I want to start with the first thing that is concepts. Some of these concepts, for example, closures in JavaScript. You might be using this all the time, but this is an important part of the language.
Another one that comes is prototyped in JavaScript and even in these concepts, you can also place in
something like new features which are new things JavaScript at the languages.
Adding the second thing you should know as a beginner or intermediate to advanced developers is how the asynchronous model in JavaScript works. Microtask queues and this is basically part of the event loop and stuff, promises and callbacks. This can also include stuff like going deep into the V8 on OS level.
Stop working with port and probably the most
important aspect of a senior developer or somebody who has worked a lot with
JavaScript and that is debugging and profiling your code.