csdn的站内短信提示是怎么实现的

<script language="javascript">
    <!--
   
function CheckNewMessage(){
    var Url = "http://message.csdn.net/newMessage.aspx";
    var nn = Math.floor( Math.random() * 2000000);
    Url += "?time="+nn;
    var ScriptName = "CheckUserHasNewMessage";
    var head= document.getElementsByTagName("head").item(0);
    var old = document.getElementById(ScriptName);
    if (old)  head.removeChild(old);

    var script = document.createElement("SCRIPT");
    script.src = Url;
    script.defer = true;
    script.type = "text/javascript";
    script.id = ScriptName;
    head.appendChild(script);
    setTimeout("CheckNewMessage()",300000);
}
   
function getElement(id, d) {
 if (!d) d = document;
 if (d.getElementById) {
  return d.getElementById(id);
 }
 if (d.layers && d.layers[id]) {
  return d.layers[id];
 }
 if (d.all && d.all[id]) {
  return d.all[id];
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}

function ShowADPic()
{
    getElement("Pre_Link").innerHTML = "<a href='http://job.csdn.net/Jobs/badaf2f8deac4dc2a573c63a06c8f7ad/viewcompany.aspx' target='_blank'><img id='Pre_Img' border='0'  name='Pre_Img' src='http://images.csdn.net/upimgs/21067_3.gif' alt='广告' width='210' height='67' /></a>";
}
   
function ShowDefaultPic(){
    // 临时推广用
    ShowDearBook();
    //getElement("Pre_Link").innerHTML = "<img id='Pre_Img' border='0'  name='Pre_Img' src='/logo/Images/eye001.gif' alt='请保护好自己的眼睛' width='210' height='67' />";
}

function ShowNewYear()
{
    getElement("Pre_Link").innerHTML = "<img id='Pre_Img' border='0'  name='Pre_Img' src='/logo/Images/newyear.jpg' alt='新年快乐、恭喜发财' width='210' height='67' />";
}

function ShowDearBook()
{
    getElement("Pre_Link").innerHTML = "<a href='http://www.dearbook.com.cn/2006/cc2e/index.htm' target='_blank'><img id='Pre_Img' border='0'  name='Pre_Img' src='/logo/images/cc2e.jpg' alt='DearBook' width='210' height='67' /></a>";
}

function ShowDearBookPic(){
/*
    var vv = Math.random()*10;
    if( vv > 5)
    {
        ShowNewYear();
    }
    else
    {
    */
    // 临时推广用
        ShowDearBook();
    //}
}

function ClickNewMessage(){
    ShowDefaultPic();
    window.open("http://message.csdn.net/ListMessage.aspx?type=newMessage");   
}
   
function ShowNewMessagePic(){
    var textt = "<a href='JavaScript:ClickNewMessage();'><img id='Pre_Img'  name='Pre_Img'  border='0' src='/logo/Images/Message001.gif' alt='您有新的短消息！' width='210' height='67' /></a><bgsound src='/Logo/Images/newMessage.wav' border='0'>";
    getElement("Pre_Link").innerHTML = textt;
}

function TimeFindNewMessage(){
    setTimeout("CheckNewMessage()",300000);
}

function YouHadNewMessage(){
    var Had = false;
    url = "http://message.csdn.net/newMessage.aspx";
    var httpRequest;
    if (typeof XMLHttpRequest != 'undefined') {
        httpRequest = new XMLHttpRequest();
    }
    else if (typeof ActiveXObject != 'undefined') {
        httpRequest = new ActiveXObject('Microsoft.XMLHTTP');
    }
    if (httpRequest) {
        httpRequest.open('GET', url, false);
        httpRequest.send(null);
        if(httpRequest.status == 200){
            var textInfo = httpRequest.responseText;
            if(textInfo == "true") Had = true;
    }}
    if(Had){
        ShowNewMessagePic();
    }
    else
    {
        ShowDearBookPic();
    }
    setTimeout("YouHadNewMessage()",300000);
}
    //-->
    </script>
慢慢看