The null value is a primitive value that represents the null, empty, or non-existent reference.
If you don't have a null keyword, you may be able to simulate a null value like this:
var null = (void 0);
The null and undefined values are subtly different. An empty thing is not the same as a non-existent thing. However in a browser it is difficult to distinguish between them.
The null value is now provided in some browsers as a built-in keyword, but the undefined value is not.
ECMA 262 edition 2 - section - 4.3.11
ECMA 262 edition 3 - section - 4.3.11