I guess like most of those projects, GameJs grew as a library while I was coding games.
Still, I have only managed to finish small games and the biggest - roguelike RTS - is still in the making http://gamejs.org/apps/rtsimple/ (rough tech demo)
If you do game programming as a hobby, then the only predictable way to finish a game is during a competition. It is much easier to finish a game within 48 hours than with unlimited time.