类型错误:无法读取未定义的属性“toString” - 为什么

这是发生这种情况的行(50(：

var meetingId = meeting._id.toString(),

以下是完整的相关代码：

var MongoClient = require('mongodb').MongoClient;
var assert = require('assert');
var ObjectId = require('mongodb').ObjectID;
var config = require('./config'),
    xlsx = require('./xlsx'),
    utils = require('./utils'),
    _ = require('lodash'),
    url = config.DB_URL;
var meetings = [];
function findNumberOfNotesByMeeting(db, meeting, callback) {
    var meetingId = meeting._id.toString(),
        meetingName = meeting.name.displayValue,
        attendees = meeting.attendees;
        host = meeting.host;
    var count = 1, pending = 0, accepted = 0;
    console.log("==== Meeting: " + meetingName + '====');
    _.each(attendees, function(item) {
      console.log(count++ + ': ' + item.email + ' (' + item.invitationStatus + ')');
      if (item.invitationStatus == 'pending') { pending++; }
      else if (item.invitationStatus == 'accepted') { accepted++; }
    });
    console.log("*** " + attendees.length + ", " + pending + "," + accepted);
    db.collection('users').findOne({'_id': new ObjectId(host)}, function(err, doc) {
        var emails = [];
        if (doc.emails) {
            doc.emails.forEach(function(e) {
                emails.push(e.email + (e.primary ? '(P)' : ''));
            });
        }
        var email = emails.join(', ');
        if (utils.toSkipEmail(email)) {
            callback();
        } else {
            db.collection('notes').find({ 'meetingId': meetingId }).count(function(err, count) {
                if (count != 0) {
                    console.log(meetingName + ': ' + count + ',' + attendees.length + ' (' + email + ')');
                    meetings.push([ meetingName, count, email, attendees.length, pending, accepted ]);
                }
                callback();
            });
        }
    });
}
function findMeetings(db, meeting, callback) {  
    var meetingId = meeting._id.toString(),
        host = meeting.host;
    db.collection('users').findOne({'_id': new ObjectId(host)}, function(err, doc) {
        var emails = [];
        if (!err && doc && doc.emails) {
            doc.emails.forEach(function(e) {
                emails.push(e.email + (e.primary ? '(P)' : ''));
            });
        }
        var email = emails.join(', ');
        if (utils.toSkipEmail(email)) {
            callback();
        } else {
            db.collection('notes').find({ 'meetingId': meetingId }).count(function(err, count) {
                if (count != 0) {
                    var cursor = db.collection('meetings').find({
                    'email': {'$regex': 'agu', '$options': 'i' }
                    });
                }
                callback();
        }); 
        }   

    cursor.count(function(err, count) {
        console.log('count: ' + count);
        var cnt = 0;
        cursor.each(function(err, doc) {
            assert.equal(err, null);
            if (doc != null) {
                findNumberOfNotesByMeeting(db, doc, function() {
                    cnt++;
                    if (cnt >= count) { callback(); }
                });
            }
        });
    });
    });
}

MongoClient.connect(url, function(err, db) {
    assert.equal(null, err);
    findMeetings(db, function() {
        var newMeetings = meetings.sort(function(m1, m2) { return m2[1] - m1[1]; });
        newMeetings.splice(0, 0, [ 'Meeting Name', 'Number of Notes', 'Emails' ]);
        xlsx.writeXLSX(newMeetings, config.xlsxFileNameMeetings);
        db.close();
    });
});

如您所见，会议变量(我几乎 100% 确定是问题所在，而不是 _id 属性(作为参数很好地传递给早期函数 findNumberOfNotesByMeeting。我在SO上找到了一些关于我的新函数可能是异步的并且需要回调的事实的信息，但是我已经尝试这样做并且不确定如何使其工作，或者即使这是我的代码的正确修复程序。