Enjoyable game. Being bound to the wall is a cool idea. I also like how you kind of slide a bit. 

I agree with the other comment about how it should get harder as you go. I would  also a score add multiplier or some combo mechanics .

Solid work!

Hey Played your game, Probably that's one of the first game you've done on Javascript, it's nice and cool to begin with. 

About the gameplay,  you should consider increasing difficulty over time, it looks like there is a constant timer spawning zombies, I would increase zombie rate over time for example. 

Anyway, still a good job behind it.