Михаил Горячкин

window.onbeforeunload = function (evt) {
    var message = "Document 'foo' is not saved. You will lost the changes if you leave the page.";
    
    if (typeof evt == "undefined") {
        evt = window.event;
    }
    if (evt) {
        evt.returnValue = message;
    }
    return message;
}