Which JavaScript library is often used to build web applications across multiple platforms?

React is a popular JavaScript library developed by Facebook for building user interfaces, particularly for single-page applications. It enables developers to create reusable UI components that can be rendered on both the client and server sides. One of its key features is the virtual DOM, which optimizes rendering performance by changing only the parts of the user interface that have changed, enhancing user experience.

The library is platform-agnostic, meaning it can be used to build applications that run on various platforms, including web browsers, mobile devices through React Native, and even desktop applications with tools like Electron. This cross-platform capability makes React a preferred choice for many developers looking to create applications that work seamlessly across different environments.

In contrast, other libraries mentioned have different focuses: Angular is a comprehensive framework that offers a complete solution for developing web applications, Vue is known for its simplicity and flexibility but is more conventionally used for web interfaces, and jQuery is mainly a library for DOM manipulation and event handling and does not have the same focus on building complex applications across platforms as React does.