complex collection of objects with async method

[pee...@gmail.com]

Hi,

I'm new to Q and promises. I'm trying to use it in my code - working with NodeJS. 

I have an object, objB, which has an async method called extract() that returns a promise (using Q.defer). extract() makes a http "get" request (hence the async nature) and when the request is successful, it updates the object's properties with the response and resolves the promise.

I have another object, objA, that contains an array of objB.

I want to iterate over a collection (array) of objA. For each objA in the array, I want to call the extract() method for only the first element in its objB array.

The issue I have is that it all works and extract is called, except the property update that extract also performs does not seem to happen.

//objA has property blist which is a collection of objB

//extract method of objB

ObjB.prototype.extract = function()

{

    var deferred = Q.defer();

    https.get(url, function(res) {

                           res.on('data', function(data) {

                                                   //do stuff with data

                                                   //update objB property

                                                   this._property1 = data.field1

                                                   deferred.resolve(data);

                                              });

                       };

    return deferred.promise;

};

I call this as:

//alist is the collection of objA

var queue = [];

for (var i=0; j<alist.length; i++)

{

var myfunc = alist[i].blist[0].extract.bind(alist[i].blist[0]);

queue.push(myfunc());

}

Q.all(queue).then(func(succ){

                              //print alist results

The issue I have is that _property1 of objB is not being updated. Extract is correctly called. But then when all the promises have been resolved and I output the original collection, alist, the property update that extract() did on objB is not reflected.

Instead of the nbind, I have tried: queue.push(Q(alist[i].blist[0]).invoke("extract")) which I saw had been suggested in this group earlier. That also does not work.

Any ideas what I am doing wrong? I'm guessing its something to do with the bind...

thanks

[Stuart Knightley]

Javascript's 'this' is not lexically scoped, and so you need to keep a reference to it to use in callbacks (i.e. in ObjB extract). Often 'var self = this;'

See http://www.2ality.com/2012/04/arrow-functions.html for more information (although arrow functions aren't yet in node)

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[pee...@gmail.com]

Thanks! Thats exactly what it was.