如何创建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>