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Question

Why is there a `null` value in JavaScript?

Answer 1

+3 A:

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.

EBGreen 2009-01-20 16:17:52

Answer 2

+17 A:

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.

Eddie Parker 2009-01-20 16:18:16

I know that `null !== undefined` - my question was why there was a need for two things which express the same semantic concept; also, your link mentions that 'null is an object' - that's wrong, it's a primitive...

Christoph 2009-01-20 16:22:51

They aren't the same semantic concept. To me at least, there is a significant difference between a property being assigned a null value, and a property not existing.

Daniel Schaffer 2009-01-20 16:31:22

But that's just it, they're not the same semantic concept. Null gets coerced when using == into implying the same thing, as a convenience to the programmer.

Eddie Parker 2009-01-20 16:33:10

Answer 3

+1 A:

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.

AnthonyWJones 2009-01-20 16:21:23

but you can use `undefined` just fine to assign to properties, so this contract (only return `undfined` if there's no such property) can easily be broken by the programmer...

Christoph 2009-01-20 16:30:47

You _can_, but you most definitely should not. Undefined is used as a very basic/simplistic form of exception in JavaScript - don't use it as a value and assign it to some property! That's crazy man.

thenduks 2009-02-12 22:18:23

Answer 4

+1 A:

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.

EndangeredMassa 2009-01-20 17:04:05

And in prototypes I think, is a member null or just undefined.

Kev 2009-01-20 17:06:56

but this only works as long as no one tries to break it - if I set `obj.userid = undefined`, it'll fail - see the last edit to my question

Christoph 2009-01-20 17:15:02

Answer 5

+1 A:

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.

Rob Stevenson-Leggett 2009-01-20 17:30:40

and this is relevant to my question in what way?

Christoph 2009-01-20 17:41:43

It's relevant because it answers your question. 'null' is a 'singleton' of sorts that means 'has no value'. 'undefined' is telling you that something is... shocking, I know... not defined. They're completely different things.

thenduks 2009-02-12 22:20:37

Answer 6

+2 A:

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.

Im 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.

Jack Ryan 2009-01-20 17:45:24

Answer 7

+6 A:

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:

when you declare 'var x;' but don't assign to it, x holds undefined;
when your function gets fewer arguments than it declares;
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 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 signalling 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.

bobince 2009-01-21 00:00:05

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Why is there a `null` value in JavaScript?

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