在页面中有条件的onbeforeunload事件

window.onbeforeunload = function(evt) {
    var message = 'Are you sure you want to leave the page. All data will be lost!';

    if (typeof evt === 'undefined') {
        evt = window.event;
    }
    if (evt && !($("#a_exit").click)) {
        evt.returnValue = message;
    }
    return message;
};
window.onbeforeunload = function(evt) {
    var