It depends on how in-depth you want to go with React JS; if you only want to learn how to implement React, an easily accessible online course or tutorial can assist. However, if you prefer to understand the principles of the React code stack, a textbook or an in-depth online course is recommended.
Nonetheless, begin with the React documentation on its official website, which is maintained by Meta, as it provides not only a basic overview but also the most recent developments in the framework.