Array challange

challange.js

Array.prototype.tail = function() {
  return Array.prototype.concat.apply(this, this);
}

Array.prototype.dublicate = function() {
  var dublicated = [],
      length = this.length;
  
  for (var i=0; i<length; i++) {
    dublicated.push(this[i], this[i]);
  }

  return dublicated;
}