How to resolve variable number of promises in node.js

Question

How to resolve variable number of promises in node.js

I am working on a function (called express.js cursor) to combine event information in the database with my Facebook copy and return it as an array of event objects.

I am having problems with the asynchronous nature of node.js and resolving a variable number of promises in a foreach loop before returning the whole object. I've tried many different ways to reorder my code (callbacks, counters, promises, etc.), but I haven't been able to solve this problem and I really would like to know why. I suspect it has something to do with swapping variables in the foreach loop, but I'm not sure how to solve this.

I am looking for three things:

What I don't understand conceptually what is needed to solve this problem?
How can I figure this out or debug this in the future?
How do I fix my code to make it work?

Here is my function:

function mergeEvents(req, res, next, events){

console.log("Merge Events");

var dfd = q.defer();

ensureAuthenticated(req, res, next).then(function(auth){
    var iEvent, event;
    var promises = [];

    if (auth){
        console.log("authenticated!");
        console.log("auth token: " + ACCESS_TOKEN);

        for (iEvent in events){
            event = events[iEvent];

            var promise = q.defer();
            promises.push(promise);

            https.get('https://graph.facebook.com/' + event.fb_id + '?access_token=' + ACCESS_TOKEN, function(response) {
                var str = '';
                response.on('data', function(chunk){
                    str += chunk;
                });

                response.on('end', function(){
                    var fb_event = JSON.parse(str);
                    event.dataValues.fb = fb_event;
                    promise.resolve(event);
                });
            });

            if (promises.length == events.length){
                console.log("last run through");
                q.all(promises).then(function(results){
                    console.log("all promises completed?");
                    console.log(results[0]); //OUTPUT BELOW
                    //more code in here... but promises haven't resolved
                    //...
                    dfd.resolve(events);
                });
            }
        }
    }else{
        console.log("Not authenticated. Redirecting to main page.");
        dfd.resolve(events);
    }
});

return dfd.promise;

}

While I am trying to get a JSON object, it returns an unresolved promise to console.log (results [0]):

{ promise: [object Object],
  resolve: [Function],
  fulfill: [Function],
  reject: [Function],
  notify: [Function] }

The code links I looked at:

Oh, and here's my function for single fb / db event pooling works, so you can compare:

function mergeEvent(req, res, next, event){
console.log("Merge Event");

var dfd = q.defer();

ensureAuthenticated(req, res, next).then(function(auth){
    if (auth){
        console.log("authenticated!");
        console.log("auth token: " + ACCESS_TOKEN);
        https.get('https://graph.facebook.com/' + event.fb_id + '?access_token=' + ACCESS_TOKEN, function(response) {
            var str = '';
            response.on('data', function(chunk){
                str += chunk;
            });

            response.on('end', function(){
                var fb_event = JSON.parse(str);
                event.dataValues.fb = fb_event;
                dfd.resolve(event);
            });
        });
    }else{
        console.log("not authenticated. redirecting to main page");
        dfd.resolve(event);
    }
});

return dfd.promise;
}

+3

javascript node.js promise asynchronous q

smileham 23 Apr 15 at 3:56

source to share

1 answer

Tomalak · Accepted Answer · 2015-04-23T07:15:28+0000

Your main problem is here:

var promise = q.defer();
promises.push(promise);

q.defer()

does not return a promise. It returns deferred .

var result = q.defer();
promises.push(result.promise);

Correctly naming variables is important, you don't notice the error because you chose the wrong variable names.

It said ...

Avoid for .. in

. Arrays have .forEach()

or .map()

.
Instead of checking, if (promises.length == events.length)

move that part out of the loop.
Your function is quite long and can use a little refactoring.
And of course, don't call your deferred objects "deferred" or your promises "promise." It's not descriptive.
Read What Is The Explicit Promise About Building Antipatterns And How To Avoid It? (let it sink, it takes a while)

This is what I will be using.

var q = require('q');
var qHttp = require("q-io/http"); // -> https://github.com/kriskowal/q-io

var FB = {
    // collect other FB API methods here, maybe transform into module
    graph: function (id) {
        var url = 'https://graph.facebook.com/' + id + '?access_token=' + ACCESS_TOKEN;
        return qHttp.read(url).then(function (data) {
            return JSON.parse(data.toString());
        });
    }
};

function mergeEvents(req, res, next, events) {
    return ensureAuthenticated(req, res, next).then(function (auth) {
        if (!auth) return q.reject("Not authenticated.");

        return q.all(events.map(function (event) {
            return FB.graph(event.fb_id).then(function (data) {
                event.dataValues.fb = data;
                return event;
            });
        }).then(function (results) {
            //more code in here...
        }));
    });
}

Note. If you wrote ensureAuthenticated

, change it to reject directly and on your own, instead of resolving with the falsy value auth

, which needs to be checked every time you use it. After that, you can delete the line if (!auth) ...

.

Also, the part //more code in here...

that deals with "extended" events should probably live out mergeEvents

.

How to resolve variable number of promises in node.js

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