How do you master React.js and Javascript?

first of all, make a lot of work on your core concepts of your java-script. Build some projects with vanilla java-script cover the topics like
1-functions and arrow functions
2-Understand the functional programming
3-Arrays and objects
4- Closures(must important)
5-high order functions
6-high order arrays
7-ES6 concepts of async-await and the promises
8-Oops concepts
then after that when you comfortable with there concepts then prceed towards the React.js but wait don't be stop your journey of learning the vanilla javascript keep polishing your skills of java-script along with react.js that's all about

By creating applications and web interfaces. Start with a simple project, then keep increasing complexity for your next projects. Try making something interactive without React, with just pure JS, then do a similar interaction with React, seeing how it simplifies the logic.

i think & as per my knowledge first you became a good with Core concepts of basic then after you must need practice,
might be you get this msg !

Thanks a lot for your suggestions. I will remind them.

It's impossible to master them because they keep evolving and nobody can remember all the documentation. Accept it and just stay up to date.