A Closure developer is primarily engaged in writing and maintaining code using Google's Closure JavaScript library. This involves creating dynamic web applications, debugging and improving existing code, and ensuring optimal performance and compatibility across various browsers. They also optimize code for efficiency and scalability, maintain documentation for future reference, and collaborate with other team members. They use Closure's advanced features like dependency management, type checking, and code optimization to deliver high-quality, efficient applications. They may also be involved in user interface design and user experience decisions, as well as testing and debugging.