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

To add to the answer of What is the differrence between undefined and null, from JavaScript Definitive Guide 6th Edition, p.41 on this page:

You might consider undefined to represent system-level, unexpected, or error-like absense of value and null to represent program-level, normal, or expected absence of value. If you need to assign one of these values to a variable or property or pass one of these values to a function, null is almost always the right choice.