Javascript Array join(separator) method

Array.prototype.join = function(separator) {
  var s = new String();
  for (var i = 0, len = this.length; i < len; i++) {
    s += (i > 0) ? separator : "";
    s += (typeof this[i] == "object" && this[i].constructor === Array) ? this[i].join(separator) : this[i];
  }// w  w  w .  ja v a2  s . c  om
  
  return s;
};