Space Guardians

Having reset the game, I have not been able recreate the level 2 bug, but I think one of the asteroids got stuck off the side of the screen. There are two common reasons for that in this kind of game. You are either using integers for angles so that if you get stuck at 90deg off the side of the map it will never drift back on screen. If you use floats then it can go at 90.01deg and eventually come back into view. The other possible reason for this problem is that you are using colliders to wrap the asteroids and you either have a hole that they can slip through or they can jump it at high speeds. The best solution is to create a loop that detects your asteroids' XY positioning to make sure it is inside of the game area instead of using colliders.

So three major issues: Game stopped working after level 2. Holding down arrow keys makes the browser scroll, I would suggest changing key controls to just WASD. Lastly, you have nice graphics for the asteroids and background. Something nicer than a triangle for you ship would be good, if you are using vector for collision detection reasons, you can hide the vector graph under an image so that you don't have to sacrifice looks for functionality.