Understanding the Problem

Large bundle sizes and broken builds in Rollup.js often occur due to misconfigured plugins, inclusion of unnecessary dependencies, or unresolved module circular dependencies. These issues can increase application load times, reduce performance, and create runtime errors.

Root Causes

1. Improper Plugin Configuration

Misconfigured plugins, such as rollup-plugin-commonjs or rollup-plugin-node-resolve, fail to handle certain module types correctly, causing build errors or larger-than-expected bundles.

2. Unoptimized Dependencies

Including unnecessary parts of libraries or failing to tree-shake dependencies bloats the bundle size.

3. Circular Imports

Modules importing each other in a circular manner lead to runtime errors or incomplete builds.

4. Multiple Output Formats

Generating multiple output formats (e.g., ESM, CJS, UMD) without proper configuration increases build time and bundle size.

5. Mismanaged External Modules

Failing to mark external dependencies as external results in unnecessary bundling of large libraries, like react or lodash.

Diagnosing the Problem

Rollup.js provides tools and practices to diagnose build and performance issues. Use the following methods:

Inspect Bundle Size

Use the rollup-plugin-visualizer plugin to analyze bundle contents:

import visualizer from 'rollup-plugin-visualizer';

export default {
  plugins: [
    visualizer({ open: true })
  ]
};

Check Build Logs

Enable verbose logging to identify misconfigured plugins or errors:

rollup --config --verbose

Analyze Circular Imports

Use the rollup-plugin-analyzer plugin to detect circular dependencies:

import analyzer from 'rollup-plugin-analyzer';

export default {
  plugins: [
    analyzer({ summaryOnly: true })
  ]
};

Inspect Tree Shaking

Verify tree-shaking effectiveness by checking which modules are included in the bundle:

rollup --config --treeshake

Check External Dependencies

Ensure that external libraries are marked correctly in the Rollup configuration:

external: ['react', 'react-dom']

Solutions

1. Optimize Plugin Configuration

Ensure plugins are correctly configured and in the right order:

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

export default {
  input: 'src/index.js',
  output: {
    file: 'dist/bundle.js',
    format: 'esm'
  },
  plugins: [
    resolve(),
    commonjs(),
    babel({ babelHelpers: 'bundled' })
  ]
};

2. Tree-Shake Dependencies

Use named imports to enable effective tree-shaking:

// Avoid
import * as lodash from 'lodash';

// Use named imports
import { debounce } from 'lodash';

Ensure your package.json specifies the sideEffects field:

{
  "sideEffects": false
}

3. Resolve Circular Imports

Refactor modules to eliminate circular dependencies:

// Avoid circular dependencies
// a.js
import { funcB } from './b.js';
export const funcA = () => funcB();

// b.js
import { funcA } from './a.js';
export const funcB = () => funcA();

// Solution: introduce a shared module
// shared.js
export const sharedFunc = () => {};

// a.js
import { sharedFunc } from './shared.js';

// b.js
import { sharedFunc } from './shared.js';

4. Configure Output Formats

Generate multiple formats without duplicating output:

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

5. Manage External Dependencies

Mark external dependencies to exclude them from the bundle:

external: ['react', 'react-dom']

For UMD builds, specify globals:

output: {
  file: 'dist/bundle.umd.js',
  format: 'umd',
  globals: {
    react: 'React',
    'react-dom': 'ReactDOM'
  }
}

Conclusion

Large bundle sizes and broken builds in Rollup.js can be resolved by optimizing plugin configurations, enabling tree-shaking, resolving circular imports, and managing external dependencies effectively. By following best practices and leveraging diagnostic tools, developers can build efficient and maintainable JavaScript applications.

FAQ

Q1: How can I reduce bundle size in Rollup.js? A1: Use tree-shaking, configure external dependencies, and optimize imports to include only the required parts of libraries.

Q2: How do I detect circular dependencies in Rollup.js? A2: Use the rollup-plugin-analyzer plugin to identify and resolve circular dependencies in your codebase.

Q3: What is the best way to manage multiple output formats in Rollup.js? A3: Use the output array in the Rollup configuration to define multiple formats without duplicating input processing.

Q4: How can I ensure effective tree-shaking? A4: Use named imports and specify the sideEffects field in package.json to help Rollup eliminate unused code.

Q5: How do I exclude external libraries from my Rollup bundle? A5: Mark external dependencies in the Rollup configuration using the external field, and define globals for UMD builds.