I think if you leap straight from old JS to ES6 it might leave you with some weird gaps, especially with regard to why some things got to be the way they are. A book I found very good on this score was:

Effective Javascript http://effectivejs.com/

Also understanding Promises properly I think is quite important (even if you don't choose to employ them yourself) and I found this book also very useful:

http://shop.oreilly.com/product/0636920032151.do