It is intended that you don't have the control over jumping via a button. It's more like Zelda where you also have similar game mechanics. The boxes are small but the controls should be tight enough to control it (The game is also fast-paced so the boxes serve mostly to get on higher grounds.).
Also (this might sound strange) but we want to keep the buttons you need to play the game at minimum.
Zelda did have a jump button though, when you equip a specific item in the Gameboy one .. >_> but whatever.
This is where we got a lot of feedback and we already started changing a lot. You did not mentioned it but you can escape attacks via dashing! Use the right mouse button to perform a dash (currently this will break your aim hard and you have a long cool down after a lot of dashes in quick series. This was already changed and now dashing is more effective). The dash will even work when you aim or throwing balls.
I tried using the right mouse button, it did nothing but bring up a right click context menu with "Save image" on it. HTML 5/Canvas strikes again.
Also, I did defeat the enemies, it just took too long.