A JavaScript MVC developer designs, codes, and modifies web applications using JavaScript and Model-View-Controller (MVC) architectural pattern. They work on both client and server-side scripting, creating user-friendly interfaces, integrating with APIs, and developing back-end components. They are responsible for maintaining data integrity and synchronization between the model (data), the view (user interface), and the controller (business logic). They also conduct performance tests, troubleshoot, fix bugs, and improve application efficiency. Their role also involves collaborating with other team members, following coding standards, and staying updated with new JavaScript libraries and MVC frameworks.