Contributing to open source on GitHub is, probably, the best way of learning, learning from great codebases and learning from great engineers.
You also can use services like CodeMentor
Talking about the JavaScript, before doing any React stuff you should learn software engineering, architecture and JavaScript itself. Here I would be able to help and mentor you, and answer your questions since I would appreciate any help at BunnyJS, you may join Gitter and PM me your first direct questions, what would you like to know and what - to do.