It's a cute little game! I think it would benefit from some sort of variety, perhaps different enemy types that might act differently, or perhaps some sort of alternate fire mode? Another thing I'd suggest is an option to return to the main menu, because as far as I can tell, there isn't really a way to return to the main menu without refreshing the page.
However, it controls with a respectable fluidity and is clearly an early attempt at game development, so I'd encourage you to keep up the good work!