Js13kGames is a JavaScript coding competition for HTML5 Game Developers. The fun part of the compo is the file size limit set to 13 kilobytes. The competition will start at 13:00 CEST, 13th August and end at 13:00 CEST, 13th September 2015. Theme for 2015 will be announced the day the competition will start.

You could certainly use CodePen to play around with ideas, if not build the whole game. Lots of prizes for the winners, including CodePen PRO accounts!