ReactJS is a JavaScript library for building user interfaces. It was developed by Facebook, and is often used for building single-page applications and mobile applications.
React allows you to build reusable UI components, and makes it easy to manage the state of your application. It follows a declarative approach, which means that you describe what your UI should look like, and React takes care of the rest.
React uses a virtual DOM (a lightweight in-memory representation of the actual DOM) to optimize updates and minimize the number of DOM manipulations required. This makes it fast and efficient for building dynamic, interactive user interfaces.
React is also designed to be easy to learn and use, with a declarative syntax that makes it easy to understand and debug your code.