Applications and websites in React.js

React is an open-source JavaScript library created by Facebook in 2013 for building dynamic user interfaces in web applications. Its key feature is a component-based architecture, where the interface is divided into independent, reusable components that manage their own state and logic. Components come in class or function versions using React Hooks, whose introduction in version 16.8 revolutionized state and side-effect management. The library uses JSX - a JavaScript syntax extension that combines logic with a presentation layer, which increases code readability. React's responsiveness is manifested in automatic DOM updates when a component's state changes, supported by virtual DOM optimizations that minimize operations on the actual document tree. One-way data flow makes debugging easy, and integration with tools like Redux and Next.js makes it a cornerstone of modern SPA applications.