I use it as a function parameter and exclude it on function execution that way I get the "real" undefined. Although it does require you to put your code inside a function. I found this while reading the jQuery source.
undefined = 2;
(function (undefined) {
console.log(undefined); // prints out undefined
// and for comparison:
if (undeclaredvar === undefined) console.log("it works!")
})()
Of course you could just use typeof though. But all my code is usually inside a containing function anyways, so using this method probably saves me a few bytes here and there.
