How to show alert on leaving page before saved data? using only javascript. I dont want windows alert msg. i need normal Alert window

Question

this below code is working correctly but i want alert of javascript one not windows alert.

unsaved = false;
            $(":input").change(function () { 
                unsaved = true;
            });

            $(document).on("submit", "form", function (event) {

                unsaved = false;
                $(window).off('unload');
            });           

            function unloadPage() {
                if (unsaved) { 
                    return "You have unsaved changes on this page. Do you want to leave page?";
                }
            }   
            window.onbeforeunload = unloadPage;