Nested async db call in for loop, wait until loop is done

Question

I am using the listCollection method in mongodb and looping through each returned collection with a query using find.

The problem is that I use the loop to construct an object that I wish to return with response.json after the loop is done but since find is async I am stuck at figuring how to "wait" until every find callback has finished before returning response.json()

var data = {};

database.listCollections({name: {$ne: 'system.indexes'}}).toArray(function(err, collections) {
    if (err) return res.json({});

    for(i=0; i<collections.length; i++){
        var collection = collections[i].name;

        database.collection(collection).find(query, limit)
        .sort({"Date": -1}).toArray(function(err, docs){

            if (err) return res.json({
            });

               /* Do stuff with docs, push stuff to data */
        });
        /** Console shows blank **/
        console.log("Data " + i+ ": " + JSON.stringify(data));
    }
  /** Response is blank.. **/
  res.json(data);
});

The problem is that the for loop returns way before the find()s are done. How do I deal with this in a JS/Node way? I can hack together a solution but I might encounter a similar problem later..

Edit: I am certain that data is actually returned and processed properly, as a console.log() inside each find on data shows that it actually has content.

rkm · Accepted Answer · 2016-11-16 16:59:01Z

1

You can use Promise if your NodeJS version supports it or use any Promise library otherwise. In this case you can wrap your code inside for loop into some function that returns Promise, then push all those promises objects into array. After loop call

Promise.all(promisesArray).then(/* Callback function that sends response */)

Another option is to use recursion instead of for loop and send request when you reach stop condition. But I don't think it's good option because in case if collections array is big you can exceed maximum stack size.

answered Nov 16, 2016 at 16:59

rkm

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