Rollup

Rollup is an open-source module bundler for JavaScript created by Rich Harris in 2015, the same developer behind Svelte, designed specifically for bundling libraries and applications using ES modules with efficient tree shaking that eliminates dead code, producing smaller and faster output bundles than traditional bundlers, adopted as the production bundler by Vite, the build tool by Astro, and by thousands of npm packages for their distribution builds. The tree shaking algorithm performs static analysis of the ES module import and export graph, tracing which exported functions, classes, variables, and constants are actually used by the consuming code and eliminating all unused exports from the output bundle, resulting in dramatically smaller bundles especially for utility libraries where only a small fraction of the API surface is typically used. The ES module first design means Rollup processes standard ES import and export syntax natively, producing output that is itself a valid ES module, enabling the bundled output to be consumed by other bundlers, run directly in modern browsers through script type module tags, or further processed by other tools, with code splitting, dynamic imports, and circular dependency handling built into the core. The plugin system provides hooks at every stage of the bundling pipeline including module resolution, loading, transformation, chunk generation, and output writing, enabling integration with Babel for transpilation, TypeScript for type stripping, CSS handling, JSON import, URL resolution, and custom transformations. The output formats include ESM, CommonJS, IIFE, UMD, AMD, and SystemJS. Designed by Rich Harris. Used by Vite and Astro. Designed for library authors and build engineers.