Why is null an object and what's the difference between null and undefined?

Question

Why is null considered an object in JavaScript?

Is checking

if ( object == null )
      Do something

the

Answer 1

For example window.someWeirdProperty is undefined, so

"window.someWeirdProperty === null" evaluates to false while

"window.someWeirdProperty === undefined" evaluates to true.

Moreover checkif if (!o) is not the same as checking if (o == null) for o being false.