A Framer JS developer uses the Framer library to design and build interactive and responsive user interfaces for web applications. They create prototypes for testing user experience, incorporating animations and transitions to enhance interaction. They work with design and development teams to integrate these prototypes into final products, translating design vision into functional code. They also debug and optimize the performance of these interfaces. Knowledge of JavaScript, HTML, and CSS is essential, as is understanding of modern UI/UX trends. They may also use Framer for user research, testing, and validation. Their goal is to improve user engagement through superior interface design.