javascript通过单体模式、闭包封装一次嗅探浏览器api

<script type="text/javascript">
var SimpleXhrFactory=(function(){
var standard={
createXhrObject:function(){

return new XMLHttpRequest();

}
};
var activeXNew={
createXhrObject:function(){
return new ActiveXObject('Msxml2.XMLHTTP');

}
};
var activeXOld={
createXhrObject:function(){
return new ActiveXObject('Microsoft.XMLHTTP');

}
};
var testObject;
try{
testObject=standard.createXhrObject();
return standard;
}catch(e){
try{
testObject=activeXNew.createXhrObject();
return testObject;
}catch(e){
try {
testObject = activeXOld.createXhrObject();
}catch (e){
throw new Error('No XHR object found in this environment.');
}
}
}
})();
var xhr=SimpleXhrFactory.createXhrObject();
alert(xhr.toString());

</script>