A React.js developer’s primary responsibility is to create UIs by writing code in JavaScript, HTML, and CSS. Furthermore, React.js developers are responsible for building reusable components that can be used across websites and applications to create a seamless user experience. They must also ensure the UI is optimized for maximum performance and keep up with the latest trends in web development technologies. Additionally, React.js developers are responsible for troubleshooting existing code and maintaining applications as well as staying up to date on industry standards.