mapImplementationOnPrototype.js

// *** Functional Programming: Implement map on a Prototype ***

// As you have seen from applying Array.prototype.map(), or simply map() earlier, the map 
// method returns an array of the same length as the one it was called on. It also doesn't 
// alter the original array, as long as its callback function doesn't.

// In other words, map is a pure function, and its output depends solely on its inputs. 
// Plus, it takes another function as its argument.

// It would teach us a lot about map to try to implement a version of it that behaves 
// exactly like the Array.prototype.map() with a for loop or Array.prototype.forEach().

// Note: A pure function is allowed to alter local variables defined within its scope, 
// although, it's preferable to avoid that as well.


// Write your own Array.prototype.myMap(), which should behave exactly like 
// Array.prototype.map(). You may use a for loop or the forEach method.


// the global Array
var s = [23, 65, 98, 5];

Array.prototype.myMap = function(callback){
  var newArray = [];
  // Add your code below this line
  // this refers to the caller of the function

  // Using a for loop
  // for(var i =0; i < this.length; i++){
  //   newArray.push(callback(this[i])); 
  // }

  this.forEach(num => {
    newArray.push(callback(num));
  })
  // Add your code above this line
  return newArray;

};

var new_s = s.myMap(function(item){
  return item * 2;
});