Of course you’ll need to be fluent in JS (syntax, rules, et cetera) but more importantly you’ll need to know how build things or solve problems. Some basic algorithms, data structure, databases, and more. You might be also expected to learn some JS libraries and frameworks.
Do you need HTML and CSS for React Native?
No, HTML and CSS are not necessary but React Native’s styling closely resembles that of CSS. For example, React Native uses the attribute backgroundColor instead of background-color that CSS uses for changing the color of the background.
Can I learn React without knowing HTML and CSS?
If you are already familiar with the basics and can build a simple web page with some interactivity, then you should probably be in good shape to get started with React and be successful in learning it. If you aren’t too familiar with HTML or CSS, you may have some difficulty getting your app to look right.