JS检测是否可以访问公网服务器功能代码

checkNet().then(function(res) {  if(res) {    //连网成功      }  } else {    //连网失败          }})

(function() {  //检测入口  checkNet = function() {    var def = $.Deferred();    xiuNet(null).then(function(res) {      netWork(null).then(function(res) {        if(res) {          def.resolve(true);        } else {          def.resolve(false);        }      })    })    return def.promise();  }  //嗅探网络，解决ios弹框接入网络问题  var checkTime = 2; //嗅探次数  xiuNet = function(def) {    if(def == null) {def = $.Deferred();}    var iframe = document.createElement("iframe");    iframe.style.cssText = "display:none;width:0px;height:0px;";    document.body.appendChild(iframe);    var ua = navigator.userAgent;    if(ua.indexOf("iPhone") != -1 || ua.indexOf("iPod") != -1 ||      ua.indexOf("iPad") != -1) {      iframe.src = "http://www.baidu.com" + "?timestap=" + new Date().getTime();      console.info("嗅探网络----");      if(checkTime > 1) {        checkTime--;        setTimeout(function() {          xiuNet(def)        }, 1000);      } else {        def.resolve(true);      }    } else {      def.resolve(false);    }    return def.promise();  }  //检测网络，是否可以连接公网  var cnetTime = 3; //检测网络次数  netWork = function(def) {    if(def == null) {def = $.Deferred();}    $.ajax({      type: "get",      timeout: 2000,      dataType: "jsonp",      jsonp: "callback",      url: 'https://jktv.tv/shphone/ver.txt',      success: function(data) {},      error: function(xhr) {        $("#ddd").html("status:"+xhr.status)        if(xhr.readyState == 4 && xhr.status == 200) {          def.resolve(true);        } else {          if(cnetTime > 1) {            cnetTime--;            setTimeout(function() {              netWork(def)            }, 200);          } else {            def.resolve(false);          }        }      }    });    return def.promise();  }})();