React is a JavaScript library for building user interfaces. It was developed by Facebook and has become one of the most popular technologies for frontend development.
Unlike the popular jQuery library, which allows you to make certain elements on a website's HTML page interactive, React is designed for building full-fledged web application interfaces. It manages application state and renders the interface basing on stored data.
React's main feature is the Virtual DOM - a lightweight copy of the real page's DOM tree that React stores in memory. When data changes, React first updates the virtual DOM tree, then compares it with the previous version (performs reconciliation) and only then makes the actual changes to the real DOM. Working with the virtual DOM is convenient for developers, but it causes runtime overhead. Therefore, JavaScript is still one of the biggest bottlenecks in achieving fast page loading.
Here are some PDF books about React:
Getting Started with React
2016 by Doel Sengupta, Manu Singhal, Danillo Corvalan

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Learning React Native: Building Native Mobile Apps with JavaScript
2015 by Bonnie Eisenman

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Full-Stack React, TypeScript and Node: Build cloud-ready web applications using React 17 with Hooks and GraphQL
2020 by David Choi

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