Javascript Promises are hard

Alternatively: Resolving multiple promises with $q.all()

For a custom Angular project we maintain for a client, we need to poll 2 server endpoints and collate the results.

It used to be a single server endpoint, and could potentially be 10 endpoints down the track (theoretically!)

So we use a javascript promise to make a bunch of server requests, wait for them all to succeed (or 1 to fail…) and then collate them together as the response.

Notice how this could just as easily be one request, or 10 identical requests, all returning as a nice consolidated array.

           var promises = [];
            for(var i = 0; i < endpoints.length; i++){
                var e = endpoints[i];
                // load up an array of promises.
                promises.push(this.fetchOrdersFromEndpoint(e));
            }

            var defer = $q.defer();

            $q.all(promises).then(function(responses){
                var ordersBack = [].concat.apply([], responses); // flatten multiple arrays into one

                defer.resolve(ordersBack);
            }, function(e){
                defer.reject(e);
            });

            return defer.promise;