js浏览器端判断当期是否在线

作者：zccst

纯浏览器端解决办法就是轮询，img轮询和ajax

批注：img轮询要加时间戳，ajax轮询要避免跨域。

			var img = new Image();
			img.id = "test_is_online";
			img.onload = function(){
				document.body.removeChild(document.getElementById("test_is_online"));
				alert("在线");
			};
			img.onerror = function(){
				document.body.removeChild(document.getElementById("test_is_online"));
				alert("断网了！");
			};
			img.src = "http://www.baidu.com/img/baidu_jgylogo3.gif?v="+(+new Date());
			img.style.display = "none";
			document.body.appendChild(img);


			//自己写的，还不完善
			$('<img src="http://www.baidu.com/img/baidu_jgylogo3.gif" />').load(function(){
				this.isOnline = true;
				alert("load"+this.isOnline);
			}).error(function(){
				this.isOnline = false;
				alert("error"+this.isOnline);
			});

//创建xmlHttp对象
			var xmlHttp = null;
			function createXMLRequest( ){
			    var msxmlhttp = new Array(
			        'Msxml2.XMLHTTP.6.0',
			        'Msxml2.XMLHTTP.3.0',
			        'Msxml2.XMLHTTP',
			        'Microsoft.XMLHTTP');
			    for(var i = 0; i < msxmlhttp .length; i++) {
			        try {
			            if(xmlHttp  = new ActiveXObject(msxmlhttp[i] )) break;
			        } catch (e) {
			            xmlHttp  = null;
			        }
			    }
			if(!xmlHttp && typeof XMLHttpRequest != "undefined")
			        xmlHttp = new XMLHttpRequest();
			}
			function getHtml(){
			     createXMLRequest( );
			     //状态调用函数
			     xmlHttp.onreadystatechange = function(){
			         if ( xmlHttp.readyState == 4 ){
			                    if (xmlHttp.status == 200){
			                                //状态成功执行，有网络
			                               reflesh();
			                               alert("有网");
			                     }else{
			                       //没有网络跳转到a.html页面  
			                        document.all.cma.src="a.html";
			                        alert("没网");
			                  }
			          }
			     }
			    //发送请求
			    xmlHttp.open( "get","http://m.weather.com.cn/m/pn2/weather.htm" ,true);
			    xmlHttp.send( null );
			}
			function reflesh(){
			      //var city  = System.Gadget.Settings.read("city");
			      var city  = "2";
			      if (city > 100000000) {
			           document.all.cma.src="http://m.weather.com.cn/m/pn2/weather.htm?id="+city+"T";
			      } else {
			          document.all.cma.src="http://m.weather.com.cn/m/pn2/weather.htm";
			     }
			}
			getHtml();


			//跨域
			    var xmlHttplink;
				function createXMLLinkHttpRequest() {
			    if (window.ActiveXObject) {
			        xmlHttplink = new ActiveXObject("Microsoft.XMLHTTP");
			    }
			    else if (window.XMLHttpRequest) {
			        xmlHttplink = new XMLHttpRequest();
			    }
			   }
			    
			function doRequest() {
				
			    createXMLLinkHttpRequest();
			    xmlHttplink.onreadystatechange = handleChange;
			    xmlHttplink.open("HEAD", "http://bbs.csdn.net/topics/320003524&date=" + new Date().getTime(), true);
			    xmlHttplink.send(null);

			}

			function handleChange() {
			    if(xmlHttplink.readyState == 4) {
					if(xmlHttplink.status == 200){
					 document.getElementById("netlink").value="连接";
					 alert("在线")
			        }else{
				    	 document.getElementById("netlink").value="断开";  
				    	alert("连接断开");   
				    }
			    setTimeout("doRequest()", 1000);
			   }
			  }
			doRequest();