angularjs - AngularJs call service function when value passed in is not null

Question

I need to call a service function when a certain value exists. I call a service, get data and then I need to call another service to make some string operations on the result of the first service call.

Actually I simly check when the calls to first service are finished, but not seems a good way to do this.

$scope.result = function(){
    Report.startQueue(url, function(response, reports){
        $scope.progress = response;
        $scope.report = reports;
        if(response === "last"){
            $scope.string= keyService.stringOperations($scope.report.MyString);
        }
    });
}

If instead I call directly

$scope.string= function(item) {
    return keyService.stringOperations(item);
}   

I got errors because the string is not ready at the moment. In the keyService I use the string replace method, that throws error if the string is empty

[PLUNKR] http://plnkr.co/edit/gcvOZL