closure.js

function add(x, y) {
  return x + y;
}

function mul(x, y) {
  return x * y;
}

function make(arg) {
  var arr = [];
  arr.push(arg);

  return function f(x) {
    if (typeof x === 'number') {
      arr.push(x);
      return f;
    }
    return exec(x);
  }

  function exec(x) {
    return arr.reduce(x);
  }
}

var s = make(1)(2)(3)(4)(5);
console.log(s(add));
console.log(s(mul));