Angular scrollTop directive

tn-scroll-top.js

angular.module('webApp')
    .directive('tnScrollTop', ['$window', function($window){
        return {
            restrict: 'A',
            link: function(scope, elem, attr, ctrl) {
                var windowEl = angular.element($window);
                var handler = function() {
                    scope.scrollTop = windowEl.scrollTop();
                };
                windowEl.on('scroll', scope.$apply.bind(scope, handler));
                handler();
            }
        };
    }]);

have you ever test the code?
angular.js:13424 TypeError: angular.element(...).scrollTop is not a function