Promise API - combining results of 2 asynchronous call

Question

With promise API, how to send two asynchronous request in parallel, and resolve the combined result as the response.

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Answer 1

I have something to add to @ForbesLindesay answer.

In our case, we wanted partial results: if a request failed (eg. server has an hiccup, we request something deleted by somebody else, etc.), we still want to collect the valid responses, and to report the errors.

I found out that we need to handle success and failure on each promise, returning a value that will be collected by $q.all.

Here is our code, simplified and made generic ('item'...):

var promiseList = _.map(itemList, function(item)
{
    return DataService.getISubtems(item.id)
        .then(
            function(response)
            {
                var subItems = response.data;
                $log.info('Received sub-item list;' + subItems.length + ';items received');
                return subItems;
            },
            function(reason)
            {
                $log.warn('Sub-item list not received for item;' + item.name + ';' + item.id);
                $scope.errorList.push('Sub-item list not received for item "' + item.name + '"');
            }
        );
});
$q.all(promiseList)
    .then(function(itemArray)
    {
        // We get an array of arrays interleaved with undefined value when an error was raised.
        // That's because error handling doesn't return anything, ie. returns undefined.
        // We remove these undefined values then put all operations at the same level.
        var allOperations = _(operationArray).reject(_.isUndefined).flatten().value();
        if ($scope.errorList.length > 0)
        {
            NotificationService.warning('Items Fetching', 'Errors while getting item list:\n' +
                $scope.errorList.join('\n'));
        }
        $scope._onItemListReceived(allItems);
    });

Note that we use Lodash (_.map, _.flatten, _.reject, _.isUndefined) but I think the usage is quite clear (that's the nice point of this library!).