Javascript 使用mongooschema.pre（'；save'；）方法时node.js中未处理的承诺拒绝

当我向/注册路由发出post请求时，我在nodejs版本8.1.2中遇到了以下错误：

（节点：8317）未处理的PromisejectionWarning:未处理的承诺拒绝（拒绝id:1）：错误：cb必须是函数或 回信无效

如果我删除mongoose pre方法，则该路由工作正常。

---

但是我想散列密码。请在这个问题上帮助我。谢谢你的预保存回调使用了一个名为“done”的函数，但是你调用的是“next”

应该是：

//This is the user model
const mongoose = require('mongoose');
const bcrypt = require('bcrypt');
const saltRounds = 10;
const userSchema = mongoose.Schema({
  email: {
    type: String,
    index: {
      unique: true
    }
  },
  password: {
    type: String
  }
});

userSchema.pre("save", function(next) {
  var user = this;
  if (!user.isModified("password")) {
    return next();
  }

  bcrypt.genSalt(saltRounds, function(err, salt) {
    if (err) {
      return next(err);
    }
    bcrypt.hash(user.password, salt, salt, function(err, hashedPassword) {
      if (err) {
        return next(err)
      }
      user.password = hashedPassword;
      next();
    });
  });
});

userSchema.methods.checkPassword = function(guess, cb) {
  bcrypt.compare(guess, this.password, function(err, isMatch) {
    cb(err, isMatch);
  });
};
const User = mongoose.model('User', userSchema);
module.exports = User;


//The code below  is the /signup route
router.post('/signup', (req, res) => {
  User.findOne({
    email: req.body.email
  }, (err, user) => {
    if (err) {
      throw err;
    }

    if (user) {
      // handle case for user already exists!!
      res.json({
        success: false,
        message: 'An account with this email already exists'
      });
    } else {
      var newUser = new User({
        email: req.body.email,
        password: req.body.password
      });

      newUser.save((err) => {
        if (err) {
          return res.send(err);
        }
        let jwtData = {
          email: req.body.email
        };

        let token = jwt.sign(jwtData, jwtSecret);

        res.json({
          success: true,
          token: token
        });
      });
    }
  });
});

userSchema.pre("save", function(next) {
    ...
}