There is no a single site/book I use, I just search for a topic that I think is interesting. Fore example, lately I've been exploring Promises and asynchronous JS [0][1]. For understanding react & flux, you can watch 3-part series from SeattleJS [2]. This post is a good general roadmap: [3].

[0] https://www.promisejs.org

[1] https://promisesaplus.com/

[2] https://www.youtube.com/watch?v=Pd6Ub7Ju2RM

[3] https://medium.com/javascript-scene/learn-javascript-b631a4a...

I use the (soon to be) language standard as a reference: http://wiki.ecmascript.org/doku.php?id=harmony:specification...