A dc.js .js developer uses both dc.js and .js JavaScript libraries to create interactive, data-driven visualizations. .js is used for manipulating documents based on data, allowing developers to bind arbitrary data to a Document Object Model (DOM), and then apply data-driven transformations to the document. Dc.js, on the other hand, is a multi-dimensional charting library built on .js, which makes it easier to create dynamic and complex charts. The developer's responsibilities include developing, testing, and debugging these visualizations, ensuring they are effective, user-friendly, and enhance data understanding. They also maintain and optimize the code, and often collaborate with other team members.