A JavaScript developer designs, implements, and tests web applications using JavaScript programming language. They build interactive elements that users engage with through their web browser. They often work with front-end technologies like HTML, CSS, and AJAX, but also back-end services using Node.js. They ensure cross-browser compatibility, troubleshoot and debug applications, and maintain code quality through best practices. Moreover, they collaborate with other developers, UX designers, and product managers to create seamless user experiences. They also stay updated on new trends and advancements in web development to enhance the functionality and user interface of applications.