Why is there a `null` value in JavaScript?

The question isn't really "why is there a null value in JS" - there is a null value of some sort in most languages and it is generally considered very useful.

The question is, "why is there an undefined value in JS". Major places where it is used:

1. when you declare var x; but don't assign to it, x holds undefined;
2. when your function gets fewer arguments than it declares;
3. when you access a non-existent object property.

null would certainly have worked just as well for (1) and (2)*. (3) should really throw an exception straight away, and the fact that it doesn't, instead of returning this weird undefined that will fail later, is a big source of debugging difficulty.

*: you could also argue that (2) should throw an exception, but then you'd have to provide a better, more explicit mechanism for default/variable arguments.

However JavaScript didn't originally have exceptions, or any way to ask an object if it had a member under a certain name - the only way was (and sometimes still is) to access the member and see what you get. Given that null already had a purpose and you might well want to set a member to it, a different out-of-band value was required. So we have undefined, it's problematic as you point out, and it's another great JavaScript 'feature' we'll never be able to get rid of.

I actually use undefined when I want to unset the values of properties no longer in use but which I don't want to delete. Should I use null instead?

Yes. Keep undefined as a special value for signaling when other languages might throw an exception instead.

null is generally better, except on some IE DOM interfaces where setting something to null can give you an error. Often in this case setting to the empty string tends to work.

Best described here, but in summary:

undefined is the lack of a type and value, and null is the lack of a value.

Furthermore, if you're doing simple '==' comparisons, you're right, they come out the same. But try ===, which compares both type and value, and you'll notice the difference.

It is entirely possible to need both. For instance if you query WMI it is entirely possible to have a class return properties that have a null value. They are defined, they just happen to hold null at the time.

I think that your conclusion that JavaScript defines undefined as "there is no such property" and null as "the property has no value" is perfectly correct. And in a language as dynamic as JavaScript it is a very important distinction. The use of duck typing means that we need to be able to differentiate between a property not existing and not having a value. It is our primary means of deriving type information. in a statically typed language there is a definite distinction between a field being null and a field not existing. In JavaScript this is no different. However it is checked at runtime, and can be modified up until that time.

I'm going to have to agree that the implementation is strange as a lot of time the distinction is blurred. However I think that in JavaScript the distinction is important. And being able to assign undefined is essential.

I remember reading a blog post a while ago about an online RPG written in JavaScript. It used examples where objects were created as copies of existing instances rather than prototypes (classes, functions, whatever), and were then altered. This really made me understand how powerful that undefined could be when modifying existing objects, but I cannot remember who wrote it.

Semantically they mean different things. The type null has exactly one value in its domain, null and a property may be assigned this specific value. Undefined represents a distinct lack of any value having been assigned.

As a Java programmer, I see a huge difference between undefined and null. Coding JavaScript, not so much, because JavaScript is not strongly typed, and the differences between undefined and null are blurred by the automatic conversions that are frequently performed by run-time. BTW, I take frequent advantage of those conversions; they make my JS code more compact and readable.

To answer your question, undefined means that a value was never set. Practically speaking, this generally points to a bug. If yourObject.property is undefined, that means you didn't set the property for some reason, or I'm looking for something that doesn't exist at all. This is a real issue when working on a project with more than one coder.

null means that "no value" was explicitly set. Practically speaking, you are telling me something about the property, perhaps that it isn't used in this context, or that a value has yet to be determined.

In Java, attempts to access a field that is undefined will always result in an exception. In fact, the compiler can be made to warn you about this in your code.

Try this example:

<html>
<head>
    <script type="text/javascript">
        function ShowObjProperties(obj) {
            var property, propCollection = "";

            for(property in obj) {
                propCollection += (property + ": " + obj[property] + "\n");
            }

            alert(propCollection);
        }

        var obj = {
            userid: 3,
            name: 'me!',
            speak: function() { alert('Hi! My name is ' + this.name + ' and my ID is ' + this.userid + '.'); }
        }

        //Shows all properties
        ShowObjProperties(obj);

        //The Speak function is no longer in the list!
        delete obj.speak;
        alert(typeof obj.speak);
        ShowObjProperties(obj);

        //The UserID is still listed, it just has no value!
        obj.userid = null;
        ShowObjProperties(obj);
    </script>
</head>
<body>

</body>
</html>

I think that there is a very real use for 2 different types here.

What it comes down to is javascripts dynamic nature.

Things may be undefined so that they can be added at a later point. This is arguably why javascript is so powerful and extensible.