谁帮写个AJAX异步通信的例子

不太懂AJAX，但是要用到，谢谢了

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function requestHttp(){
var request;
if(window.XMLHttpRequest) {
request = new XMLHttpRequest();
if(request.overrideMimeType) {request.overrideMimeType('text/xml');
}
} else if(window.ActiveXObject) {
var versions = ['Microsoft.XMLHTTP', 'MSXML.XMLHTTP', 'Microsoft.XMLHTTP', 'Msxml2.XMLHTTP.7.0', 'Msxml2.XMLHTTP.6.0', 'Msxml2.XMLHTTP.5.0', 'Msxml2.XMLHTTP.4.0', 'MSXML2.XMLHTTP.3.0', 'MSXML2.XMLHTTP'];
for(var i=0; i <versions.length; i++) {try
{request = new ActiveXObject(versions[i]);break;}
catch(e) {}
}}
return request;
}
function xmlhttp(){
this.r=requestHttp();
}
xmlhttp.prototype.postopen=function(url,data){
this.r.open('POST',url,false);
this.r.setrequestheader("content-type","application/x-www-form-urlencoded");
this.r.onreadystatechange = ReadyStateChange(this);
if(typeof(data)=='undefined')
  this.r.send();
else
    this.r.send(data);
}
xmlhttp.prototype.getopen=function(url){
if(window.XMLHttpRequest) {this.r.open('GET',url);
this.r.onreadystatechange = ReadyStateChange(this);
this.r.send(null);
} else {
this.r.open("GET", url, true);
this.r