Add map function to HTMLCollection

htmlcollection-map.js

HTMLCollection.prototype.map = function(f) {
  var result = [];
  for (var i = 0; i < this.length; i = i + 1) {
    el = elements[i];
    result.push(f(el));
  };
  return result;
}

-     el = elements[i];
+     el = this[i];