Functional Programming - Implement map on a Prototype

JavaScript
#1

Im hoping for some clarification on what this challenge is asking me to do.

**THE TASK: Write your own Array.prototype.myMap(), which should behave exactly like Array.prototype.map(). You should not use the built-in map method. The Array instance can be accessed in the myMap method using this.

I think it is asking me to write a for loop that is equivalent to this line of code:

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

Is that correct?

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

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

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

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Challenge: Functional Programming - Implement map on a Prototype

Link to the challenge:

#2

They aren’t specifically asking for a for.
But they said

It is recommended you use a for loop or Array.prototype.forEach() .

However you can ignore this if you wish.

#3

So just use one of those 2 methodologies that is functionally identical to this specific line of code, correct?

Array.prototype.myMap = function(callback) {
  const newArray = [];
#4

No. That line is a partial function definition. You are trying to fill in the function definition so that it replicates the behavior of .map

#5

It wants you to loop over this inside the myMap function which is the array the method was called on (this in a method invocation).

Then pass in each array element to the callback function.

It might be helpful to not inline everything and add a few logs

function myMap(callback) {
  console.log('"this" is ->', this);
  console.log('"callback" is ->', callback.toString());
  this.forEach((item) => {
    console.log('"callback(item)" is ->', callback(item));
  });
}

Array.prototype.myMap = myMap;

function callbackFunction(item) {
  return item * 2;
}

const new_s = s.myMap(callbackFunction);
1 Like
#6

Thank you, this really clarified my confusion for me