The funny way to tell difference between null and undefined in JavaScript

You: What is name?
JavaScript: name? What's a name? I don't know what you're talking about. You haven't ever mentioned any name before. Are you seeing some other scripting language on the (client-)side?

name = null;
You: What is name?
JavaScript: I don't know.
In short; undefined is where no notion of the thing exists; it has no type, and it's never been referenced before in that scope; null is where the thing is known to exist, but it's not known what the value is.
One thing to remember is that null is not, conceptually, the same as false or "" or such, even if they equate after type casting, i.e.

name = false;
You: What is name?
JavaScript: Boolean false.

name = '';
You: What is name?
JavaScript: Empty string


http://stackoverflow.com/questions/801032/why-is-null-an-object-and-whats-the-difference-compared-to-undefined