Array .push is not working inside a promise function

Question

I have a nodejs Mongodb code like this . I'm trying to push into a global array and return all my results but the array contains only one value . Any idea why ?

var s = [];
var p = function(){
return new Promise(function(resolve, reject) {
    MongoClient.connect(url, function (err, db) {

   db.listCollections().forEach(function(collname) {

   db.collection(collname.name).find().sort( { duration_in_seconds: 1 }).limit(1).toArray(

  function(err, docs) {

        assert.equal(null, err);
        s.push(docs);
        resolve();
      });


});

 });

});

}


p().then(function() {

  console.log(s);

});

You cannot reference a variable outside of the scope of the promise like that. Work with it "inside" instead. Read the duplicate and understand it. — Neil Lunn
– Neil Lunn, Commented Jun 15, 2017 at 1:46
@NeilLunn the problem here is not the promise, it's the forEach loop — Bergi
– Bergi, Commented Jun 15, 2017 at 2:09

Bergi · Accepted Answer · 2017-06-15 02:35:59Z

2

You are resolving the promise when the first collections returns its document. You'd need to wait for all of them. Instead of wrapping everything in a large new Promise, promisify every asynchronous function on its own, and make the returned promise fulfill with the result value instead of putting it in a global variable:

function connect(url) {
    return new Promise(function(resolve, reject) {
        MongoClient.connect(url, function (err, db) {
            if (err) reject(err);
            else resolve(db);
        });
    });
}
function getArray(cursor) {
    return new Promise(function(resolve, reject) {
        cursor.toArray(function(err, docs) {
            if (err) reject(err);
            else resolve(docs);
        });
    });
}

Now you can write your p using these helpers, put the promises for each collection in an array, and await them with Promise.all, which yields a promise right for the array of all results:

function p() {
    return connect(url).then(function(db) {
        return getArray(db.listCollections()).then(function(collections) {
            var promises = collections.map(function(collname) {
                return getArray(db.collection(collname.name).find().sort({duration_in_seconds: 1 }).limit(1));
            });
            return Promise.all(promises);
        });
    });
}

p().then(function(s) {
    console.log(s);
});

edited Jun 15, 2017 at 2:35

answered Jun 15, 2017 at 2:07

Bergi

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3 Comments

karthik006 Over a year ago

Thank you so much for the solution . I tried it but i get this error "TypeError: db.listCollections(...).map is not a function "

Bergi Over a year ago

Oh drat, I was expecting listCollections to return an array (with standard forEach and map methods), not an asynchronous cursor that needs to be awaited. But we can adjust to that as well.

karthik006 Over a year ago

Oh yea thats what I thought . It worked Thank you Bergi

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