How to fix Error: Cannot set headers after they are sent to the client in Node.js

// WRONG
app.get('/user', (req, res) => {
  res.json({ id: 1, name: 'John' });
  res.setHeader('X-Custom', 'value'); // Error: headers already sent
});

// CORRECT
app.get('/user', (req, res) => {
  res.json({ id: 1, name: 'John' });
  return; // Prevents further execution
});

// OR more idiomatically
app.get('/user', (req, res) => {
  return res.json({ id: 1, name: 'John' });
});

// WRONG - both paths execute
app.get('/data', (req, res) => {
  if (req.query.format === 'json') {
    res.json({ data: 'value' });
  }
  if (!req.query.format) {
    res.send('No format specified');
  }
});

// CORRECT - only one path executes
app.get('/data', (req, res) => {
  if (req.query.format === 'json') {
    return res.json({ data: 'value' });
  }
  if (!req.query.format) {
    return res.send('No format specified');
  }
  res.status(400).send('Invalid format');
});

// WRONG
app.get('/data', (req, res) => {
  res.send('Hello World');
  res.setHeader('Content-Type', 'text/plain'); // Too late!
});

// CORRECT
app.get('/data', (req, res) => {
  res.setHeader('Content-Type', 'text/plain');
  res.send('Hello World');
});

// Also correct - headers implicitly set by json()
app.get('/api/data', (req, res) => {
  res.json({ message: 'Hello' }); // Sets Content-Type: application/json
});

// WRONG - response could be sent twice
app.get('/user/:id', (req, res) => {
  User.findById(req.params.id, (err, user) => {
    if (err) {
      res.status(500).send('Error finding user');
    }
    res.json(user); // Might execute even if error was sent
  });
});

// CORRECT - early return prevents double responses
app.get('/user/:id', (req, res) => {
  User.findById(req.params.id, (err, user) => {
    if (err) {
      return res.status(500).send('Error finding user');
    }
    return res.json(user);
  });
});

// With async/await (cleaner)
app.get('/user/:id', async (req, res) => {
  try {
    const user = await User.findById(req.params.id);
    return res.json(user);
  } catch (err) {
    return res.status(500).send('Error finding user');
  }
});

app.get('/data', (req, res) => {
  // ... some processing ...

  if (!res.headersSent) {
    res.setHeader('X-Custom', 'value');
  }

  // Send response
  res.json({ data: 'value' });
});

// In middleware
app.use((req, res, next) => {
  // Only add custom header if response hasn't been sent yet
  if (!res.headersSent) {
    res.setHeader('X-Request-ID', generateId());
  }
  next();
});

// WRONG - header middleware comes after route that sends response
app.get('/data', (req, res) => {
  res.send('Data');
});

app.use((req, res, next) => {
  res.setHeader('X-Custom', 'value');
  next();
});

// CORRECT - header middleware before routes
app.use((req, res, next) => {
  res.setHeader('X-Custom', 'value');
  next();
});

app.get('/data', (req, res) => {
  res.send('Data');
});