What is the difference between undefined value and null value?

Question

I search most of the article available in the internet, but i am able to clear on this two statement

1/0 is undefined or infinity.
1/infinite is zero or undefined.

But i am clear about the things if we will use lim (x->infinite) 1/x approaches to zero

Answer 1

Undefined: a variable has been declared but has not yet been assigned a value.
null: It can be assigned to a variable as a representation of no value.

Example (Undefined):

 var a;
 alert(a); //shows undefined
 alert(typeof a); //shows undefined

Example (null):

 var a = null;
 alert(a); //shows null
 alert(typeof a); //shows object