how to handle async function that return promise?

Question

I want to get a user from MongoDB. so I should use the async function to get the right result. but it return me [object , promise]. how can I use this data?

async function getUsers() {
    var allUsers = null;
    await MongoClient.connect(url, function(err, db) {
        if (err) throw err;
        var dbo = db.db('mydb');
        dbo.collection("users").find({})
            .toArray(function(err, results) {
                if (err)
                    throw error;
                db.close();
                allUsers = results;
            });
    });
}


app.get('/user/', function(req, res) {
    MongoClient.connect(url, function(err, db) {
        if (err) throw err;
        var dbo = db.db('mydb');
        var id = req.params.id;
                    var allUsers = getUsers();
             res.render('pages/user', {users: allUsers });
            db.close();
        });
});

Use await or .then and mark your function as async.

Christopher Moore
– Christopher Moore

2020-07-03 21:12:13 +00:00
Commented Jul 3, 2020 at 21:12 — Christopher Moore
– Christopher Moore, Commented Jul 3, 2020 at 21:12
where use them? I used them.

Saeedeh
– Saeedeh

2020-07-03 21:13:14 +00:00
Commented Jul 3, 2020 at 21:13 — Saeedeh
– Saeedeh, Commented Jul 3, 2020 at 21:13
See my answer for where to use it

Christopher Moore
– Christopher Moore

2020-07-03 21:17:02 +00:00
Commented Jul 3, 2020 at 21:17 — Christopher Moore
– Christopher Moore, Commented Jul 3, 2020 at 21:17

jfriend00 · Accepted Answer · 2020-07-04 00:35:25Z

2

You are mixing up promises and plain callbacks. You need to pick one or the other and consistently use it. Here's how to code it with promises - using the promises that MongoDb can return if used in the right way:

async function getUsers() {
    const db = await MongoClient.connect(url);
    try {
        const dbo = db.db('mydb');
        const results = await dbo.collection("users").find({});
        return results;
    } finally {
        db.close();
    }
}


app.get('/user/', function(req, res) {
    getUsers().then(allUsers => {
        res.render('pages/user', {users: allUsers });
    }).catch(err => {
        console.log(err);
        // send some sort of error response here
        res.sendStatus(500);
    });
});

I removed the database operations from inside your route handler because they weren't actually doing anything and the db stuff was already inside of getUsers().

edited Jul 4, 2020 at 0:35

answered Jul 3, 2020 at 23:55

jfriend00

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2 Comments

Saeedeh Over a year ago

where can i learn more about this promises and callbacks?

jfriend00 Over a year ago

@Saeedeh - I don't have a specific tutorial recommendation. There are tons of articles on Javascript promises on the web. You can start here.

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