Sapper is a universal web application framework and server-side rendering framework for Svelte. It enables the development of applications that render views on the server for fast initial loads and hydrate them on the client for subsequent interactivity.
The project features a file-based routing engine that maps URL paths to components based on the file system structure. It also functions as a static site generator, exporting production-ready HTML, CSS, and JavaScript files for hosting in environments without a live server.
The framework covers full-stack development capabilities, including the creation of server routes for API endpoints, shared layouts for consistent navigation, and universal data fetching. It manages performance and security through code-splitting, prefetching, and the configuration of content security policies.