Undefined checks

Undefined in js

# JavaScript undefined

> The cause of headaches.

> The bringer of tears.

> The destroyer of functions.

> The slayer of equations.

## The Problem

JavaScript has a few problems. One of which being the fact that everybody can override the ```undefined``` value.

This, when occurring, brings a lot of tears to developers eyes. Especially when the developer has not prepared for such an event.

## Example of the problem

To display the problem i have created a simple [jsfiddle][jsfiddle] demonstration.

In that [demo][jsfiddle] i will overwrite undefined value in scope and do numerous equations to check the values. Surprising results???

As the [demo][jsfiddle] clearly shows then JavaScript is uncap-able of detecting bogus ```undefined``` values.

## The cure

The cure itself is simple. Use a ```typeof``` operator.

Example:

```
    // Keep in mind that i am not using type conversions.
    // Also the "quotes".
    if (typeof x === "undefined") {
        // magic
    }
```

## Aftermath

This issue itself is quite simple. However I've seen people failing to-do proper ```undefined``` checks for ages.

I can just imagine the amount of joy a developer has when somebody, who has a sick sense of humor, preforms a successful [persistent XSS][xss] injection and overwrites ```undefined``` value in some scope...

 [jsfiddle]: http://jsfiddle.net/kirstein/KY4tV/
 [xss]: http://en.wikipedia.org/wiki/Cross-site_scripting