---

name: rollup-bundler description: Best practices and guidelines for Rollup.js module bundler configuration, ES modules, and library bundling metadata: maintainer: Mindrally source: https://github.com/Mindrally/skills

Rollup Bundler

You are an expert in Rollup.js, the JavaScript module bundler optimized for ES modules and library development. Follow these guidelines when working with Rollup configurations.

Core Principles

• Rollup is designed for ES modules and produces cleaner, smaller bundles
• Superior tree-shaking through deep execution path analysis
• Ideal for libraries and packages that will be consumed by other projects
• Focus on producing efficient, readable output code

Project Structure

project/
├── src/
│   ├── index.js          # Main entry point
│   ├── modules/          # Internal modules
│   └── utils/            # Utility functions
├── dist/                 # Build output
│   ├── bundle.esm.js     # ES module format
│   ├── bundle.cjs.js     # CommonJS format
│   └── bundle.umd.js     # UMD format
├── rollup.config.mjs     # Configuration file
└── package.json

Configuration Basics

Basic Configuration

// rollup.config.mjs
export default {
  input: 'src/index.js',
  output: {
    file: 'dist/bundle.js',
    format: 'esm',
    sourcemap: true
  }
};

Multiple Output Formats

export default {
  input: 'src/index.js',
  output: [
    {
      file: 'dist/bundle.esm.js',
      format: 'esm',
      sourcemap: true
    },
    {
      file: 'dist/bundle.cjs.js',
      format: 'cjs',
      sourcemap: true
    },
    {
      file: 'dist/bundle.umd.js',
      format: 'umd',
      name: 'MyLibrary',
      sourcemap: true
    }
  ]
};

Output Formats

ES Modules (esm)

• Keeps bundle as ES module file
• Best for modern bundlers and browsers
• Supports tree-shaking in consuming projects

CommonJS (cjs)

• For Node.js environments
• Compatible with require() syntax

UMD (Universal Module Definition)

• Works as AMD, CJS, and IIFE
• Best for libraries that need broad compatibility

IIFE (Immediately Invoked Function Expression)

• Self-executing function
• Suitable for script tags in browsers

ES Modules Best Practices

Use Named Exports

// Prefer named exports for better tree-shaking
export const add = (a, b) => a + b;
export const subtract = (a, b) => a - b;

// Avoid default exports when possible
// export default { add, subtract }; // Less tree-shakeable

Static Imports

// Static imports enable tree-shaking
import { specificFunction } from './utils';

// Avoid dynamic requires in library code
// const utils = require('./utils'); // CommonJS - no tree-shaking

Essential Plugins

Node Resolve

import resolve from '@rollup/plugin-node-resolve';

export default {
  plugins: [
    resolve({
      browser: true,
      preferBuiltins: false
    })
  ]
};

CommonJS Conversion

import commonjs from '@rollup/plugin-commonjs';

export default {
  plugins: [
    commonjs()
  ]
};

Babel Transpilation

import babel from '@rollup/plugin-babel';

export default {
  plugins: [
    babel({
      babelHelpers: 'bundled',
      presets: ['@babel/preset-env']
    })
  ]
};

TypeScript Support

import typescript from '@rollup/plugin-typescript';

export default {
  input: 'src/index.ts',
  plugins: [
    typescript({
      tsconfig: './tsconfig.json'
    })
  ]
};

Minification

import terser from '@rollup/plugin-terser';

export default {
  plugins: [
    terser()
  ]
};

Environment Variables

import replace from '@rollup/plugin-replace';

export default {
  plugins: [
    replace({
      preventAssignment: true,
      'process.env.NODE_ENV': JSON.stringify('production')
    })
  ]
};

External Dependencies

Marking Dependencies as External

export default {
  input: 'src/index.js',
  external: ['react', 'react-dom', 'lodash'],
  output: {
    file: 'dist/bundle.js',
    format: 'esm',
    globals: {
      react: 'React',
      'react-dom': 'ReactDOM'
    }
  }
};

Peer Dependencies Pattern

import pkg from './package.json';

export default {
  external: [
    ...Object.keys(pkg.peerDependencies || {}),
    ...Object.keys(pkg.dependencies || {})
  ]
};

Code Splitting

Multiple Entry Points

export default {
  input: {
    main: 'src/index.js',
    utils: 'src/utils/index.js'
  },
  output: {
    dir: 'dist',
    format: 'esm'
  }
};

Dynamic Imports

// Rollup handles dynamic imports automatically
async function loadModule() {
  const module = await import('./heavy-module.js');
  return module.default;
}

Tree Shaking Optimization

Pure Function Annotations

// Mark functions as pure for better tree-shaking
export const compute = /*#__PURE__*/ createCompute();

Side Effects Configuration

{
  "name": "my-library",
  "sideEffects": false
}

Watch Mode

// rollup.config.mjs
export default {
  watch: {
    include: 'src/**',
    clearScreen: false
  }
};

Command line:

rollup -c -w

Package.json Configuration

{
  "name": "my-library",
  "version": "1.0.0",
  "main": "dist/bundle.cjs.js",
  "module": "dist/bundle.esm.js",
  "browser": "dist/bundle.umd.js",
  "types": "dist/index.d.ts",
  "exports": {
    ".": {
      "import": "./dist/bundle.esm.js",
      "require": "./dist/bundle.cjs.js"
    }
  },
  "files": ["dist"],
  "sideEffects": false
}

Best Practices

Do

• Use ES6 module syntax consistently
• Mark third-party dependencies as external
• Generate source maps for debugging
• Use named exports for better tree-shaking
• Test all output formats
• Use watch mode during development

Avoid

• Bundling peer dependencies
• Using CommonJS modules when ES modules are available
• Ignoring bundle size
• Skipping TypeScript declaration files for libraries

Common Configuration Patterns

Library Configuration

import resolve from '@rollup/plugin-node-resolve';
import commonjs from '@rollup/plugin-commonjs';
import typescript from '@rollup/plugin-typescript';
import terser from '@rollup/plugin-terser';
import pkg from './package.json';

export default {
  input: 'src/index.ts',
  external: Object.keys(pkg.peerDependencies || {}),
  plugins: [
    resolve(),
    commonjs(),
    typescript({ tsconfig: './tsconfig.json' }),
    terser()
  ],
  output: [
    { file: pkg.main, format: 'cjs', sourcemap: true },
    { file: pkg.module, format: 'esm', sourcemap: true }
  ]
};

Debugging and Analysis

• Use --configDebug flag for configuration debugging
• Generate source maps for all formats
• Use rollup-plugin-visualizer for bundle analysis
• Check output code readability