I'd personally love to see a frontend-heavy codebase that is using relatively new tooling (for example, ES2015 with Babel, React on a significant scale, etc.) if anyone would like to share :).
Derek recommends Backbone and Underscore - at least they can be found nicely annotated here:<p><a href="https://sivers.org/learn-js" rel="nofollow">https://sivers.org/learn-js</a><p><a href="http://underscorejs.org/docs/underscore.html" rel="nofollow">http://underscorejs.org/docs/underscore.html</a><p><a href="http://backbonejs.org/docs/backbone.html" rel="nofollow">http://backbonejs.org/docs/backbone.html</a>
I'd start with the repos for the libraries/framework(s) you've been using lately. Or for something a little different, take a look at Babel's codebase (<a href="https://github.com/babel/babel" rel="nofollow">https://github.com/babel/babel</a>).