ajax异步判断网站是否可以访问到的解决方法(XMLHttpRequest)

原文地址：https://www.cnblogs.com/lyl6796910/archive/2011/11/16/2461342.html

    function getXMLHttpRequest(){   
      var client = getXMLHttpRequestFromIE();   
       if(client == null){   
            client = new XMLHttpRequest();   
       }   
      return client;   
    }   
      
    function getXMLHttpRequestFromIE(){   
      var namePrefixes = ["Msxml3","Msxml2","Msxml","Microsoft"];   
      for(var i = 0; i < namePrefixes.length; i++){   
           try{   
               var name = namePrefixes[i] + ".XMLHTTP";   
               return new ActiveXObject(name);   
           }catch(e){   
           }   
      }   
      return null;   
    }   
    var client = null;   
    //是否可以访问   
    var isRun = false;   
    //返回值   
    var stateString="";   
    //请求url的网站   
    function connUrl(url){   
        client = getXMLHttpRequest();   
        var actionURL = convertURL(url);   
        try{   
            client.open("GET", actionURL, true);   
            client.onreadystatechange = callBack;   
            client.send(null);   
        }catch(e){}   
        finally{   
        }   
    }   
    function callBack(){   
        if(client.readyState == 4){   
            stateString = client.responseText;   
            if(stateString && client.status==200)   
                isRun = true;   
        }   
    }   
    //是否可达   
    function isReach(url){   
        connUrl(url);   
        return isRun;   
    }   
    //给url地址增加时间戳，骗过浏览器，不读取缓存       
    function convertURL(url) {       
            //获取时间戳       
            var timstamp = (new Date()).valueOf();       
            //将时间戳信息拼接到url上   
            if (url.indexOf("?") >= 0) {       
                    url = url + "&t=" + timstamp;       
            } else {       
                    url = url + "?t=" + timstamp;       
            }       
            return url;       
    }   
    //因为是异步，先调用，此时isRun的值是false   
    isReach("http://www.baidu.com");   
    //3秒钟后判断isRun的值，如果为false认为网站不可达，类似超时时间   
    setTimeout(function(){      
            //alert(stateString);    
            //alert(isRun);   
            if(isRun==true){   
                document.getElementById("ceshi").innerText = "网站是通的！";   
            }else{   
                document.getElementById("ceshi").innerText = "网站不通！";   
            }   
        },3000);  

<div id="ceshi"></div>