Bundle Analyzer

Analyzes JavaScript bundle sizes, identifies optimization opportunities, and helps reduce bundle size for faster page loads.

When to Use

• "Analyze my bundle size"
• "Why is my bundle so large?"
• "Optimize webpack bundle"
• "Reduce bundle size"
• "Find large dependencies"
• "Setup bundle analysis"

Instructions

1. Detect Build Tool

Check which bundler is being used:

# Check package.json
grep -E "(webpack|vite|rollup|parcel|esbuild)" package.json

# Check config files
[ -f "webpack.config.js" ] && echo "Webpack"
[ -f "vite.config.js" ] && echo "Vite"
[ -f "rollup.config.js" ] && echo "Rollup"

2. Install Analysis Tool

For Webpack:

npm install --save-dev webpack-bundle-analyzer

For Vite:

npm install --save-dev rollup-plugin-visualizer

For Rollup:

npm install --save-dev rollup-plugin-visualizer

Cross-platform:

npm install --save-dev source-map-explorer

3. Configure Analysis

Webpack

webpack.config.js:

const { BundleAnalyzerPlugin } = require('webpack-bundle-analyzer')

module.exports = {
  // ... other config
  plugins: [
    new BundleAnalyzerPlugin({
      analyzerMode: 'static',
      reportFilename: 'bundle-report.html',
      openAnalyzer: true,
      generateStatsFile: true,
      statsFilename: 'bundle-stats.json'
    })
  ]
}

Or for conditional analysis:

const { BundleAnalyzerPlugin } = require('webpack-bundle-analyzer')

module.exports = {
  plugins: [
    process.env.ANALYZE && new BundleAnalyzerPlugin()
  ].filter(Boolean)
}

package.json scripts:

{
  "scripts": {
    "build": "webpack",
    "build:analyze": "ANALYZE=true webpack",
    "analyze": "webpack-bundle-analyzer dist/stats.json"
  }
}

Vite

vite.config.js:

import { defineConfig } from 'vite'
import { visualizer } from 'rollup-plugin-visualizer'

export default defineConfig({
  plugins: [
    visualizer({
      open: true,
      gzipSize: true,
      brotliSize: true,
      filename: 'dist/stats.html'
    })
  ],
  build: {
    rollupOptions: {
      output: {
        manualChunks: {
          vendor: ['react', 'react-dom'],
          utils: ['lodash', 'date-fns']
        }
      }
    }
  }
})

Next.js

next.config.js:

const { ANALYZE } = process.env

module.exports = {
  webpack: (config, { isServer }) => {
    if (ANALYZE) {
      const { BundleAnalyzerPlugin } = require('webpack-bundle-analyzer')
      config.plugins.push(
        new BundleAnalyzerPlugin({
          analyzerMode: 'static',
          reportFilename: isServer
            ? '../analyze/server.html'
            : './analyze/client.html'
        })
      )
    }
    return config
  }
}

Or use @next/bundle-analyzer:

npm install --save-dev @next/bundle-analyzer

const withBundleAnalyzer = require('@next/bundle-analyzer')({
  enabled: process.env.ANALYZE === 'true'
})

module.exports = withBundleAnalyzer({
  // Next.js config
})

package.json:

{
  "scripts": {
    "analyze": "ANALYZE=true next build"
  }
}

Create React App

npm install --save-dev source-map-explorer

package.json:

{
  "scripts": {
    "analyze": "source-map-explorer 'build/static/js/*.js'"
  }
}

4. Analyze Bundle

Run analysis:

npm run build:analyze
# or
npm run analyze

Generate report showing:

• Total bundle size
• Breakdown by module
• Treemap visualization
• Gzipped sizes
• Duplicate dependencies

5. Identify Issues

Common issues to look for:

1. Large Dependencies

   • Moment.js (use date-fns or dayjs instead)
   • Lodash (use lodash-es or individual functions)
   • Full libraries when only using small parts

