Quickly Get Started with Debugging the Next.js Source Code

In daily development, we often need to delve deeper into Next.js’s internal implementation to solve problems or optimize performance. This article will introduce a simple and practical method for debugging the Next.js source code.

Environment Setup

First, we need to obtain the Next.js source code and set up the development environment.

1. Clone Repository

Use --depth=1 to clone only the latest code, which speeds up the download:

git clone [email protected]:vercel/next.js.git --depth=1
cd next.js

2. Install Dependencies and Start Development Mode

pnpm install
pnpm dev

At this point, Next.js source code is in watch mode, and any modifications will automatically trigger recompilation.

Start Debugging

1. Modify Source Code

Now we can modify the Next.js source code. For example, let's add a log at the entry point of request handling:

1. Open file: packages/next/src/server/base-server.ts
2. Find the handleRequest method
3. Add a log line:

async handleRequest(req: BaseNextRequest, res: BaseNextResponse, parsedUrl?: UrlWithParsedQuery) {
  console.log('hello'); // Add this line
  // ... original code
}

2. Use Local Next.js in Example Project

To test our modifications, we need to make the example project use our local Next.js version:

1. Enter example project:

cd examples/hello-world

2. Modify package.json to point next dependency to local file:

{
  "dependencies": {
    "next": "file:../.."
  }
}

3. Reinstall dependencies and start the project:

pnpm install
pnpm dev

3. Verify Changes

Now when you visit http://localhost:3000, the console should print "hello", indicating our modification is working.

Debugging Tips

1. Hot Reload: In development mode, source code modifications will automatically recompile without requiring service restart.

2. Core Files to Focus On:

   • packages/next/src/server/ - Server-side related code
   • packages/next/src/client/ - Client-side related code
   • packages/next/src/build/ - Build related code

3. Use console: Adding console.log at key points is the most direct debugging method.

Summary

By using this approach, you can conveniently debug and understand the Next.js source code. This is particularly helpful for:

• Troubleshooting complex problems
• Understanding the framework’s underlying principles
• Contributing code to Next.js

Remember, the source code is the best documentation. When you encounter issues, directly looking at the source often leads you to the answer.