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JavaScript Asynchronous Programming with JavaScript Understanding Promises Handle Multiple Promises with Promise.all

Why use map() when you could just use forEach()?

Guil uses map() in the generateHTML function, but map returns a new array. Why doesn't he just use forEach() which simply iterates over each item in the array? Am I missing something?

3 Answers

Steven Parker
Steven Parker
229,005 Points

If you used "forEach", you'd need to first create a new empty array, and then inside the body of the forEach you would push the current item onto the new array.

Using "map" saves you a couple of steps since it's designed to return a new array.

Yeah but in the video he didnt need a new array. He just needed to iterate over each object in the array and generate html for it and that's it.

Steven Parker
Steven Parker
229,005 Points

You didn't give a time index, but I hunted down the part you are referring to — and yes, a "forEach" should work just fine there.

For anyone else here, I believe .forEach() to be a better implementation than .map() (with respect to Guil), since he is appending the values to the DOM within the callback.

  • .forEach() is best to perform a task with each item of the array in turn (as per this exercise)
  • .map() is best to return a new array where each index contains a value which is some modification of the value at the same index of the original array.

Quick example:

const array = [1, 2, 3, 4, 5];
const doubledArray = array.map(num => num * 2); // [2, 4, 6, 8, 10];
let total = 0;
doubledArray.forEach(num => total += num);
console.log(total); // 30;
// Note: this would be better achieved with '.reduce()', but that's for another time ;)

In my opinion, a more appropriate use of .map() for this lesson would be something like:

function generateHTML(data) {
  const sectionsHTML = data.map(person => `
    <div class="section">
      <img src=${person.thumbnail.source}/>
    </div>`) // Now each person object has been 'mapped' over with a HTML block distributing its data
      .join(''); // Remove commas between each 
  peopleList.innerHTML = sectionsHTML; // Only one write to the DOM :)

I'm not saying this is the best way of doing it, but I understand this to be a better usage of .map().

My thoughts were the same. The purpose was to iterate over each array and 'do something... period', instead of iterate over each array to return a new modified array.