When something doesn't exist

In JavaScript, we have two ways to say that something doesn't exist. These are the keywords null and undefined. These keywords are a nightmare for some, but if you master them, you'll be fine.

Both means that there is no value there, and if you try to see if they are equal... Well, here's an example:

undefined == null

>> true

So that's fine, they both represent "nothing". But are they of the same data type?

undefined === null

>> false

They are not. We can use the unary operator `typeof` to find out what they really are.

typeof undefined

>> "undefined"

typeof null

>> "object"

undefined means a variable has been declared but has not yet been assigned a value. null is an assignment value, and can be assigned to a variable. More often than not they can be used interchangeable. If you try to print the value of a variable that doesn't have a value, you will get undefined in return. You can also declare a variable with the value of undefined or null if you need to assign it a value later. This can often lead to errors though, i.e. if you think it has a value and it doesn't, so use it with care.

If you need to use a variable that at some point have been undefined or null, it might be a good idea to check for it. Check for it how? Well, that's a perfect segue to the next section!