I could see it was undefined in Firebug. When the page loads it's not not undefined. I click on it, and boom it loses its definition. How could that be.
Well, you could have assigned it an undefined value. Let's look for an assignment. Found it.
Search on Google. Aha. The weirdness. You can call a function with fewer parameters than it defines. The unspecified parameters are (ta dahh) undefined.