我对MERN堆栈开发还是个新手。我有一个http：//localhost：8000/api/signin端点，在那里提交电子邮件和密码。后端代码首先检查用户是否已经保存在MongoDB中，然后将端点中提交的密码与数据库（mongoDB）中保存的密码进行比较。

/controllers/auth.js

exports.signin = (req, res) => {
  const { email, password } = req.body;
  const signInUser = async () => {
    try {
      // Check if user exists
      let findUser = await User.findOne({ email }); // promise is the new way of doing this instead of callback as of express 4.18 and mongoose 7.0.3

      console.log('findUser', findUser);

      if (!findUser) {
        return res.status(400).json({
          error: 'User with that email does not exist. Please signup'
        })
      }

      // authenticate
      if (!findUser.authenticate(password)) {
        return res.status(400).json({
          error: 'Email and password do not match'
        })
      }

      // generate a token and send to client
      const token = jwt.sign({_id: findUser._id}, process.env.JWT_SECRET, { expiresIn: '7d' });
      const {_id, name, email, role} = existingUser;

     return res.json({
      message: 'Yey!'
     })
    } catch (error) {
      console.log(error); // Sometimes this error is not displayed on postman

      return res.status(400).json({
        error: 'Something went wrong'
      })
    }
  }

  signInUser();
};

问题：我现在看到一个ReferenceError: Cannot access 'email' before initialization错误。我认为错误从const {_id, name, email, role} = existingUser;行开始
你知道我如何更新代码来克服这个错误吗？
任何帮助都非常感谢。谢谢。