使用快遞。我正在嘗試創(chuàng)建一個(gè) .js 文件來(lái)處理 POST 請(qǐng)求，以在插入到我的 MongoDB 之前檢查用戶是否存在。我建立了 2 個(gè) MongoClient 連接來(lái)處理不同的情況。首先是檢查用戶是否存在。如果用戶不存在，那么它將轉(zhuǎn)到第二個(gè)連接。我無(wú)法理解我做錯(cuò)了什么代碼是：`router.post('/', function(req, res, next) {  console.log("inside post method profile")  ssn = req.session;  ssn.firstName = req.body.fname;  ssn.lastName = req.body.lname;  ssn.userEmail = req.body.email;  ssn.userPass = req.body.pass;  let userExists = false;  MongoClient.connect(url, function(err, db) {    if (err) throw err;    let dbo = db.db("projectOne");    let myInfoLog = {      email: ssn.userEmail,      pass: ssn.userPass    };    // TRYING TO COUNTERCHECK IF A USER ALREADY EXISTS    dbo.collection("userInfo").findOne(myInfoLog, function(err, data) {      if (data.email) {        userExists = true;        ssn.signUpError = "User email already exists";        console.log("data returns an active email");        db.close();        res.redirect('/');      }    });  });  if (userExists == false) {    MongoClient.connect(url, function(err, db) {      if (err) throw err;      let dbo = db.db("projectOne");      let myInfoLog = {        email: ssn.userEmail,        pass: ssn.userPass      };      let myInfo = {        fname: ssn.firstName,        lname: ssn.lastName,        email: ssn.userEmail,        pass: ssn.userPass      };      dbo.collection("userInfo").insertOne(myInfo, function(err, data) {        if (err) throw err;        console.log("collection inserted");        // console.log(data.ops[0].fname);        // console.log(data.ops[0].lname);        // console.log(data.ops[0].email);        // console.log(data.ops[0].pass);        ssn.firstName = data.ops[0].fname;        ssn.lastName = data.ops[0].lname;        ssn.userEmail = data.ops[0].email;        ssn.userPass = data.ops[0].pass;        console.log("welcome! " + ssn.firstName + " " + ssn.lastName)        db.close();      });      res.redirect('/profile');    });  }});module.exports = router;