port-already-in-use.md

Error: Port Already in Use (EADDRINUSE)

The Error

Error: listen EADDRINUSE: address already in use :::3000
Error: listen EADDRINUSE :::8080

Common Causes

1. Previous server process still running — didn't properly stop it
2. Another application using the port — database, system service
3. Multiple terminal tabs running the same server
4. Zombie process — server crashed but OS didn't release port

Quick Fix

# Kill whatever is on port 3000
# macOS / Linux:
kill $(lsof -t -i:3000)
# or
fuser -k 3000/tcp

# Windows:
netstat -ano | findstr :3000
taskkill /PID <PID> /F

Detailed Solution

Solution 1: Find and Kill the Process

# Find process using port (macOS/Linux)
lsof -i :3000
# COMMAND   PID   USER   TYPE
# node     1234   alice  TCP *:3000

kill 1234        # graceful
kill -9 1234     # force kill

# Find process using port (Windows)
netstat -ano | findstr :3000
# TCP    0.0.0.0:3000    0.0.0.0:0    LISTENING    5678
taskkill /PID 5678 /F

Solution 2: Use a Different Port

// app.js — use environment variable with fallback
const PORT = process.env.PORT || 3001;  // try 3001 instead
app.listen(PORT, () => console.log(`Server running on port ${PORT}`));

Solution 3: Add Graceful Shutdown

const server = app.listen(PORT);

// Handle Ctrl+C
process.on('SIGINT', () => {
  console.log('Shutting down...');
  server.close(() => process.exit(0));
});

// Handle nodemon restart
process.on('SIGTERM', () => {
  server.close(() => process.exit(0));
});

Prevention

• Always stop your dev server before closing the terminal (Ctrl+C)
• Use environment variables for ports to avoid hardcoded conflicts
• nodemon automatically handles restarts cleanly

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