rollup

Rollup is an open-source JavaScript module bundler designed to produce small, optimized bundles from multiple ES modules. It is particularly well suited for building libraries and applications where code size and predictable output matter. Rollup analyzes the import and export statements across a project's modules to build a dependency graph and generate a single bundled file or a set of bundles.

A core feature of Rollup is tree-shaking, which removes unused code by leveraging the static structure of ES modules. This often results in smaller bundles compared to other bundlers. Rollup can emit bundles in several formats, including ES module (esm), CommonJS (cjs), UMD, and IIFE, and supports code splitting through multiple entry points and dynamic imports. It also preserves the original module boundaries in the output when possible, aiding library authors who want clear and tree-shaken APIs.

Rollup is configured via a rollup.config.js file that specifies input files, output options, and a plugins array. The plugin system extends Rollup’s capabilities to handle non-JavaScript assets, transpile code, or transform modules. Common plugins provide support for resolving node modules, converting CommonJS modules to ES modules, Babel transpilation, and minification with terser. Rollup also supports sourcemaps and various output configurations to suit different environments.

Historically, Rollup was created by Rich Harris and has since grown a community-driven ecosystem. It is widely used by JavaScript libraries and projects that prioritize small, predictable bundles, with notable adoption in projects such as Svelte's tooling. While it competes with Webpack in broader application bundling, Rollup is often favored for library distribution due to its efficient tree-shaking and straightforward output formats.