node-generators.js

const authenticateUser = function(req, res, next) {
  Promise.coroutine(function*() {
    try {
      let user = yield User.findOne({ username: req.body.username });

      if (user) {
        let isPasswordMatch = yield user.verifyPassword(req.body.password);

        if (isPasswordMatch) {
          let token = yield issueToken(user._id);
          res.status(200).send(token);
          return;
        }
      }
      res.status(401).send();
    } catch (err) {
      next(err);
    }
  })();
}