为什么在尝试从mongodb查找所有数据时返回未定义的原因？

原因？"/>

为什么在尝试从mongodb查找所有数据时返回未定义的原因？

Express在控制台日志中返回undefined。我想获取如下所示的数据。

app.get('/api/leaders/', async (req, res) => {
    try {
        const client = await MongoClient.connect('mongodb://localhost:27017', {useNewUrlParser: true, useUnifiedTopology: true});
        const db = client.db('leaders');

        const leadersList = await db.collection('leaders').find({}).toArray(function (err, result) {
            console.log("Connected correctly to server");
            console.log(result);
        });

        res.status(200).json(leadersList);

        client.close();

    } catch (error) {
        res.status(500).json({message: 'Error connecting to db', error});
    }
})

这是我想从数据库返回的数据：

db.leaders.find({})
{ "_id" : ObjectId("5eb8922d17d98c2bb6f98a5a"), "username" : "kk", "score" : 20 }
{ "_id" : ObjectId("5eb8922d17d98c2bb6f98a5b"), "username" : "mj", "score" : 18 }
{ "_id" : ObjectId("5eb8922d17d98c2bb6f98a5c"), "username" : "vr", "score" : 15 }
{ "_id" : ObjectId("5eb8922d17d98c2bb6f98a5d"), "username" : "mdb", "score" : "25" }

您正在尝试将.Js callbacks与async-await混合使用，这会引起问题。尝试下面的代码：

app.get('/api/leaders/', async (req, res) => {
    const client = null;
    try {
        client = await MongoClient.connect('mongodb://localhost:27017', {useNewUrlParser: true, useUnifiedTopology: true});
        const db = client.db('leaders');
        console.log("Connected correctly to server");

        const leadersList = await db.collection('leaders').find({}).toArray();

        console.log('leadersList ::',leadersList);
        /** I would not recommend to open & close DB connections for each API call. Find a better way to manager connections */
        client.close(); 
        res.status(200).json(leadersList);

    } catch (error) {
        if(client){ console.log('Error at DB find operation'); client.close(); res.status(500).json({message: 'Error at DB find operation', error}); }
        res.status(500).json({message: 'Error connecting to db', error});
    }
})