1. Next.js Build Failures on CodeSandbox

Understanding the Issue

When running a Next.js project on CodeSandbox, builds may fail due to missing dependencies, incorrect module resolution, or sandbox limitations.

Root Causes

• CodeSandbox uses a containerized environment that may not have all dependencies pre-installed.
• Package.json might be missing required dependencies or using incorrect versions.
• CodeSandbox's default settings may not support Next.js's custom Webpack configurations.

Fix

Ensure all dependencies are explicitly listed in package.json:

{
  "dependencies": {
    "next": "latest",
    "react": "latest",
    "react-dom": "latest"
  }
}

Manually trigger dependency installation by running:

npm install

2. Environment Variables Not Loading

Understanding the Issue

Next.js applications often require environment variables, but CodeSandbox does not automatically load .env files like local development environments.

Root Causes

• CodeSandbox does not support environment variables natively for security reasons.
• Next.js environment variables must be prefixed with NEXT_PUBLIC_ to be exposed to the frontend.

Fix

Store environment variables in the sandbox.config.json file:

{
  "env": {
    "NEXT_PUBLIC_API_URL": "https://api.example.com"
  }
}

Alternatively, define environment variables directly in next.config.js:

module.exports = {
  env: {
    NEXT_PUBLIC_API_URL: "https://api.example.com",
  },
};

3. API Routes Not Working in CodeSandbox

Understanding the Issue

Next.js API routes may return 404 errors or fail to execute in the CodeSandbox environment.

Root Causes

• CodeSandbox's containerized environment may not support certain backend configurations.
• API routes require a proper local server setup that may not be available.

Fix

Ensure API routes are placed inside /pages/api:

// pages/api/hello.js
export default function handler(req, res) {
  res.status(200).json({ message: "Hello from API!" });
}

Manually start the Next.js development server in CodeSandbox by adding this script to package.json:

{
  "scripts": {
    "dev": "next dev -p 3000"
  }
}

4. Slow Performance or Sandbox Freezing

Understanding the Issue

CodeSandbox sometimes becomes unresponsive or experiences high latency when running Next.js applications, especially those with large dependencies.

Root Causes

• Next.js projects require significant resources, which may exceed CodeSandbox's limitations.
• Too many dependencies loaded at runtime.

Fix

Reduce unnecessary dependencies and use lightweight builds:

npm ci --production

Disable telemetry to prevent extra network requests:

npx next telemetry disable

5. Deploying Next.js from CodeSandbox

Understanding the Issue

Deploying a Next.js project from CodeSandbox to Vercel or another cloud provider may fail due to missing configurations.

Root Causes

• CodeSandbox's automated deployments may not match Next.js's production build requirements.
• Environment variables may not persist across deployments.

Fix

Export the Next.js project and deploy manually:

npx next build && npx next export

Ensure Vercel detects the correct build command:

{
  "buildCommand": "npm run build"
}

Conclusion

CodeSandbox is a powerful cloud-based development tool, but Next.js projects require additional configuration to run smoothly. By ensuring proper dependency management, handling API routes correctly, optimizing performance, and configuring environment variables, developers can avoid common pitfalls and streamline their Next.js development experience on CodeSandbox.

FAQs

1. Why is my Next.js project not building in CodeSandbox?

Ensure that all dependencies are explicitly listed in package.json and try running npm install manually.

2. How do I fix missing environment variables in CodeSandbox?

Use sandbox.config.json or define variables inside next.config.js to ensure they are loaded properly.

3. Why are my API routes returning 404 in CodeSandbox?

Ensure API routes are inside /pages/api and that the development server is running on the correct port.

4. How can I improve the performance of Next.js on CodeSandbox?

Disable unnecessary telemetry, reduce dependencies, and optimize build processes using npm ci --production.

5. How do I deploy a Next.js project from CodeSandbox?

Export the project manually using npx next build && npx next export and deploy it to Vercel or another cloud provider.