How to fix Error: Server already listening on port 3000 (server.listen called twice) in Node.js

// ❌ Wrong - listen() called twice
const server = app.listen(3000);
server.listen(3000);  // Error!

// ✅ Correct - listen() called once
const server = app.listen(3000);

// Close the existing server
if (server) {
  await new Promise<void>((resolve, reject) => {
    server.close((err) => {
      if (err) reject(err);
      else resolve();
    });
  });
}

// Now you can create a new listen() call
const newServer = app.listen(3000);

const server = app.listen(3000);

// Later, close it
await new Promise<void>((resolve) => {
  server.close(() => resolve());
});

// Now you can call listen() again on a new instance
const newServer = app.listen(3000);

let serverInstance = null;

async function getServer() {
  if (serverInstance === null) {
    serverInstance = app.listen(3000, () => {
      console.log('Server started on port 3000');
    });
  }
  return serverInstance;
}

// Always use getServer() instead of calling listen() directly
const server = await getServer();
const server2 = await getServer();  // Returns the same instance

// In your module that sets up the server
if (import.meta.hot) {
  import.meta.hot.dispose(() => {
    // Cleanup code runs when the module is replaced
    server.close();
  });
}

// Only call listen() if not already listening
if (!server.listening) {
  server.listen(3000);
}

// Check if module is being reloaded
if (module.hot) {
  module.hot.dispose((data) => {
    if (server) {
      server.close();
    }
  });
}

// Only listen once
if (!server || !server.listening) {
  server = app.listen(3000);
}

// ❌ Wrong - listen() called in multiple places
describe('API Tests', () => {
  beforeEach(() => {
    server = app.listen(3000);  // Called multiple times!
  });

  test('GET /health', () => {
    // ...
  });
});

// ✅ Correct - listen() called once per test suite
describe('API Tests', () => {
  let server;

  beforeAll(() => {
    server = app.listen(3000);  // Called once
  });

  afterAll((done) => {
    server.close(done);  // Properly closed after tests
  });

  test('GET /health', () => {
    // ...
  });
});

const server = app.listen(3000);

server.on('listening', () => {
  console.log('Server is listening on port 3000');
});

server.on('error', (err) => {
  if (err.code === 'ERR_SERVER_ALREADY_LISTENING') {
    console.error('Server is already listening. Close it first before calling listen() again.');
    process.exit(1);
  } else {
    throw err;
  }
});