In the documentation (version 1.1) of AngularJS about interceptors, the interceptor functions all return something like this

return response || $q.when(response);

However, in my app, 'response' is always defined, so $q.when(response) is never executed. So the question is in what situation will the 'response' be undefined and what will

$q.when(response) // == $q.when(null)

do! because response is undefined/null ?

In the documentation (version 1.1) of AngularJS about interceptors, the interceptor functions all return something like this

return response || $q.when(response);

However, in my app, 'response' is always defined, so $q.when(response) is never executed. So the question is in what situation will the 'response' be undefined and what will

$q.when(response) // == $q.when(null)

do! because response is undefined/null ?

• javascript
• angularjs
• return-value
• interceptor
• promise

• What is response in your code, does it promise or some data? – Maxim Shoustin Commented Nov 4, 2013 at 15:37
• it is the response from the server. In my interceptor I do things like "response.headers('someProperty)" – Jeanluca Scaljeri Commented Nov 4, 2013 at 15:42

2 Answers 2

• $q.when(promise) → promise
• $q.when(nonPromise) → a new promise, that will asynchronously resolve to the given value nonPromise.

Lets see what is $q.when:

$q.when = function (foreignPromise) {
    var deferred = $q.defer();
    foreignPromise.then(function (data) {
        deferred.resolve(data);
        $rootScope.$digest();
    }, function (reason) {
        deferred.reject(reason);
        $rootScope.$digest();
    });
    return deferred.promise;
}

Factory return $q.when(data)

As we can see $q.when receives promise or nonPromise and wrap it with.

Factory example:

fessmodule.factory('Data', ['$resource','$q',  function($resource, $q) {
    var data = [
        {
            "PreAlertInventory": "5.000000",
            "SharesInInventory": "3.000000",
            "TotalSharesSold": "2.000000",
            "TotalMoneySharesSold": "18.000000",
            "TotalSharesBought": "0.000000",
            "TotalShareCost": "0.000000",
            "EstimatedLosses": "0.000000"
        }
    ]; 

    var factory = {
        query: function (selectedSubject) {                             
            return $q.when(data);
        }    
    }
    return factory;
}]); 

Now we can call it from controller:

Data.query()
           .then(function (result) {
               $scope.data = result;                           
           }, function (result) {
               alert("Error: No data returned");
           });

Demo Fiddle

Factory returns $q.when(data) || data

From this example we return promise. So lets change it a bit:

Instead return $q.when(data); we will write:

return $q.when(data) || data;

It will work as well. But not vice versa.

As I understand Angular knows that controller waits from Data service the promise and above mentioned statement will use 1st off $q.when(data).

Demo 2 Fiddle

Factory returns data || $q.when(data)

Now lets call our Data service by this way:

$scope.data =  Data.query();

No promises, the call is sync.

Out factory seems like:

fessmodule.factory('Data', ['$resource','$q',  function($resource, $q) {
    var data = [
        {
            "PreAlertInventory": "5.000000",
            "SharesInInventory": "3.000000",
            "TotalSharesSold": "2.000000",
            "TotalMoneySharesSold": "18.000000",
            "TotalSharesBought": "0.000000",
            "TotalShareCost": "0.000000",
            "EstimatedLosses": "0.000000"
        }
    ]; 

    var factory = {
        query: function (selectedSubject) {                             
            return  data || $q.when(data);
        }
    }
    return factory;
}]);

Demo 3 Fiddle

My Conclusion

The return data || $q.when(data) means that our service can return single value or promise. But since we know what type of data our service returns , there is no sense in this statement. Or data or promise.

The statements:

return config || $q.when(config);

//AND

return response || $q.when(response);

Are shown in V1.1 of the Documentation of interceptors. It was removed from later versions of the documentation. I think it was a misguided attempt to document that interceptor functions can return either a value or a promise.

The statements otherwise make no sense as config and response always have a reference.