if (navigator.onLine)
{ //正常工作}
else { //執行離線狀態時的任務}

var EventUtil = {
addHandler: function (element, type, handler) {
if (element.addEventListener) {
element.addEventListener(type, handler, false);
} else if (element.attachEvent) {
element.attachEvent("on" + type, handler);
} else {
element["on" + type] = handler;
}
}
};
EventUtil.addHandler(window, "online", function () {
alert("Online");
});
EventUtil.addHandler(window, "offline", function () {
alert("Offline");
});