创建ajax对象并兼容多个浏览器

<script>
function createAjax(){
var request=false;
//window对象中有XMLHttpRequest存在就是非IE，包括（IE7以上版本）
if(window.XMLHttpRequest){
request=new XMLHttpRequest();
if(request.overrideMimeType){
request.overrideMimeType("text/xml");
}
//window对象中有ActiveXObject属性存在就是IE
}else if(window.ActiveXObject){
var versions=['Microsoft.XMLHTTP', 'MSXML.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]);
if(request){
return request;
}
}catch(e){
request=false;
}
}
}
return request;
}
var ajax=createAjax();
alert(ajax);
</script>