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);

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

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