It's a bit tough to know what you mean by "advanced js developer." However, here are a few ideas for areas to explore in more depth:
Learn how to develop applications using several different front-end frameworks, such as Vue, React, Angular
Write a back-end web site/api with Node/Express (or similar)
Develop a strong understanding of the browser security model, including xss and csrf exploits
Know how the cooperative concurrency model works in detail in Javascript - look into the details of how the event loop in the browser and node work (also see v8 tasks and microtasks )
It's a bit tough to know what you mean by "advanced js developer." However, here are a few ideas for areas to explore in more depth: