Help me Understand: Implement map on a Prototype

Tell us what’s happening:
Okay so I’m trying to make sure that I understand what is happening with these functions and not just accepting that it does work. For this reason I was very pleased to see this lesson that breaks down the

I want to check my understanding of the order of events in the code:

  • The loop goes through the first time and i = 0
  • That brings 23 down into the function and it looks like this: newArray.push(callback(23));
  • callback refers to the word function down below in s.myMap(function(item)and 23 becomes the item that gets multiplied
  • Finally the result of 23 * 2 is pushed to the newArry

I think I still struggle to understand what is happening with this section of the code:

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

Your code so far

// The global variable
var s = [23, 65, 98, 5];

Array.prototype.myMap = function(callback){
var newArray = [];
// Only change code below this line
for (let i = 0; i < this.length; i++) {
// Only change code above this line
return newArray;


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

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Challenge: Implement map on a Prototype

Link to the challenge:


But you explained it here:

Maybe if you paste the code here to see how it works may help :slight_smile:.

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here the myMap method is being executed on the s array, with the callback this time being function(item){return item * 2;}, and storing the result in new_s

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Thank you so much for that link. That will be very useful!

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