Implementing your version of popular JavaScript functions is a fairly trivial interview question, so to prepare for it, I decided to implement all of them on my own as a part of the interview process. Array.map() is a starting point in the series. The next ones in the queue are Array.filter() and Array.reduce().

What is Array.map()?

To begin with, let’s first figure out what Array.map() is. Array.map() is a built-in JavaScript function for array manipulations and transformations based on the existing array. Array.map() iterates each element of the existing array and applies a callback function for each element. Array.map() doesn't mutate the original array. Instead, it returns a new one.

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1const arr = [1, 2, 3];
2
3const doubled = arr.map((elem) => elem * 2);
4
5console.log(doubled);
6// Output: [2, 4, 6]

Implementing your version of Array.map()

Now, let’s implement myMap() function, which will act as your Array.map() replacement.

The myMap() function takes an arr (array) and a cb (callback) function as arguments. It returns a new array that results from applying the callback function to each input array element.

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1const myMap = (arr, cb) => {
2  let temp = [];
3
4  for (let i = 0; i < arr.length; i++) {
5    if (arr.hasOwnProperty(i)) {
6      temp.push(cb(arr[i]));
7    }
8  }
9
10  return temp;
11};

Let’s quickly go through the code to figure out what it does.

We start by creating a new empty array temp

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1let temp = [];

Then we loop through each element of the input array arr

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1for (let i = 0; i < arr.length; i++) {
2  // loop code
3}

We do `hasOwnProperty`` check ensures that the callback is only executed on elements that actually exist in the array, and it won't be invoked for undefined elements or holes in the array.

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1if (arr.hasOwnProperty(i)) {
2  ...
3}

Inside the loop, it applies the callback function cb to the current element of the input array arr[i]. Finally, the result is pushed into a temporary array temp.

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1temp.push(cb(arr[i]));

After we finish with the loop, we return the temp array.

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1return temp;

That's it! Now you can use myMap() as an alternative replacement to the existing Array.map() function.

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1const arr = [1, 2, 3];
2
3const doubled = myMap(arr, (elem) => elem * 2);
4
5console.log(doubled);
6// Output: [2, 4, 6]

Appendix

• Array.map()