ES6 modules with many exports
DevOps & Tooling / Knowledge library
Rollup
es modules tree shaking and library output formats
Why learn Rollup?
Useful for publishing libraries and controlling bundle structure. Side effects and package exports must be represented correctly for consumers.
Where it came from
Rollup combines JavaScript modules into distributable bundles. It supports multiple output formats and tree-shaking of eligible unused code.
Build your understanding
Work through these exercises in order. Try the task before opening a reference.
- Bundle an ES module library.
- Generate an appropriate consumer output format.
- Mark a peer dependency as external.
- Inspect whether unused exports are removed.
- Test the published-style artifact in a separate sample project.
Documentation and sources
- Official project documentation (rollupjs.org)
Catalog research notes
- Research snapshot: September 9, 2026. This entry summarizes the linked primary documentation; it does not assert a latest release version.
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Real World Example
Optimizing Data Visualization Library Distribution
A team developing a modular data visualization library for web developers.
The business problem
The library contains dozens of specialized chart types, but users only need one or two. Bundling the entire library forces end-users to download massive, unused code blocks, increasing page load times.
How this topic solves it
Rollup utilizes ES module static analysis to perform tree-shaking, which identifies and excludes unused functions from the final bundle, ensuring only the necessary code is shipped.
- Refactor the library codebase to use ES6 import and export statements for all internal modules.
- Configure the Rollup build process to target multiple output formats, specifically ES modules for modern bundlers and CommonJS for legacy Node.js compatibility.
- Define the entry point in the package.json file to point to the compiled CommonJS output.
- Add a module field in package.json that points to the ES module version to enable tree-shaking for end-users.
- Execute the Rollup build command to generate the optimized bundles.
This diagram illustrates how Rollup processes source code into optimized formats.
Static analysis and tree-shaking
Clean, tree-shakable code
Legacy compatible bundle
- Source Code → imports → Rollup Engine
- Rollup Engine → generates → ES Module Output
- Rollup Engine → compiles → CJS/UMD Output
Expected business outcome
End-users receive significantly smaller bundles because Rollup strips out the unused chart components during the build process.
Limitations and checks
Rollup's tree-shaking relies on static analysis of ES modules; dynamic imports or side-effect-heavy code can prevent the bundler from safely removing unused exports, which can be verified by inspecting the final bundle output for unexpected code remnants.