Unable to handle onbeforeunload event using Angular Service

I have the following callback for my onbeforeUnload event in one of my service. In my app.run block I have:

window.onbeforeunload = Services.beforeWindowClose;

and this method is in a service:

this.beforeWindowClose = function (event) {
            var currentState = $state.current.name;
            if (currentState !== constant.LOGOUT) {
                $rootScope.$broadcast("close");
                if (Utilities.isSavePending)
                    event.returnValue = constant.MSG;
            }
        }

In my controllers I have :

$scope.$on("close", function () {
            Utilities.isSavePending = vm.save; //sets it to true
        })

Now given the events are synchronous in angular, this code should give me a popup on window close. However,this directly closes my window.

My intention is that whenever the user closes window, I raise an event and see if there is any unsaved data in my controller. If there is some unsaved data, the browser should not close and give a popup, while if there is no unsaved data, the browser should close.

Am I doing or understanding something wrong?