NodeJS中await报错：仅在async函数及模块顶层有效，如何解决？

问题重现 #

在开发NodeJS结合MongoDB的用户登录注册功能时，login模块出现以下错误：

existingUser = await User.findOne({email: email});
                     ^^^^^

SyntaxError: await is only valid in async functions and the top level bodies of modules.

相关代码 #

user-controller.js文件代码：

const User = require('../model/User');
const bcrypt = require('bcryptjs');

// next is used to move to the next middleware task
const signup = async (req, res, next) => {
  const { name, email, password } = req.body;

  let existingUser;
  try {
    existingUser = await User.findOne({ email: email });
  } catch (err) {
    console.log(err);
  }
  if (existingUser) {
    return res.status(400).jason({ message: 'User already exists! Login Instead' })
  }

  const hashedPassword = bcrypt.hashSync(password);

  const user = new User({
    name,
    email,
    password: hashedPassword,
  });

  try {
    await user.save();
  } catch (err) {
    console.log(err);
  }

  return res.status(201).json({ message: user })
};

const login = (req, res, next) => {
  const { email, password } = req.body;

  let existingUser;
  try {
    existingUser = await User.findOne({ email: email });

  } catch (err) {
    return new Error(err);
  }
  if (!existingUser) {
    return res.status(400).json({ message: "User not found. Signup Please" })
  }
  const isPasswordCorrect = bcrypt.compareSync(password, existingUser.password);
  if (!isPasswordCorrect) {
    return res.status(400).json({ message: "Invalid Email / Password" })
  }
  return res.status(200).json({ message: "Successfully logged in" })
}

exports.signup = signup;
exports.login = login; 

解决步骤 #

1. 核心问题修复：login函数未标记为async，而await关键字仅能在async函数内部使用。修改login函数定义，添加async关键字：

const login = async (req, res, next) => {
  const { email, password } = req.body;

  let existingUser;
  try {
    existingUser = await User.findOne({ email: email });

  } catch (err) {
    return res.status(500).json({ message: "Something went wrong" });
  }
  if (!existingUser) {
    return res.status(400).json({ message: "User not found. Signup Please" })
  }
  const isPasswordCorrect = bcrypt.compareSync(password, existingUser.password);
  if (!isPasswordCorrect) {
    return res.status(400).json({ message: "Invalid Email / Password" })
  }
  return res.status(200).json({ message: "Successfully logged in" })
}

补充：原代码中catch块返回new Error(err)无法向客户端返回有效错误响应，建议改为返回500状态码的JSON响应。

2. 额外小问题修复：signup函数存在一处拼写错误，res.status(400).jason应改为res.status(400).json，否则会导致响应失败。

内容的提问来源于stack exchange，提问作者Tech World