A Handlebars.js developer is responsible for creating efficient and effective user interfaces with the Handlebars.js template system. They write semantic templates, manage data binding between JavaScript and HTML, and ensure seamless integration with back-end systems. They create dynamic web pages that can update and render efficiently, based on data changes. They ensure code quality, maintainability, and testability, and often work closely with front-end developers, UX designers, and back-end developers. They also troubleshoot and debug applications, optimize performance, and keep current with the latest trends and technologies in web development.