Error: Cannot set headers after they are sent to the client

What causes this error

Once Express sends a response, the HTTP request is considered complete. Any further attempt to call `res.send()`, `res.json()`, `res.redirect()`, etc. on that same response object throws this error — Node is telling you it already flushed headers to the client.

The Fix

// Wrong — missing return, both branches can execute
app.get('/user/:id', async (req, res) => {
  const user = await getUser(req.params.id);
  if (!user) {
    res.status(404).json({ message: 'Not found' });
  }
  res.json(user); // still runs even after the 404!
});

// Correct
app.get('/user/:id', async (req, res) => {
  const user = await getUser(req.params.id);
  if (!user) {
    return res.status(404).json({ message: 'Not found' });
  }
  return res.json(user);
});

Common causes / variations

• Missing `return` before a conditional response inside an async handler
• Calling `next()` after already sending a response
• A timeout/error handler firing after the main handler already responded

Related errors

See also: req.body is undefined.