node-callbacks.js

var authenticateUser = function(req, res, next) {
  User.findOne({ username: req.body.username }, function(errFindUser, user) {
    if (errFindUser) return next(errFindUser);

    if (user) {
      return user.verifyPassword(req.body.password, function(errVerifyPassword, isPasswordMatch) {
        if (errVerifyPassword) return next(errVerifyPassword);

        if (isPasswordMatch) {
          return issueToken(user._id, function(errIssueToken, token) {
            if (errIssueToken) return next(errIssueToken);

            res.status(200).send(token);
          });
        }
        res.status(401).send();
      });
    }
    res.status(401).send();
  });
}