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.

Answer 1

alert(sam); // returns undefined

Nope, that’s an exception.

You get undefined when you access an unset property; you get an error when you use an unset name directly.

Global variables are interesting because they can be accessed either using a simple variable name, or by using properties of the window global object:

alert(window.sam);      // undefined
alert(window['sam']);   // undefined
alert('sam' in window); // false
alert(sam);             // ERROR

If sam is declared but not initialised, accessing window.sam still gets you undefined, but for a different reason: there is an entry in the window object for sam, but it points to the same undefined object as you get when you access a non-existant property.

var sam;
alert(window.sam);      // undefined
alert(window['sam']);   // undefined
alert('sam' in window); // ** true
alert(sam);             // ** undefined

This is of course a confusing bloody mess; undefined is one of the worst mistakes in the design of the JavaScript language.

null on the other hand is fine and works pretty much the same as null/nil/void/None values in other languages. It doesn’t come into any of the above.