Is React.js a Front-End library? And what's the usage of it?

Answer 1

React allows developers to create large web applications which can change data, without reloading the page. The main purpose of React is to be fast, scalable, and simple.
source: http://www.c-sharpcorner.com/article/what-and-why-reactjs/

Answer 2

I'm really confused, since it's written by JavaScript... which means it's client side language, but it's also used as server side.. so what it is? Front or Back?

Answer 3

i think this may help you..
https://www.quora.com/Is-React-js-a-front-end-library-only/answer/Marcus-Geduld?share=77dd9b4a&srid=hu92d