When JavaScript returns null & undefined?

Question

I have been using JavaScript for couple of years and never cared about the difference between null & undefined earlier, I always use undefined to validate the object existence.

But recently I came through this article. Here they said

JavaScript distinguishes between null, which is an object of type 'object' that indicates a deliberate non-value, and undefined, which is an object of type 'undefined' that indicates an uninitialized value — that is, a value hasn't even been assigned yet. We'll talk about variables later, but in JavaScript it is possible to declare a variable without assigning a value to it. If you do this, the variable's type is undefined.

I am completely confused now, what exactly is non-value here. How this non-value differs from undefined. And what are the circumstances javascript returns null.

I have tried the below sample

var sam;
alert(sam);  // returns undefined

And

try {
    //var sam;
    alert(sam);  
} catch(ex) { }   // exception says: sam is undefined

And I am not sure about when js returning nulls. Can someone clarify me.

When JavaScript returns null & undefined?

4 Answers