What is ReactJS ?

React is a UI library developed at Facebook to facilitate the creation of interactive, statefull & reusable UI components. It is used at Facebook in production, and Instagram.com is written entirely in React.

It's just a view library

Let's get the basics out of the way. React is not another MVC framework, or any other kind of framework. It's just a library for rendering your views. If you're coming from the MVC world, you need to realize that React is just the 'V', part of the equation, and you need to look elsewhere when it comes to defining your 'M' and 'C', otherwise you're going to end up with some really yucky React code

Why learn ReactJS ?

ReactJS presents graceful solutions to some of front-end programming's most persistent issues. It's fast, scalable, flexible, powerful, and has a robust developer community that's rapidly growing. 

React is the "View" in the application, a fast one. It also provides different ways to organize your templates and gets you think in components. In a React application, you should break down your site, page or feature into smaller pieces of components. It means that your site will be built by the combination of different components. These components are also built on the top of other components and so on. When a problem becomes challenging, you can break it down into smaller ones and solve it there. You can also reuse it somewhere else later. Think of it like the bricks of Lego