ReactJS is a front-end library, used to handle the MVC's V (View) layer for web and mobile apps, with which you can build reusable UI components.
I use https://www.tutorialspoint.com/reactjs/ to learn, and install these packages to practice the exerices:
- Node.js (also there: https://www.tutorialspoint.com/nodejs)
- Node Package Management
- express (small introduction)