Arrays in JavaScript

What are Arrays?

Array in JavaScript is a type that is used to store multiple items or a list of items where the order matters. Keep in mind that array is of typeof object 🤷🏽‍♂️

Every item in the array has an index. The index is the position of the element in the array. Arrays have a 0 based index. The first element in the array has the index 0, the second element in the array has the index 1 and so on.

The total number of items in the array is the length of the array.

How can you create an array?

// Using the square bracket notation
const carArray = ["Honda", "BMW", "Ferrari", 27, true];

// Using new keyword
const bikeArray = new Array("Honda", "Ducati", "Yamaha");

🚨 Warning: It is not recommended to use the new keyword to create an array. See below.

How to know if a variable is an array?

Since the typeof array is object, when you do the following:

const carArray = ["Honda", "BMW", "Ferrari", 27, true];
console.log(typeof(carArray)); // output will be object!

instead you need to do

const carArray = ["Honda", "BMW", "Ferrari", 27, true];
console.log(Array.isArray(carArray)); // true

We can also do

const carArray = ["Honda", "BMW", "Ferrari", 27, true];
carArray instanceof Array; // true

How do I access the properties of an array when they don't have keys?

Use the index!

const carArray = ["Honda", "BMW", "Ferrari", 27, true];
console.log(carArray[2]); // Ferrari

Keep in mind that even though Ferrari is the 3rd element in the array, it is referred to by index 2 because arrays are 0 based index.

Changing the elements in an array

You can set the values in an array the same way as you retrieve the values. Both actions use an index.

const carArray = ["Honda", "BMW", "Ferrari", 27, true];
carArray[2] = "Mazda";
console.log(carArray); // [ 'Honda', 'BMW', 'Mazda', 27, true ] 

How many items are in an array?

Use the length property!

const carArray = ["Honda", "BMW", "Ferrari", 27, true];
console.log(carArray.length); // 5

And if you need to access the last element of an array, you can do:

const carArray = ["Honda", "BMW", "Ferrari", 27];
console.log(carArray[carArray.length - 1]); // 27

Common array methods

We will discuss a ton of methods in the next few blog posts as we go in-depth with arrays. For now, there are two common methods I would like to discuss:

1. Push: This allows you to add an element at the end of the array

   const carArray = ["Honda", "BMW", "Ferrari", 27, true];
   carArray.push("Mazda");
   console.log(carArray); // [ 'Honda', 'BMW', 'Ferrari', 27, true, 'Mazda' ] 

1. Sort: This sorts the array in place. It is a mutable method. It will change the original array!

   const carArray = ["Honda", "BMW", "Ferrari", 27, true];
   carArray.sort();
   console.log(x); // [ 27, 'BMW', 'Ferrari', 'Honda', true ] 

Why not to use the new keyword when creating an array?

The square bracket [] notation and the new keyword do the same thing by creating an empty array or create an array with the passed value.

The new keyword however, has unexpected results at times.

let score = new Array(5, 10);
console.log(score); // [ 5, 10 ]

score = new Array(5);
console.log(score); // [ , , , ,  ]

The first example creates an array with items 5 and 10. The second example however, creates an array with 5 undefined elements instead of an array with element 5!