A Common.js developer primarily works with the CommonJS module system, which is often used in server-side JavaScript development, specifically Node.js. They write and maintain code that follows the CommonJS specification, allowing for the import and export of functionalities across different JavaScript files. This includes creating reusable code modules, debugging and testing these modules, and integrating them into larger codebases. They also ensure the smooth interaction between these modules and other parts of the application, such as the front-end interface or the database. Their goal is to enhance application functionality and improve code maintainability.