A HammerJS developer is responsible for implementing user interface interactions and functionalities using the HammerJS library. They integrate touch gestures, such as swipe, pan, pinch, and rotate into web applications, enhancing user experience especially on mobile devices. They troubleshoot and resolve issues related to touch events and ensure the smooth operation of these features across different platforms and browsers. Additionally, they may also collaborate with other developers to integrate HammerJS functionalities into larger projects and ensure the overall performance and usability of the application. Lastly, they are also expected to stay updated with the latest updates and improvements in the HammerJS library.