Rollup is an ES module bundler and JavaScript module transpiler. It functions as a build tool that combines JavaScript modules into optimized files and converts modern JavaScript and JSX into various output formats for different runtime environments.
The project is distinguished by its static-analysis tree shaking engine, which removes unused code and dead exports to minimize bundle size. It also acts as a code splitting orchestrator, partitioning modules into separate chunks to optimize network requests and load times.
The tool provides a hook-based plugin system for extending build logic, transforming source code, and managing module resolution. Additional capabilities include multi-format output generation for library distribution, source map generation for debugging, and a command-line interface with file system watching.