Duplicating an Array

duplicator.js

// Given an array, var arr = [1,2,3,4,5] how do you implement a function duplicator such that
// arr.duplicator() == [1,2,3,4,5,1,2,3,4,5]?

Array.prototype.duplicator = function() {
  return this.concat(this); 
};

var arr = [1,2,3,4,5], 
dupe = arr.duplicator(); 

console.log(dupe);​