make-this-work.js

[1, 2, 3, 4, 5].duplicate();

// My initial attempt
Array.prototype.duplicate = function() {
  var arr = this.slice(0);

  arr.forEach(function(e) {
    arr.push(e);
  });

  return arr;
}

// Easy way
Array.prototype.duplicate = function() {
  return this.concat(this);
};