A .js developer utilizes the .js JavaScript library for producing dynamic, interactive data visualizations in web browsers. They manipulate documents based on data for creating complex graphical layouts and animations. These developers bind arbitrary data to a Document Object Model (DOM), and then apply data-driven transformations to the document. They are responsible for designing, implementing, and maintaining user interfaces, translating business requirements into technical specs, and integrating with APIs to fetch data. They also ensure cross-browser compatibility, optimize application for speed, and maintain visual consistency across web pages. They often work with other developers, designers, and stakeholders.