Fix "Express Server Not Starting" Issues

# macOS/Linux - Find what's using port 3000
lsof -i :3000

# Kill the process (replace PID with actual process ID)
kill -9 PID

# Or kill all processes on port 3000
lsof -ti:3000 | xargs kill -9

# Windows - Find what's using port 3000
netstat -ano | findstr :3000

# Kill the process (replace PID)
taskkill /PID PID /F

// In your Express app (app.js or server.js)
const express = require('express');
const app = express();

// Use environment variable with fallback
const PORT = process.env.PORT || 3001;

app.get('/', (req, res) => {
  res.send('Hello World!');
});

app.listen(PORT, () => {
  console.log(`Server running on port ${PORT}`);
});

# Create .env file in project root
echo "PORT=3001" > .env

# Install dotenv
npm install dotenv

# Load in your app (at the very top of app.js)
require('dotenv').config();

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

# Install Express
npm install express

# Verify it's in package.json
cat package.json | grep express

# Check if node_modules exists
ls node_modules

# If node_modules is missing or corrupted:
rm -rf node_modules package-lock.json
npm install

// Common causes and fixes:

// ❌ Wrong - file doesn't exist
const users = require('./routes/users');

// ✓ Check if file exists
// Make sure routes/users.js exists

// ❌ Wrong - incorrect path
const users = require('./users');  // but file is in routes/

// ✓ Correct - use proper relative path
const users = require('./routes/users');

// ❌ Wrong - case sensitivity (on Linux/macOS)
const users = require('./routes/Users');  // but file is users.js

// ✓ Correct - match exact case
const users = require('./routes/users');

# Check for missing dependencies
npm list

# Install all dependencies from package.json
npm install

# Install specific missing package
npm install package-name

# Clear npm cache if issues persist
npm cache clean --force
rm -rf node_modules package-lock.json
npm install

// ❌ Missing closing bracket
app.get('/users', (req, res) => {
  res.json({ users: [] });
  // Missing }

// ✓ Correct
app.get('/users', (req, res) => {
  res.json({ users: [] });
});

// ❌ Missing semicolon or comma
const config = {
  port: 3000
  host: 'localhost'  // Missing comma
}

// ✓ Correct
const config = {
  port: 3000,
  host: 'localhost'
};

// ❌ Using await without async
app.get('/data', (req, res) => {
  const result = await fetchData();  // Error!
});

// ✓ Correct
app.get('/data', async (req, res) => {
  const result = await fetchData();
  res.json(result);
});

# Run your app directly to see errors
node app.js

# Use a linter
npm install --save-dev eslint
npx eslint app.js

# For TypeScript
npm install --save-dev typescript
npx tsc --noEmit

const express = require('express');
const app = express();

// ❌ Bad - No error handling
const db = require('./db');
db.connect();  // If this fails, server crashes

// ✓ Good - With error handling
const db = require('./db');

db.connect()
  .then(() => {
    console.log('Database connected');
    startServer();
  })
  .catch(err => {
    console.error('Database connection failed:', err);
    process.exit(1);  // Exit gracefully
  });

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

// Add global error handlers
process.on('uncaughtException', (err) => {
  console.error('Uncaught Exception:', err);
  process.exit(1);
});

process.on('unhandledRejection', (err) => {
  console.error('Unhandled Promise Rejection:', err);
  process.exit(1);
});

const express = require('express');
const app = express();
const PORT = process.env.PORT || 3000;

// Try to connect to database, but don't block server startup
if (process.env.DATABASE_URL) {
  const db = require('./db');
  db.connect()
    .then(() => console.log('Database connected'))
    .catch(err => console.error('Database connection failed:', err));
} else {
  console.log('Running without database (development mode)');
}

// Start server regardless of database status
app.listen(PORT, () => {
  console.log(`Server running on port ${PORT}`);
});

// ✓ Correct - Load dotenv FIRST, before anything else
require('dotenv').config();

const express = require('express');
const app = express();

// Now environment variables are available
const PORT = process.env.PORT || 3000;
const DB_URL = process.env.DATABASE_URL;

console.log('Port:', PORT);  // Verify it's loaded

app.listen(PORT, () => {
  console.log(`Server running on port ${PORT}`);
});

# .env file in project root
PORT=3000
NODE_ENV=development
DATABASE_URL=mongodb://localhost:27017/myapp
JWT_SECRET=your-secret-key
API_KEY=your-api-key

# No quotes needed for values
# No spaces around = sign

# Check if .env is in .gitignore (it should be)
cat .gitignore | grep .env

# Make sure .env file exists in project root
ls -la | grep .env

# If .env doesn't exist, create it
cat > .env << EOF
PORT=3000
NODE_ENV=development
EOF

# macOS/Linux - Show all Express internal logs
DEBUG=express:* node app.js

# Windows CMD
set DEBUG=express:* && node app.js

# Windows PowerShell
$env:DEBUG="express:*"; node app.js

# Show only router logs
DEBUG=express:router node app.js

# Show multiple debug namespaces
DEBUG=express:*,myapp:* node app.js

require('dotenv').config();
console.log('1. Environment loaded');

const express = require('express');
console.log('2. Express loaded');

const app = express();
console.log('3. Express app created');

const PORT = process.env.PORT || 3000;
console.log('4. Port set to:', PORT);

app.use(express.json());
console.log('5. Middleware configured');

app.get('/', (req, res) => {
  console.log('6. Root route hit');
  res.send('Hello World!');
});

app.listen(PORT, () => {
  console.log('7. Server started successfully on port', PORT);
});

// If you don't see "7", the issue is before app.listen()

# Start with inspector
node --inspect app.js

# Or with breakpoint on first line
node --inspect-brk app.js

# Open Chrome and go to:
chrome://inspect

# Click "inspect" on your Node process
# You can now set breakpoints and step through code

# Check current Node.js version
node -v

# Express 4.x requires Node.js 0.10 or higher
# Express 5.x (beta) requires Node.js 14.x or higher

# If version is too old, update Node.js:
# macOS (using Homebrew)
brew upgrade node

# Linux (using nvm)
nvm install --lts
nvm use --lts

# Windows - Download from nodejs.org

# Create .nvmrc file
echo "18.17.0" > .nvmrc

# Team members can then use:
nvm use

# This ensures everyone uses same Node version