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 undefin…

Accepted Answer

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.

When JavaScript returns null & undefined?

Solution

Problem