Some code is so bad that it's hard to have sympathy for the people who wrote it when stuff breaks. Overriding undefined is one of those things.
If we're getting that nitpicky, having an isDef method at all is adding an unnecessary function call that will slow down your program. And while you certainly could create an undefined variable for checking against each time, I'd hardly say not doing so is a "rookie mistake".
If we're getting that nitpicky, having an isDef method at all is adding an unnecessary function call that will slow down your program. And while you certainly could create an undefined variable for checking against each time, I'd hardly say not doing so is a "rookie mistake".