React.js is popular for several reasons:

Component-Based Architecture:

React allows developers to build reusable components, which leads to more organized and maintainable code. This component-based structure is perfect for building complex UIs while keeping code modular and scalable.

Virtual DOM:

React uses a virtual DOM to improve performance. Instead of updating the entire web page when something changes, React updates only the parts that need to be changed. This makes UI updates faster and more efficient.

Declarative UI:

React's declarative approach makes it easier to predict and debug UI states. Developers can describe how the UI should look based on the current application state, and React handles the rendering and updates.

Strong Ecosystem and Community:

React has a vast ecosystem with a wide array of libraries and tools, making it easy to integrate with other frameworks, manage state (like Redux or Context API), and handle tasks such as routing (React Router). The large community ensures continuous improvements, support, and a wealth of resources.

Cross-Platform Development:

With tools like React Native, developers can use React to build mobile apps for iOS and Android, which adds to its appeal as a cross-platform solution.

Backed by Facebook:

Since React was created and is maintained by Meta (Facebook), it has the backing of a large organization, ensuring its continued development and widespread adoption.

Rich Developer Tools:

React comes with a powerful set of development tools (React Developer Tools), which makes debugging and inspecting components much easier for developers.