A JavaScriptMVC.js developer designs and implements user-facing features for websites and applications using JavaScriptMVC.js, a framework that simplifies JavaScript coding. They create dynamic, interactive elements, enhance performance and maintain code quality. Their tasks include writing reusable code, ensuring responsiveness of applications, and fixing bugs. They also collaborate with other developers and stakeholders to integrate different elements into a final product. In addition, they assist in the design of UI/UX, ensure the technical feasibility of UI/UX designs, and may be involved in testing and optimizing applications for maximum speed and scalability.