I enjoyed the mechanic, I Think it's probably the first time I've ever seen it, nice work. I think being able to kill aliens offscreen was a bit cheesy, however a few lines of code should do. another optional thing you could add are things like coyote time and jump buffering, that way the game feels more responsive, however don't worry too much if you don't use it.
overall nice first game for the time you had cut off👍