There's a lot of good here, the intro was cool and the game moves quickly. There are a few things that could be added to improve the feel for the player:
The ammo counter is bugged. Once you full screen it disappears and does not return, even if you exit fullscreen.
The enemies could use a more significant hit sound or effect, like is on the player.
The gun sprite should be flipped when aiming to the left, currently it just turns upside down from the rotation.
On death it seems like the game just freezes, with no restart button other than reloading the page.
Overall what you have is pretty fun, actually pretty incredible for a first game, and with some small tweaks it could be even better.