React is a JavaScript library (MIT License) made by Facebook to create interactive UIs. It's used to create dynamic and reusable components. The most powerful thing about React is that can be used in the client, server, mobile applications, and even VR applications. 

In the modern web, we need to manipulate the DOM constantly; the problem is that doing this a lot may affect the performance of our application seriously. React uses a Virtual DOM, which means that all updates occur in memory (this is faster than manipulating the real DOM directly). The learning curve of React is short in comparison with other JavaScript frameworks such as Angular, Vue, or Backbone, mainly because the React code is mostly written with modern JavaScript (classes, arrow functions, string templates, and so on) and does not have too many patterns used to write code, like Dependency Injection, or a template system, like in Angular.

Companies such as Airbnb, Microsoft, Netflix, Disney, Dropbox, Twitter, PayPal, Salesforce, Tesla, and Uber are extensively using React in their projects. In this book, you will learn how to develop your React applications in the way they do, using best practices.