How to check an undefined JavaScript value in Facebook?

In normal JavaScript, I can check either

if (i == undefined)

or

if (i === undefined)

or the "typeof".

However, within Facebook, it will have the api_key appended as the prefix in undefined.

That is, it will become

if (a12345_i == a12345_undefined)

which is NOT undefined.

So, what can I do to find out if the variable is undefined or not?

Well, thanks for all answers, but I thi开发者_如何转开发nk I should emphasize one thing. I have no problem to detect the null value in a normal JavaScript environment. The probem I face is when it is with Facebook as an FBML application.

typeof(var) returns a string, you should be checking:

typeof(var) == 'undefined'

Try this ...

if (i == null) or if (i === null)
if (i == '') or if (i === '')

You can use x == null, which will check for null or undefined. However, the check you do should work. While a12345_undefined doesn't look like undefined, it is. For instance, this would work just as well:

var notDefined;
if (x === notDefined) { // ...

As long as you never assign anything into the variable undefined (which becomes a12345_undefined) you should be fine. undefined is just a normal variable in JavaScript, it just starts with undefined value. You could use any variable without a value as a comparison for undefined.

Hope that makes sense.