name: rollup-bundler
description: Best practices and guidelines for Rollup.js module bundler configuration, ES modules, and library bundling

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