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

In JavasSript there are 5 primitive data types String , Number , Boolean , null and undefined. I will try to explain with some simple example

lets say we have a simple function

 function test(a) {

     if(a == null){
        alert("a is null");
     } else {
        alert("The value of a is " + a);
     }
  }

also in above function if(a == null) is same as if(!a)

now when we call this function without passing the parameter a

   test(); it will alert "a is null";
   test(4); it will alert "The value of a is " + 4;

also

var a;
alert(typeof a); 

this will give undefined; we have declared a variable but we have not asigned any value to this variable; but if we write

var a = null;
alert(typeof a); will give alert as object

so null is an object. in a way we have assigned a value null to 'a'