rollup

Rollup is a JavaScript module bundler that compiles small pieces of code into larger, more complex bundles optimized for distribution. Rollup pioneered tree shaking — the process of eliminating unused code by analyzing ES module import/export statements to determine which exports are actually consumed. This produces smaller bundles than tools that cannot perform dead code elimination at the module level. Rollup is particularly well-suited for building JavaScript libraries because it generates clean, readable output in multiple formats (ESM, CommonJS, UMD, IIFE) from a single source, and its tree-shaking ensures consumers only include the code they actually use. Rollup uses a plugin architecture for extending functionality including code transformation (Babel, TypeScript), module resolution (node_modules), CommonJS interop, JSON handling, and asset management. Vite uses Rollup as its production bundler, exposing a compatible plugin interface. While Rollup excels at library bundling, its lack of built-in code splitting for applications and slower development server experience compared to Vite mean it is less commonly used directly for application development. Rollup's output quality and tree-shaking capabilities remain best-in-class.