I just wanted to give a shout out to this free online resource, Ilya Kantor, and all the other contributors.
The Modern JavaScript Tutorial
This book has helped me plug some gaps in my still inadequate knowledge of JS, and has been a great complementary piece to FCC. Every lesson I’ve looked at so far also includes exercises that reinforce an understanding of the concepts being taught.
It’s up to date with ES6, and helped me complete quite a few of the challenges and projects in the JavaScript section. I’m still going through the book methodically while I complete the next FCC section. The material can be dense but going over it twice really helps.