A Coffeescript.js developer specializes in using Coffeescript, a programming language that transcompiles into JavaScript, to develop client-side and server-side web applications. They write clean, readable, and efficient code, utilizing Coffeescript's lightweight syntax. They also maintain and debug existing codebases, and help optimize applications for maximum speed and scalability. Furthermore, they collaborate with other team members, like UI/UX designers and other developers, to integrate user-facing elements with server-side logic. They are also responsible for implementing automated testing platforms and unit tests to ensure the robustness and reliability of the web applications.