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Exploring Array Map in JavaScript

Mar 24, 2020 5 mins read

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Follow along with the Exploring JavaScript Array Methods series!

What is Array Map?

Array Map is a method that exists on the Array.prototype that was introduced in ECMAScript 5 (ES5) and is supported in all modern browsers.

Array Map allows us to loop our array, access each value and return a new value for each iteration - which in turn creates a new array.

Think of Array Map as: “I want a new array containing new copies, or changes, of each array element”

You could, for example, use Map to return a specific property from an object, which would result in an array of just those properties in the order your looped them.

Exploring JavaScript Array Methods cover

⚡️ FREE eBook: 🔥 ForEach, Map, Filter, Reduce, Some, Every, Find

Todd Motto “This book is straight to the point, syntax exploration, comprehensive guide, real-world examples, tips and tricks - it covers all you need Todd Motto, author of Exploring JavaScript Array Methods

Here’s the syntax for Array Map:

const returnValue = array.map((value, index, array) => {...}, thisArg);

Our returnValue will contain our new array of, potentially new, return values.


Array Map syntax deconstructed:

  • Map’s first argument is a callback function that exposes these parameters:
    • value (the current element)
    • index (the element’s index - sometimes used with Map)
    • array (the array we are looping - rarely used)
    • Inside the body of the function we need to return a value, this could be your array element, a modified version of it, or a completely new calculated value, this will then tell Map what to return after completing the loop
  • Map’s second argument thisArg allows the this context to be changed

See the ECMAScript Array Map specification!


In its simplest form, here is how Map behaves:

const mapped = [1, 2, 3, 4, 5].map((x) => x * 2);
// [2, 4, 6, 8, 10]
console.log(mapped);

I’m using x to identify whatever the value is and simply multiplying it by 2, giving us a new array of exactly each number doubled from the previous array. The original array would remain untouched and still accessible.

It’s common to deal with all kinds of data with Map, as Arrays allow any value type, from primitive values through to Objects - giving us great programming flexibility.

So that’s the basics of Map, let’s take a look at a more real-world scenario where we’ve tasked with mapping an Array of Objects.

Using Array Map

Here’s our data structure that we’ll be using Array Map with:

const items = [
  { id: '🍔', name: 'Super Burger', price: 399 },
  { id: '🍟', name: 'Jumbo Fries', price: 199 },
  { id: '🥤', name: 'Big Slurp', price: 299 }
];

Let’s assume we’ve just applied a coupon which applies HALF OFF our 'Jumbo Fries'. We’d need to loop through our data and update that specific object.

Here’s how we could solve that via Map by conditionally returning a new representation of the item object, with an updated price, otherwise we just return the item:

const halfOffFries = items.map(item => {
  if (item.id === '🍟') {
    return {
      ...item,
      price: item.price / 2
    };
  }
  return item;
});

// log the return value
console.log(halfOffFries);

Using Array Map is an immutable pattern as it creates a new array from an existing array. We are also using the ...spread operator to return a new object instead of mutating the existing one. Both operations do not mutate existing data structures and are considered immutable ways of achieving state change.

This would then give us some half price fries (which can only be good news):

[
  { id: '🍔', name: 'Super Burger', price: 399 },
  { id: '🍟', name: 'Jumbo Fries', price: 99.5 },
  { id: '🥤', name: 'Big Slurp', price: 299 }
]

Interestingly, our original items array remains unmodified, and we have a new collection to deal with now in our halfOffFries variable. This practice is called an immutable operation as we don’t mutate the initial array.

Give the live demo a try:

Bonus: Map-ing without Map

Let’s check out a for…in loop example that mimics the behaviour of Array Map:

const halfOffFries = [];

for (let i = 0 ; i < items.length; i++) {
  const item = items[i];
  if (item.id === '🍟') {
    halfOffFries.push({
      ...item,
      price: item.price / 2
    });
  } else {
    halfOffFries.push(item);
  }
}

First we declare halfOffFries as an empty array. Inside the loop we use pretty much the same logic, but instead of a return statement we use the Array.prototype.push method which adds each item to the new halfOffFries array.

Once the loop as finished, you’re free to work with your new halfOffFries array.

This also demonstrates us the power and flexibility of using Map and other array prototype methods. The code is far smaller, promotes better practices, is easier to read and far more contained.

Summary

You’ve now learned how to use Array Map to map your array to a new set of values.

Map is the next best place to begin after getting started with array ForEach. Moving from a traditional for...in loop, the Map method can be introduced to bring a more functional approach and style to your programming.

If you are serious about your JavaScript skills, your next step is to take a look at my JavaScript courses, they will teach you the full language, the DOM, the advanced stuff and much more!

Exploring JavaScript Array Methods cover

⚡️ FREE eBook: 🔥 ForEach, Map, Filter, Reduce, Some, Every, Find

Todd Motto “This book is straight to the point, syntax exploration, comprehensive guide, real-world examples, tips and tricks - it covers all you need Todd Motto, author of Exploring JavaScript Array Methods

Further tips and tricks:

  • Use Map to create a new collection with changed values of your initial collection
  • Don’t forget to return or your values will be undefined
  • Map will shallow copy your object references into the new array
  • Don’t encourage bad habits by using .map() over .forEach() just because it can have the same effect and is ‘shorter’ - use the right tool for the right job or you will confuse people!
  • You can access the array you’re looping in the third argument of the callback
  • You can change the this context via a second argument to .map(callback, thisArg) so that any references to this inside your callback point to your object
  • You can use arrow functions with Map but remember that this will be incorrect if you also supply a thisArg due to arrow functions not having a this context
  • Like ForEach and friends, you cannot Map in reverse or break a Map, use for...in or for...of
  • Using Map will skip empty array slots
  • You shouldn’t need to in this day and age of evergreen browsers, but use a polyfill for older browsers if necessary

Thanks for reading, happy Mapping!

Go to the next article in Exploring JavaScript Array Methods - Array Filter!