A crossfilter.js developer uses the Crossfilter JavaScript library to construct highly interactive and dynamic data-driven web applications. They utilize the library's capabilities for handling multi-dimensional datasets, enabling users to explore and manipulate large amounts of data in real-time. The developer designs and implements features such as data visualization, filtering, grouping, and sorting. They also ensure that the application remains responsive and efficient, even with significant amounts of data. Their role often includes troubleshooting, improving existing systems, and collaborating with other team members to deliver high-quality web applications.