You could try implementing some of the methods in underscore/lodash - http://underscorejs.org/? Each of those are pretty simple and can build off each other - you'll learn how to deal with arrays, objects, functions, arguments, apply/call, this, etc. You could write your own tests or use their unit tests as well to get some practice with testing. I was planning on doing this myself at some point haha.
There's an annotated source for underscore: http://underscorejs.org/docs/underscore.html.
There's also just implementing data structures/algs: https://github.com/felipernb/algorithms.js