An array is an ordered list of values. It can keep any number of values inside. These values can even be of any type — Numbers, Strings, Objects, even other Arrays.

let animals = ['cat', 'dog', 'horse'];
let randomThings = [2, 'book', 'today is Saturday', 10];
let numbers = [1, 2, 8, 19];

Challenge:

Create your own array, named favouriteFood, and write in a couple of things you like.

Array Length

We can easily check how many items we have in our array with a property by looking at the property .length.

let randomThings = [2, 'book', 'today is Saturday', 10];
console.log(randomThings.length); // We will get 4 because there are 4 things in the array.

Challenge:

Check how many values you have in your array favouriteFood and console.log the result.

Array Usage

It’s useful to keep all of these values in one place. But what if we want to use only 1 item from the array?

We can get them out using ‘bracket notation’. Thanks to a guy named Edsger Dijkstra, array indices start counting from 0. The usage looks like this.

let randomThings = [2, 'book', 'today is Saturday', 10];
let firstItem = randomThings[0];
let secondItem = randomThings[1]; // and so on

Challenge:

Get the third element from your array favouriteFood and console.log it.

Changing Values in Arrays

We also can replace values inside of the arrays by assigning a new value to a specific index.

let animals = ['cat', 'dog', 'horse'];

// let's replace 'dog' with 'fish'
animals[1] = 'fish';

// now our animals array will be ['cat', 'fish', 'horse']

Challenge:

Take your array favouriteFood and replace the first value with anything else. Console.log the whole array to see the change.

Array.push()

If you want to add new values to an existing array you can use the method .push(). Push will add a new value to the end of the array.

let animals = ['cat', 'dog', 'horse'];
animals.push('rabbit');

// animals will be ['cat', 'dog', 'horse','rabbit']

Challenge:

Let’s extend your list of favouriteFood and add one more value to it. Console.log the whole array to see your changes.

Constant Arrays

An array is what is known as a reference type. What this means is that even if an array is declared (created) using ‘const’, the values inside that array can still be changed; only the array itself cannot be overwritten.

const animals = ['cat', 'dog', 'horse'];

// These are both legal, and works the same way as when animals is a variable:
animals[1] = 'fish';
animals.push('rabbit');

// This is illegal, and will return an error the same as changing any constant:
animals = [ 'mouse', 'elephant' ];

Challenge:

Try creating an array as a constant and modifying the values in it.

Challenge:

See what happens if you add something with .push(), change something with bracket notation (array[1]) and assigning a new whole new array to the constant.