A MarionetteJS developer specializes in using the Marionette.js library to build complex JavaScript applications. They design and implement user interfaces, manage application workflows, and handle events and behaviors. They utilize MarionetteJS to simplify the code, reduce the boilerplate, and make the application more manageable. They also engage in testing, debugging, and optimizing application performance. Their work involves a deep understanding of Backbone.js, as MarionetteJS is built on it. They often collaborate with other developers, UI/UX designers, and stakeholders, ensuring the application meets user needs and business requirements. They also maintain and update the application as needed.