Some of the really cool Website especially for improving Javascript Knowledge and code skills

https://www.jschallenger.com/
(Do code challenges & improve the logic implementation)

https://www.java5cript.com/
(Any thing you can learn about JavaScript & in that you will find the other options like Books, Courses, Videos, Fundamentals specific for JavaScript)

I would say this will help individuals who are working in JS framework OR looking to move in JS