Js navigator.onLine 获取设备是否可以上网、连接网络

http://zccst.iteye.com/blog/2194229

获取用户的联网状态

if (navigator && navigator.onLine === false) {
  alert("无法连接网络")
}
else
{
 alert("正常上网")
}

监听事件

var el = document.body;
if (el.addEventListener) {
   window.addEventListener("online", function () {
     alert("online");}, true);
   window.addEventListener("offline", function () {
     alert("offline");}, true);
}
else if (el.attachEvent) {
   window.attachEvent("ononline", function () {
     alert("online");});
   window.attachEvent("onoffline", function () {
     alert("offline");});
}
else {
   window.ononline = function () {
     alert("online");};
   window.onoffline = function () {
     alert("offline");};
}