map

var arr = [23, 65, 98, 5];

Array.prototype.myMap = function(callback){
  var newArray = [];

  for (let i = 0, l = this.length; i < l; i++) {
    newArray.push( callback(this[i]) );
  }

  return newArray;
};

var new_arr = arr.myMap(function(item){
  return item * 2;
});

console.log(new_arr);