更改行，因为即使在该行之后程序仍继续运行

if (existingUser) res.status(400).json({message: 'User already exists!!'});

结束日期

if (existingUser) return res.status(400).json({message: 'User already exists!!'});

您需要return密钥

你能试着用Else方法这样修改代码吗？

export const signUp = async (req, res) => {
  const {email, password, username, roomid} = req.body;
  try {
    
    console.log(req.body);
    
    let existingUser = User.findOne({email});
    
    console.log(existingUser);
    
    if (existingUser){
        return res.status(400).json({message: 'User already exists!!'});

    }else{
      const hashedPassword = await bcrypt.hash(password, 12);

      const result = await User.create({
        email,
        password: hashedPassword,
        username,
        roomid,
      });
  
      const token = jwt.sign(
        {email: existingUser.email, id: existingUser._id},
        'test',
        {expiresIn: '1hr'}
      );
      return res.status(201).json({result, token});
    }

  } catch (err) {
    console.log(err);
  }
};

注意：不要忘记关闭服务器，然后再打开。

试试这个

const existingUser = db.collection('users').findOne({ email: email }); 

if (existingUser) { 
  res.json({ 
    message: 'User already exists', 
    existingUser: existingUser 
  }); 
  client.close(); 
  return; <-- stop executing code if user already exists 
} //checks if user already exists