When you start your Node.js server and see this error, another process is already using that port.

**Error Message:**
```
Error: listen EADDRINUSE: address already in use :::3000
```

## Quick Solution

### Mac/Linux

Find the process:
```bash
lsof -i :3000
```

Output shows:
```
node    12345 user   23u  IPv6 ... TCP *:3000 (LISTEN)
```

Kill it:
```bash
kill -9 12345
```

One-liner:
```bash
kill -9 $(lsof -t -i:3000)
```

### Windows

Find the process:
```bash
netstat -ano | findstr :3000
```

Kill it:
```bash
taskkill /PID 12345 /F
```

## Better Solution: Auto Port Selection

Instead of hardcoding port 3000, try the next available port:

```javascript
const http = require('http');

function startServer(port) {
  const server = http.createServer(app);
  
  server.listen(port)
    .on('listening', () => {
      console.log(`Server on port ${port}`);
    })
    .on('error', (err) => {
      if (err.code === 'EADDRINUSE') {
        console.log(`Port ${port} busy, trying ${port + 1}`);
        startServer(port + 1);
      }
    });
}

startServer(3000);
```

## Environment Variables

Use flexible port configuration:

```javascript
const PORT = process.env.PORT || 3000;
app.listen(PORT, () => {
  console.log(`Server running on port ${PORT}`);
});
```

Run with custom port:
```bash
PORT=3001 node server.js
```

## Common Causes

| Cause | Solution |
|-------|----------|
| Previous server still running | Kill the process |
| Multiple server instances | Check running processes |
| Docker container active | Stop container |
| nodemon didn't clean up | Restart nodemon |

## Proper Server Cleanup

Add signal handlers to close server properly:

```javascript
const server = app.listen(3000);

process.on('SIGINT', () => {
  server.close(() => {
    console.log('Server closed');
    process.exit(0);
  });
});

process.on('SIGTERM', () => {
  server.close(() => {
    process.exit(0);
  });
});
```

Now Ctrl+C properly shuts down the server.

## NPM Script Helper

Add to package.json:

```json
{
  "scripts": {
    "kill-port": "kill -9 $(lsof -t -i:3000) 2>/dev/null || true",
    "start": "npm run kill-port && node server.js"
  }
}
```

## Docker Issues

If using Docker and port is still busy:

```bash
docker ps                    # List containers
docker stop <container-id>   # Stop specific container
docker rm <container-id>     # Remove container
```

## Prevention Tips

1. Always handle SIGINT and SIGTERM signals
2. Use environment variables for ports
3. Implement auto port selection for development
4. Clean up Docker containers regularly
5. Use process managers like PM2 in production

This error usually means a previous instance didn't shut down cleanly. The solutions above handle both the immediate fix and prevention.

Getting EADDRINUSE error? Your port is taken. Here's how to find what's using it and fix it properly.

Error: listen EADDRINUSE - Port Already in Use in Node.js

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Error: listen EADDRINUSE - Port Already in Use in Node.js