Basic JavaScript

Arrays, null, undefined

JavaScript and NodeJS Fundamentals

Arrays, null, undefined

Arrays and index access
Undefined
Null and how to treat it

In this section, we’ll cover three more types that are very common in JavaScript programs:

Arrays: sequences of values undefined: a special value that means "nothing was assigned" null: another special value that means "intentionally empty"

Arrays and index access

An array is a type that can hold multiple values in a list.

You create an array by using square brackets [] and separating the items with commas.

Here’s a basic example:

const numbers = [10, 2, 88]
console.log(numbers)

This prints:

[ 10, 2, 88 ]

You can store anything in an array, not just numbers:

const things = ["apple", 42, true]
console.log(things)

This prints:

[ 'apple', 42, true ]

To access a specific item in the array, we use an index. The index is the position of the item, starting from 0.

So in this array:

const colors = ["red", "green", "blue"]

colors[0] is "red" colors[1] is "green" colors[2] is "blue"

Let’s try:

const colors = ["red", "green", "blue"]
console.log(colors[0])
console.log(colors[1])
console.log(colors[2])

This will print:

red
green
blue

You can assign a value to a specific index of an array

const colors = ["red", "green", "blue"]

colors[1] = "yellow"

console.log(colors)

This will print:

[ 'red', 'yellow', 'blue' ]

You can use any natural number as an index, even one stored in a variable


const i = 1
const colors = ["red", "green", "blue"]
console.log(colors[i])

This will print:

green

But if you try to access an index that doesn’t exist, you will get undefined:

const colors = ["red", "green", "blue"]
console.log(colors[3])

This prints:

undefined

What's that??

undefined

The special value undefined means “no value was assigned”.

If you create a variable but don’t give it a value, it will be undefined:

const name
console.log(name)

This prints:

undefined

Because we didn’t assign anything to name, JavaScript sets it to undefined by default.

As seen before, you can also get undefined when you access an array index that doesn’t exist:

const fruits = ["banana", "apple"]
console.log(fruits[2]) // There is no index 2

This prints:

undefined

null and how to treat it

null is also a special value. It means “nothing is here, and I did that on purpose.”

Unlike undefined, which is automatic, null is something you set yourself.

For example:

const currentUser = null
console.log(currentUser)

This prints:

null

Why use null? Maybe you expect a value later, but it’s not ready yet:

let winner = null

// Later in the program:
winner = "Alice"

console.log(winner)

This prints:

Alice

So null is useful when you want to say, for example, “There should be something here later, but right now it’s empty.”