Very cool idea! Cute art, great music and a nice forgiving swing that feels pretty good to use. Enemies having friendly fire was a great touch too!
The only bit of feedback I can think of is maybe give the player an incentive to sometimes *not* be swinging the bat (I found myself sometimes constantly mashing it) - maybe they move slower for a short period of time while/after swinging? A little bit of telegraphing on enemy attacks would be nice too. Great stuff overall!
(I had a weird issue though where sometimes my keys got stuck and the character kept on moving without me pressing anything until I hit a bunch of buttons - might just be my browser though!)