2. Duplicate Dependencies

   • Same package included multiple times
   • Different versions of same package

3. Unused Code

   • Dead code not tree-shaken
   • CSS/JS not actually used

4. Large Images/Assets

   • Images not optimized
   • SVGs not compressed

5. Development Code in Production

   • Console logs
   • Dev-only packages
   • Source maps in production

6. Suggest Optimizations

Optimization Strategies

1. Code Splitting

// Dynamic imports
const HeavyComponent = lazy(() => import('./HeavyComponent'))

// Route-based splitting
const Dashboard = lazy(() => import('./pages/Dashboard'))
const Settings = lazy(() => import('./pages/Settings'))

// Webpack magic comments
const module = import(
  /* webpackChunkName: "my-chunk" */
  /* webpackPrefetch: true */
  './module'
)

2. Tree Shaking

// ❌ BAD: Imports entire library
import _ from 'lodash'

// ✅ GOOD: Import only what you need
import debounce from 'lodash/debounce'
import throttle from 'lodash/throttle'

// ✅ BETTER: Use lodash-es for tree-shaking
import { debounce, throttle } from 'lodash-es'

3. Replace Large Libraries

// ❌ BAD: Moment.js (heavy)
import moment from 'moment'

// ✅ GOOD: date-fns (modular)
import { format, parseISO } from 'date-fns'

// ✅ GOOD: dayjs (lightweight)
import dayjs from 'dayjs'

4. Lazy Load Routes (React Router)

import { lazy, Suspense } from 'react'
import { BrowserRouter, Routes, Route } from 'react-router-dom'

const Home = lazy(() => import('./pages/Home'))
const About = lazy(() => import('./pages/About'))
const Dashboard = lazy(() => import('./pages/Dashboard'))

function App() {
  return (
    <BrowserRouter>
      <Suspense fallback={<div>Loading...</div>}>
        <Routes>
          <Route path="/" element={<Home />} />
          <Route path="/about" element={<About />} />
          <Route path="/dashboard" element={<Dashboard />} />
        </Routes>
      </Suspense>
    </BrowserRouter>
  )
}

5. Manual Chunks (Vite/Rollup)

// vite.config.js
export default {
  build: {
    rollupOptions: {
      output: {
        manualChunks(id) {
          // Vendor chunk for node_modules
          if (id.includes('node_modules')) {
            if (id.includes('react') || id.includes('react-dom')) {
              return 'react-vendor'
            }
            if (id.includes('@mui')) {
              return 'mui-vendor'
            }
            return 'vendor'
          }
        }
      }
    }
  }
}

6. Externalize Dependencies (CDN)

// webpack.config.js
module.exports = {
  externals: {
    react: 'React',
    'react-dom': 'ReactDOM',
    lodash: '_'
  }
}

<!-- index.html -->
<script src="https://cdn.jsdelivr.net/npm/react@18/umd/react.production.min.js"></script>
<script src="https://cdn.jsdelivr.net/npm/react-dom@18/umd/react-dom.production.min.js"></script>

7. Optimize Images

// next.config.js
module.exports = {
  images: {
    formats: ['image/avif', 'image/webp'],
    deviceSizes: [640, 750, 828, 1080, 1200, 1920],
  }
}

// Use next/image
import Image from 'next/image'

<Image
  src="/photo.jpg"
  width={500}
  height={300}
  alt="Photo"
/>

8. Remove Unused CSS

# Install PurgeCSS
npm install --save-dev @fullhuman/postcss-purgecss

// postcss.config.js
module.exports = {
  plugins: [
    require('@fullhuman/postcss-purgecss')({
      content: ['./src/**/*.{js,jsx,ts,tsx}'],
      defaultExtractor: content => content.match(/[\w-/:]+(?<!:)/g) || []
    })
  ]
}

9. Compression

// webpack.config.js
const CompressionPlugin = require('compression-webpack-plugin')

module.exports = {
  plugins: [
    new CompressionPlugin({
      algorithm: 'gzip',
      test: /\.(js|css|html|svg)$/,
      threshold: 10240,
      minRatio: 0.8
    })
  ]
}

