Unit test with promises: How to mock $q or know deferred has been called

I have a service that are using promises. Here is a simplified example of my service:

angular.module('myService').service('popup', [
    '$q', function(
     $q ) {

    this.alert = function(myValue) {
        var deferred = $q.defer();

        if (myValue = 0) {
            deferred.resolve('myValue is null');
        } else {
            deferred.reject('myValue isn\'t null');
        }
        return deferred.promise;
    }
}]);

I would like a unit test like:

• Given: myValue = 0


• When: myService.alert()


• Then: deferred.resolve has been called

The problem is, I understand since 'deferred' is a var inside my service, I can't access it in my unit tests (to mock it). So I need to mock $q, and I don't have any idea how to do that (I've tried some ways without success).

Anyone know how I can mock $q or know if deferred.resolve() has been called?