Angularjs unit test scope value

I have a directive that look like this:

angular.directive('myDirective', ['$compile', function($compile){
    return {
        link: function(scope, element, attr, controller) {
            function update(){
                if(scope.value == "A"){
                    scope.desc = "Welcome";
                }else{
                    scope.desc = "Not Welcome";
                }
            }

            scope.$watch('value', function(newValue, oldValue) {
                update();
            },true);

            update();
        }
    };
}]);

How can I write an unit test that can verify the changes in scope.value will return the correct scope.desc?