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.

I don't think there's any reason to have both null and undefined, because the only reason many people have suggested ("undefined means there's no such variable/property") is not valid, at least in JavaScript. undefined can't tell you whether the variable/property exists or not.

console.log(foo);               // "ReferenceError: foo is not defined"
                                // foo does not exist
var foo;
console.log(foo);               // "undefined", a different response
console.log(foo === undefined); // "true", but it does exist

var obj = {};
console.log(obj.hasOwnProperty("foo")); // "false", no such property
obj.foo = undefined;
console.log(obj.hasOwnProperty("foo")); // "true", it exists and has the value "undefined"
console.log(obj.foo === undefined);     // "true", but it does exist

obj.bar = "delete me";
obj.bar = undefined;
console.log(obj.hasOwnProperty("bar")); // "true", not actually deleted
delete obj.bar;
console.log(obj.hasOwnProperty("bar")); // "false", deleted

As you can see, checking foo === undefined does not tell you whether foo exists, and setting obj.bar = undefined does not actually delete bar.

It may be the JavaScript author's original intent that undefined should represent "nonexistence". However, the implementation didn't turn out that way.