Topic: Safari javascript error makes no sense

In case I have a popup window
lurking under my screen,
I want it closed.

This is the line I use to do it:
if ( apptWindow ) {apptWindow.close();}

Yet Safari (3.1.2) chokes and tells me:
Can't find variable: apptWindow

Does it work in other browsers?

How does the window get opened, how is the variable set?