ReactJS is a popular JavaScript library for building user interfaces, specifically single-page applications. Developed and maintained by Facebook (now Meta), React allows developers to create reusable UI components and efficiently update and render them based on changing data using a virtual DOM. It uses a declarative programming style, making it easier to reason about the application's state and behavior. React supports both JavaScript and JSX (a syntax extension that resembles HTML) to define component structures.

Example using JSX:

function WelcomeMessage() {
  return <h1>Hello, React!</h1>;
}

function WelcomeMessage() {
  return <h1>Hello, React!</h1>;
}

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