我无法使mongoclient.connect在继续之前等待结果。我试图将db从Server.js（连接mongoclient的地方）传递到routes/api.js（处理post请求的地方）。但它不起作用，我总是得到:

TypeError:无法读取未定义的属性“Collection"

下面是我的routes/api.js:

var db = require("../server");

router.post('/video_url', async (req, res) => {
    const cursor = db.collection('movie').findOne({ link: req.body.videoURL }, function (findErr, result) {
        if (findErr) throw findErr;
    console.log(cursor)
});

server.js:

var db = async function () {
    return await MongoClient.connect(MONGODB_URI, function(err, client) {
        try {
            if(err) throw err;

            db = client.db('sub-project');

            // Start the application after the database connection is ready
            app.listen(PORT, () => console.log(`Server listening on port ${PORT}`));
            return db;
        }
        catch(ex) {
            console.log(ex)
        }
    });
}

module.exports = db;

编辑:

var dbObject = (async function() {
    var connection = await new Promise(function(resolve, reject) {
        MongoClient.connect(MONGODB_URI, { useNewUrlParser: true }, function(err, client) {
            try {
                if (err) throw err;

                db = client.db('sub-project');

                // Start the application after the database connection is ready
                app.listen(PORT, () => console.log(`Server listening on port ${PORT}`));
                resolve(db);
            } catch (ex) {
                console.log(ex)
                reject(ex);
            }
        });
    });
    return connection;
})();

    console.log("TYPEOF DB IN", typeof(dbObject))
    console.log("TYPEOF DB.COLLECTION IN", typeof(dbObject.collection))

两个console.log（）都未定义。。。这正常吗？