YOU CAN REDEFINE UNDEFINED IN JAVASCRIPT????

I like to draw and code and sometimes do other things.
YOU CAN REDEFINE UNDEFINED IN JAVASCRIPT????
Userscript developers, I seek thy wisdom: how do I get the current signed in users session id. Looking to make a userscript to remove bookmarked tags
As a proof of concept for my silly pictochat idea I made a lil limited drawing program. Feel free to mess with it here
Idk how many of these I'm doing but I wanted to do a funny