const username = req.body.username;
const password = req.body.password;

if (!username) {
  throw new Error('Missing username');
}

if (!password) {
  throw new Error('Missing password');
}

User.findOne({ username, password }).then(user => {
  res.json({ user });
}).catch(err => {
  res.json({ err });
});
	
const username = req.body.username;
const password = req.body.password;

if (!username) {
  res.json({ err: 'Missing username' });
}

if (!password) {
  res.json({ err: 'Missing password' });
}

User.findOne({ username, password }).then(user => {
  res.json({ user });
}).catch(err => {
  res.json({ err });
});