codewars.com its a great site that uses the classic coding kata approach to learning J.S it gives you the opportunity to think for yourself and problem solve, then when your finished you can compare you code with other users on the site.
dailyjs.com/framework-index this tutorial series teaches the fundamentals of how to build a js library/framework.
Finally I would look on github, npm and bower and look for small packages that you can digest quickly and move up from there, this guy has a ton of smaller J.S repos github.com/bendrucker