JavaScript detects whether the public network server function code can be accessed

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();
  }
})();