How do you handle a resource not found in a REST API?

Respond with HTTP status 404 and an error message body when the requested resource does not exist.

app.get('/users/:id', (req, res) => {
  const user = users.find(u => u.id === req.params.id);
  if (!user) {
    return res.status(404).json({
      error: 'User not found'
    });
  }
  res.json(user);
});

Use appropriate status codes: 400 (bad request), 401 (unauthorized), 404 (not found), 500 (server error).