Michael Geary said:

“Now you can code this in any browser:

if( foo === undefined ) whatever;”

Now, this is unfortunately not true. IE5 on Windows does not support undefined as an object (the error you get tells you that in fact ‘undefined’ is undefined). There may be other browsers that don’t support it. IE5 on Windows is enough on its own to put me off, given that I still have to write code to work on it.