Fix: Error when starting dev server: port already in use error

The ‘Error when starting dev server: port already in use’ error occurs when trying to start a development server on a port that’s already being used by another process or server instance.

How the Error Happens

❌ Error Scenario:

# ❌ This causes the error
npm run dev
# Error: Port 3000 is already in use

✅ Quick Fix - Resolve Port Conflicts

Solution 1: Find and Kill Process Using the Port

# ✅ Find process using port 3000 (Linux/Mac)
lsof -i :3000

# ✅ Kill the process
kill -9 PID  # Replace PID with actual process ID

# ✅ On Windows
netstat -ano | findstr :3000
taskkill /PID PID /F  # Replace PID with actual process ID

Solution 2: Use Different Port

# ✅ Start server on different port
npm run dev -- --port 3001

# ✅ For Next.js
PORT=3001 npm run dev

# ✅ For Vite
vite --port 3001

# ✅ For React
PORT=3001 npm start

Solution 3: Configure Port in Environment

// ✅ In package.json scripts
{
  "scripts": {
    "dev": "vite --port 3002",
    "dev:alt": "vite --port 4000"
  }
}

Solution 4: Auto-assign Available Port

// ✅ For custom servers, auto-assign port
const express = require('express');
const app = express();

const PORT = process.env.PORT || 3000;

app.listen(PORT, () => {
  console.log(`Server running on port ${PORT}`);
}).on('error', (err) => {
  if (err.code === 'EADDRINUSE') {
    console.log(`Port ${PORT} is busy, trying ${PORT + 1}`);
    app.listen(PORT + 1);
  }
});