rollup

When to use this skill

Use this skill whenever the user wants to:

• Bundle JavaScript libraries with tree shaking and multi-format output (ESM/CJS/UMD)
• Configure Rollup input, output, plugins, and external dependencies
• Optimize bundle size with code splitting and dead code elimination
• Build libraries for npm distribution

How to use this skill

Workflow

1. Configure — create rollup.config.js with input, output, and plugins
2. Build — run rollup -c to generate bundles
3. Validate — check output formats, bundle size, and tree shaking effectiveness
4. Publish — set main, module, and exports in package.json

Quick Start Example

// rollup.config.js
import resolve from '@rollup/plugin-node-resolve';
import commonjs from '@rollup/plugin-commonjs';
import terser from '@rollup/plugin-terser';

export default {
  input: 'src/index.js',
  output: [
    {
      file: 'dist/index.cjs.js',
      format: 'cjs',
      sourcemap: true,
    },
    {
      file: 'dist/index.esm.js',
      format: 'es',
      sourcemap: true,
    },
  ],
  external: ['react', 'react-dom'], // Don't bundle peer deps
  plugins: [
    resolve(),
    commonjs(),
    terser(), // Minify for production
  ],
};

# Build
npx rollup -c

# Watch mode for development
npx rollup -c --watch

// package.json — library distribution fields
{
  "main": "dist/index.cjs.js",
  "module": "dist/index.esm.js",
  "exports": {
    ".": {
      "import": "./dist/index.esm.js",
      "require": "./dist/index.cjs.js"
    }
  },
  "files": ["dist"],
  "sideEffects": false
}

Code Splitting Example

// Dynamic imports create separate chunks
export default {
  input: 'src/index.js',
  output: {
    dir: 'dist',
    format: 'es',
    chunkFileNames: 'chunks/[name]-[hash].js',
  },
  plugins: [resolve(), commonjs()],
};

Best Practices

• Mark peer dependencies as external to avoid bundling them
• Enable sourcemaps for debugging; disable in production if not needed
• Use sideEffects: false in package.json to enable maximum tree shaking
• For large dependencies, externalize them to reduce bundle size
• Use code splitting with dynamic import() for on-demand loading

Reference

• Official documentation: https://rollupjs.org/

Keywords

rollup, bundler, ESM, CJS, tree-shaking, code splitting, library bundling, plugins