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...