Javascript doesn't exist in a vacuum, and focusing on becoming an expert on just the language is IMHO a misguided effort.
The language is a means to an end and the most useful bits are exactly in the API's that the runtime provide, be they the browser or something like node / react-native.
The advice I often give for "how do I learn ___" is find something you want to make, do what you need to do to arrive at the most basic version that you possible can, then iterate.
If you have someone who can help direct you when you get stuck, or can point out ways things can be vastly better with minimal effort that will be a real boon to your efforts.
Focusing on arcane corners of the language just isn't productive. If you have a mission, you can deal with those when they come up (which due to their arcane nature, will be rarely if at all).