An Alt.js developer works with the Alt.js library for managing data within JavaScript applications. They create and manage data stores, handle server responses, and manage application state. They use the Flux architecture to handle data flow in React applications, ensuring unidirectional data flow to maintain predictability and control. Alt.js developers also handle asynchronous actions and data loading, making the application more responsive and efficient. They work closely with front-end developers to seamlessly integrate UI components with the underlying data structure. In essence, they leverage Alt.js to build robust, scalable, and complex web applications.