User.pre('save', function (next) {
  const user = this;
  if (this.isModified('password') || this.isNew) {
    bcrypt.genSalt(10, function (err, salt) {
      if (err) {
        return next(err);
      }
      bcrypt.hash(user.password, salt, function (err, hash) {
        if (err) {
          return next(err);
        }
        user.password = hash;
        next();
      });
    });
  } else {
    return next();
  }
});

User.pre('save', (next) => {
  const user = this;
  if (this.isModified('password') || this.isNew) {
    bcrypt.genSalt(10, (err, salt) => {
      if (err) {
        return next(err);
      }
      bcrypt.hash(user.password, salt, (err, hash) => {
        if (err) {
          return next(err);
        }
        user.password = hash;
        next();
      });
    });
  } else {
    return next();
  }
});