if javascript is never defined

case 1:
=================

case 2:
========


we see alert box : undefined

case 3
===============


we see alert box "undefined"


case 4
===============


No error. Alert box shows undefined

Case 5
============
var a;
if(null==a)
alert("its null");
else
alert("its not null");

Ans: Its Null

case 6
==========
if(null==a)
alert("its null");
else
alert("its not null");

Ans: error , a is not defined


Case 7
==============
var a;
alert(a);
if(null==a)
alert("its null");
else
alert("its not null");

Ans: alert box "undefined"
its null