A JSX developer uses JSX, a syntax extension for JavaScript, to write user interfaces in React, a popular JavaScript library. Their tasks include creating and maintaining UI components, managing state and props, handling events, and implementing React lifecycle methods. They also ensure the UI works seamlessly across different browsers and devices. In addition, they may work with other technologies like HTML, CSS, and Redux, and integrate APIs for dynamic content. They must write clean, efficient code that follows best practices, and often collaborate with designers, backend developers, and other team members.