Lucid is a 2D Javascript game-engine, primarily for the platforming genre. Heavily based on the game-engine Dissimulate posted on both GitHub and CodePen. It's quite evolved from that engine though, as things are much more broken apart now, and easier to extend onto.
There's a few example maps that showcase what's possible with the game engine.
Lucid is open source and free to use, check out the GitHub repo to learn more about how to use it.