Javascript Array arraySum()

a = [1,2,3,4] // => 10

Array.prototype.arraySum = function () {
  if (this.length <= 1) {
    return this[0];
  }/*  www  .  jav  a  2 s .  c  om*/

  return this.slice(0,-1).arraySum() + this.slice(-1)[0]
};

console.log(a.arraySum());