Interesting toys for learning
Space Mahjong a simple Card Game (there's no dedicated repository for sources, but you can see it in the browser).
Or watch presentation about exploration of maze building algorithms.
Frozen Defence simple Tower Defence game, play it or learn its sources (there's no dedicated repository for sources, but you can see it in browser).
So, choose whatever looks interesting to you, open its source code and work with it. Try to alter it or made something similar. It would take just a couple of hours or days and can improve you JS skills significantly.
If you are just learning JS, you may also choose to start from basics first and checkout these toys later.
You may also be interested in more advanced open source HTML5 games, but its source code is more complicated and requires much more time to learn.
By Alex Craft