A Handlebar.js developer designs and implements user-facing features for websites and applications using the Handlebar.js templating engine. They create dynamic HTML templates that can be filled with values from a JavaScript object or function, enhancing user interactivity and experience. They also ensure the seamless integration of these templates with backend services. Their role includes debugging applications, maintaining code quality, and optimizing performance. They often work with other JavaScript frameworks and libraries, collaborating with other developers and designers. Knowledge of web standards, cross-browser compatibility, and responsive designs are also integral to their job.