(10)(12)(-5)(-13)(100)(1000)(-250)(9)(10)); // 874