Duplicate Array without Assignment

duplicate2.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]?
// Duplicate array elements by modifying array instead of assigning the returned array to a new var.

Array.prototype.duplicator = function(obj) {
    this.map(duplicate);
};

function duplicate(element, index, array){                    
    return array.push(element);
}

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

console.log(arr);​