This game scratches the Vampire Survivors and Seraph's Last Stand itch :D Feels surprisingly polished for a ~3-day jam game. I'll definitely be playing more of this.
Unfortunately, I've attained immortality, so I can't die to submit my score, and this far in, the game lags so much that I basically only get a frame rate when I level up, during upgrade selection. Between level-ups I just get a frozen screen. This kind of lag can be remedied by limiting how many fixed-step updates you do per "frame", and if there's still time left in the accumulator after that, zero it out. This'll have the effect of slowing the game down when it can't keep up, but I think that's better than runaway lag. I noticed my browser's memory usage shot up from ~1GB to 8+GB as well, so there may also be a memory leak, or perhaps the game is just running so many updates that garbage collection can't keep up.
I think the main source of the lag might be the particles associated with Arson and Flamethrower, but what it's worth.
A minor cosmetic issue I noticed is that the timer can only fit 4 digits before it gets a scrollbar xP An even more minor issue is that in full screen, the game stretches to fit the screen, even if that messes up its aspect ratio. Black bars would probably look nicer.
I hope you'll work on this game after the jam! With a little more polish, I think it'll be great. Aside from the lag (some of which I imagine is inevitable) and missing weapon upgrade descriptions, an options menu where the music and sound volume can be controlled would be handy. An option to use simplified particles might also help with lag.