This snippet results in a JavaScript runtime error: (foo is not defined)

if (foo) {

    // ...

}

I have to define foo first, like so:

var foo = foo || null // or undefined, 0, etc.

... And only then can I do:

if (foo) {

    // ...

}

Why is that?






Update:

This was somewhat of a brainfart on my side of things: 'fcourse you can't access a variable which is not allocated.

Fun stuff that you can do a typeof() on an undefined variable thou. I'm gonna accept miccet's answer since I think it's the most elegant solution.