Code for add(1)(2)...(n)()

A simpler code for simpler problem
Well written article about currying

function add(x) {
  return function (y) {
    if (typeof y !== "undefined") {
      x = x + y;
      return arguments.callee;
    } else {
      return x;
    }
  };
}

Alternative to arguments.callee

function add(x) {
  return function (y) {
    if (typeof y !== "undefined") {
      x = x + y;
      return add(x);
    } else {
      return x;
    }
  };
}

Alternative Syntax

const add = (x) => (y) => typeof y === "undefined" ? x : add(x + y);