What is the difference between null and undefined in JavaScript?

Question

I want to know what the difference is between null and undefined in JavaScript.

Answer 1

Undefined means a variable has been declared but has no value:

var var1;
alert(var1); //undefined
alert(typeof var1); //undefined

Null is an assignment:

var var2= null;
alert(var2); //null
alert(typeof var2); //object