ajax 对象创建 兼容各个浏览器

<script>
function createAjax(){
var request=false;

//window对象中有XMLHttpRequest存在就是非IE，包括（IE7，IE8）
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>