window.addEventListener("beforeunload", function (e) {
    var confirmationMessage = 'It looks like you have been editing something. '
                            + 'If you leave before saving, your changes will be lost.';

    (e || window.event).preventDefault && (e || window.event).preventDefault();
    (e || window.event).returnValue = confirmationMessage; //Gecko + IE

    return confirmationMessage; //Gecko + Webkit, Safari, Chrome etc.
});