What is ReactJS?

Question

can anyone tell me in detail please

Answer 1

React is front end library developed by Facebook. ReactJS allows us to create reusable UI components.
React uses virtual DOM which is JavaScript object. This improves apps performance since JavaScript virtual DOM is faster than the regular DOM.
React can be used on client and server side.
Unlike Angular React can be used with other frameworks as it is a library not a framework.

I recommend you to to there website directly and read about it. https://facebook.github.io/react/

It is quite easy if you understand the component building approach.