Debugging Chunk Issues After Turbopack Migration in Next.js 15
Since the issues only started after adding Turbopack and migrating to Next.js 15, this significantly narrows down the potential causes. Turbopack was marked stable in Next.js 15 1 and has different bundling strategies compared to webpack.
Understanding Turbopack vs Webpack Differences
- Different chunking strategies: Turbopack uses a different algorithm for code splitting than webpack
- Module resolution differences: Turbopack may resolve dependencies differently
- Optimization differences: Turbopack has different default optimizations
Debugging Turbopack-Specific Issues
1. Check if Turbopack is the cause
First, confirm if Turbopack is indeed causing the issue by temporarily disabling it:
// next.config.js
module.exports = {
// Disable Turbopack for testing
webpack: (config, { isServer }) => {
// Your existing webpack config
return config;
},
// Explicitly disable Turbopack
experimental: {
turbo: false
}
}If the chunk loading errors disappear when reverting to webpack, you've confirmed Turbopack is the cause.
2. Use Turbopack-specific debugging
Next.js 15 includes improved debugging tools for Turbopack 2:
# Enable Turbopack debug logs
TURBOPACK_DEBUG=1 yarn dev
# For more verbose output
TURBOPACK_DEBUG_TRACE=1 yarn dev3. Check for known Turbopack issues
Look at the Next.js GitHub repository for known issues with Turbopack in Next.js 15. There have been several reported issues with chunk loading after migration.
4. Analyze Turbopack output
Turbopack generates different output files than webpack. Check:
# Turbopack output directory
ls -la .next/server/chunks
ls -la .next/static/chunks5. Create a Turbopack-specific error monitor
app/turbopack-monitor.tsx
Solutions for Turbopack Chunk Issues
Option 1: Revert to webpack temporarily
If you need a quick fix while investigating:
// next.config.js
module.exports = {
// Explicitly use webpack instead of Turbopack
experimental: {
turbo: false
}
}Option 2: Adjust Turbopack settings
Next.js 15 allows some Turbopack customization:
// next.config.js
module.exports = {
experimental: {
turbo: {
// Turbopack-specific options
resolveAlias: {
// Custom module resolutions if needed
}
}
}
}Option 3: Optimize client components
Since Turbopack handles client/server component splitting differently:
- Review large client components (with
'use client'directive) - Consider moving more logic to server components
- Use dynamic imports with
next/dynamicfor large client components
Option 4: Update dependencies
Some third-party packages may have compatibility issues with Turbopack:
# Update all dependencies to latest versions
yarn upgrade-interactive --latestOption 5: Use Next.js 15's optimizePackageImports
This feature can help with large packages:
// next.config.js
module.exports = {
experimental: {
optimizePackageImports: [
'lodash', 'date-fns', 'react-icons',
// Add other large libraries you use
]
}
}Next Steps for Debugging
- Identify the specific components causing issues by adding the TurbopackErrorMonitor
- Compare bundle output between webpack and Turbopack builds
- Check for dynamic imports that might be handled differently by Turbopack
- Review third-party dependencies that might be incompatible with Turbopack
Since this is a Turbopack-specific issue that wasn't occurring before, focusing on the differences between webpack and Turbopack will be key to resolving it.
Debugging "ChunkLoadError: Loading chunk 6603 failed" in Next.js 15 with Turbopack
This specific error indicates that your browser tried to load chunk ID 6603 but failed. Let's identify what's in this chunk and fix the issue.
1. Identify What's in Chunk 6603
Create a script to find what's in this specific chunk:
scripts/inspect-chunk-6603.js
Run this script to see what's in chunk 6603:
node scripts/inspect-chunk-6603.js2. Common Causes of Chunk Load Failures in Turbopack
- Large dependencies: Turbopack might be creating chunks that are too large
- Network issues: The chunk might be failing to load due to network problems
- Caching issues: Browser cache might be serving a stale version of the chunk
- Code splitting differences: Turbopack splits code differently than webpack
- Dynamic imports: Issues with dynamically imported modules
3. Add Error Monitoring to Capture More Details
Add this to a client component that's loaded on all pages:
app/components/chunk-error-logger.tsx
Add this component to your layout:
// In your layout.tsx
import { ChunkErrorLogger } from './components/chunk-error-logger';
export default function Layout({ children }) {
return (
<html>
<body>
<ChunkErrorLogger />
{children}
</body>
</html>
);
}4. Specific Fixes for Chunk 6603 Error
Solution 1: Disable Turbopack temporarily
// next.config.js
module.exports = {
// Your existing config
experimental: {
// Disable Turbopack
turbo: false
}
}Solution 2: Adjust chunk size limits
If you identify that chunk 6603 is too large, adjust your webpack configuration:
// next.config.js
module.exports = {
webpack: (config, { isServer }) => {
if (!isServer) {
config.optimization.splitChunks = {
chunks: 'all',
maxInitialRequests: 25, // Increased from 20
minSize: 20000, // Decreased from 25000
maxSize: 400000, // Decreased from 500000
// Rest of your config...
};
}
return config;
}
}Solution 3: Identify and lazy-load the problematic component
If you identified a specific component in chunk 6603, convert it to use dynamic import:
// Before
import LargeComponent from './large-component';
// After
import dynamic from 'next/dynamic';
const LargeComponent = dynamic(() => import('./large-component'), {
loading: () => <div>Loading...</div>
});Solution 4: Clear browser caches
Have users clear their browser cache or use incognito mode to test. You can also add a version query parameter to force a fresh load:
// next.config.js
module.exports = {
generateBuildId: async () => {
// This forces new chunk URLs on each build
return Date.now().toString();
}
}Solution 5: Check for circular dependencies
Circular dependencies can cause issues with code splitting. Use a tool to check:
npx madge --circular --extensions js,jsx,ts,tsx .5. Prevent This Issue in the Future
- Monitor chunk sizes: Regularly check your bundle sizes
- Implement progressive loading: Load less critical components on demand
- Use React.lazy and Suspense: For client components that might be large
- Consider server components: Move more logic to server components
- Add error boundaries: Catch and handle chunk load errors gracefully
app/components/chunk-error-boundary.tsx
By following these steps, you should be able to identify what's in chunk 6603, why it's failing to load, and implement a solution to fix the issue.