One thing that threw me off when I started learning JavaScript (with a background in Java) was the fact that JavaScript has both undefined and null as possible values for a variable.
In Java, all we have is just null which means that the variable has no value.
So when I saw this additional value of undefined in JavaScript, I was confused… what did it mean?
Undefined Means “Nothing”
When a variable has the value of undefined it means two things:
- A variable has been declared
- The variable that has been declared has not yet been assigned ANY value whatsoever
An example of this in JavaScript would be as follows:
var person;
So as you can see, I've declared a variable, but I haven't assigned a value.
Now if I were to inspect that person variable to see what value it contained, you'd see that it says “undefined”.
var person;
alert("The person variable has the value: " + person);
Here's what the output of this code would be:
Now it's also interesting to note that when we inspect the type that is assigned to our person variable, we'll get the type undefined. This makes sense if you remember reading this article on the different data types in JavaScript.
Okay, fair enough then right?
undefined means that there's nothing assigned to our variable… so then, what the heck does null mean?
Null is a Place-holder for Nothing
In JavaScript null means that you've intentionally assigned a value to your variable, and therefore you're intentionally assigning the value of nothing to your variable.
You can think null like a place-holder, meaning that the variable has nothing inside of it, but you intend on assigning it a value later.
The interesting thing to note about assigning null to a variable is what type JavaScript gives to a variable that has been assigned the value of null. Let's take a look:
var person = null;
alert("The person variable has the data type: " + (typeof person));
Here's the result:
What!?
The type of our person variable is object?
Believe it or not, this is a bug in JavaScript that they've just sort of “leaned into” and stuck with over the years. It's part of the way the JavaScript engine works and if they changed it now, it would likely cause some side-effects to a lot of code out there on the interwebs.
The common justification for this bug is that when you're using the value of null for a variable, it's because you're using it as a place-holder for an object that will be assigned to this variable later.
Not the greatest justification, but that's just how the cookie crumbles in this case.
In Summary
The value of undefined is assigned to any variable that has not yet been assigned a value.
The value of null can be assigned by YOU if you like, and it represents a lack of a value (aka. nothing!)
Clear as mud?
Good!
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