10. Environment-Specific Code

// webpack.config.js
const webpack = require('webpack')

module.exports = {
  plugins: [
    new webpack.DefinePlugin({
      'process.env.NODE_ENV': JSON.stringify('production')
    })
  ]
}

// In code
if (process.env.NODE_ENV !== 'production') {
  // This will be removed in production build
  console.log('Development mode')
}

7. Size Budgets

webpack.config.js:

module.exports = {
  performance: {
    maxEntrypointSize: 250000, // 250kb
    maxAssetSize: 250000,
    hints: 'warning'
  }
}

package.json with size-limit:

npm install --save-dev size-limit @size-limit/preset-app

{
  "size-limit": [
    {
      "path": "dist/bundle.js",
      "limit": "300 KB"
    },
    {
      "path": "dist/vendor.js",
      "limit": "200 KB"
    }
  ]
}

8. CI/CD Integration

GitHub Actions:

name: Bundle Size Check

on: [pull_request]

jobs:
  size:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v3
      - uses: actions/setup-node@v3
      - run: npm ci
      - run: npm run build

      - name: Check bundle size
        uses: andresz1/size-limit-action@v1
        with:
          github_token: ${{ secrets.GITHUB_TOKEN }}

9. Monitoring

Set up continuous monitoring:

// Report to analytics
if (typeof window !== 'undefined' && window.performance) {
  const perfData = window.performance.getEntriesByType('resource')

  perfData.forEach(entry => {
    if (entry.name.includes('.js')) {
      console.log(`${entry.name}: ${(entry.transferSize / 1024).toFixed(2)} KB`)
    }
  })
}

Lighthouse CI:

# .lighthouserc.js
module.exports = {
  ci: {
    assert: {
      assertions: {
        'total-byte-weight': ['error', { maxNumericValue: 1000000 }],
        'resource-summary:script:size': ['error', { maxNumericValue: 500000 }]
      }
    }
  }
}

10. Generate Report

Create comprehensive report:

# Bundle Analysis Report

## Summary
- Total bundle size: 450 KB (gzipped: 150 KB)
- Number of chunks: 5
- Largest chunk: vendor.js (200 KB)

## Top 10 Largest Dependencies
1. moment.js - 72 KB (❌ Consider replacing with date-fns)
2. lodash - 65 KB (⚠️ Use lodash-es for tree-shaking)
3. @mui/material - 120 KB (✅ Already code-split)
4. react-dom - 40 KB (✅ Essential)
5. chart.js - 35 KB (⚠️ Lazy load if not on first page)

## Optimization Opportunities
- [ ] Replace moment.js with date-fns (-50 KB)
- [ ] Use lodash-es instead of lodash (-30 KB)
- [ ] Lazy load chart.js (-35 KB)
- [ ] Code split routes (-80 KB initial load)
- [ ] Remove unused CSS (-15 KB)

## Estimated Savings
Total potential reduction: 210 KB (47% smaller)
New bundle size: 240 KB

## Action Items
1. Immediate: Replace moment.js
2. Short-term: Implement route-based code splitting
3. Long-term: Audit and remove unused dependencies

Best Practices

DO:

• Analyze on every major release
• Set size budgets and enforce them
• Use code splitting for routes
• Lazy load below-the-fold content
• Tree-shake effectively
• Monitor bundle size in CI/CD

DON'T:

• Import entire libraries
• Ignore duplicate dependencies
• Skip production optimizations
• Forget to compress assets
• Include dev dependencies in production
• Use large polyfills unnecessarily

Size Targets

General Guidelines:

• Main bundle: < 200 KB (gzipped)
• Vendor bundle: < 300 KB (gzipped)
• Total page weight: < 1 MB
• Time to Interactive: < 3 seconds on 3G

Mobile-first:

• First bundle: < 100 KB (gzipped)
• Critical CSS: < 14 KB (first TCP round)
• Above-fold images: < 200 KB

Analysis Checklist

• Bundle analysis tool installed
• Baseline measurements taken
• Large dependencies identified
• Duplicate dependencies found
• Code splitting implemented
• Tree shaking verified
• Size budgets set
• CI/CD checks added
• Production build optimized
• Monitoring in place