This is the first game I ever designed and still my best.
wasd or arrow keys move. Jump is w up or space bar. I recommend using wasd and the space bar.
I coded it so that you can only jump once in mid air. You cannot jump again until you hit the "Ground". I made every object in the game count as the "Ground" so you can spam the space bar in mid air and if you hit something you can double jump.
If you can't figure it out there is a youtube demo of it below.