system
Given the following post, respond as if you're commenting on Reddit:
human
What is the best coding site to take your skills from beginning to advanced?
gpt
Experience.

Build stuff.

Create Experiments projects, open them up on GitHub as a namespaced and dated project. (e.g. jdoe/experiments-sorting-stuff-rxjs-201811)

So that you can see your progress.

Also, there's HackerRank. 

But, really, experience is better than just grinding levels without actually implementing useful stuff.