Ajax XMLHttpRequest 异步请求

ajax异步请求即XMLHttpRequest请求：
兼容firefox,mozillar,safari,opera,IE等主流浏览器
以get请求为例子：

var xmlHttp;
var bgArgument;
var responseFunction;
/**
* url:request path
* argName: argument name
* argName: argument value
*/
function httpRequest(url,argName,argValue){
    if(window.XMLHttpRequest){
             //support firfox, mozillar,safari,opera,IE7 later version
try{
xmlHttp = new XMLHttpRequest();
//mozillar bug
if(xmlHttp.overrideMimeType){
xmlHttp.overrideMimeType("text/xml");
}
}catch(e){
    alert('do not support '+e);
}
    }else if(window.ActiveXObject){
        //support IE6 previous version
        var activexName = ["Msxml2.XMLHTTP","Microsoft.XMLHTTP"];
        for(var i=0; i<activexName.length; i++){
            try{
              xmlHttp = new ActiveXObject(activexName[i]);
              break;
            }catch(e){
             alert('do not support IE '+e);
            }
        }
    }
    if(!xmlHttp){
        alert('create XMLHttpRequest failed');
        return;
    }

    //regist callback function
    xmlHttp.onreadystatechange = callback;
    xmlHttp.open("GET",url+"?"+argName+"="+argValue,true);
    xmlHttp.send(null);
   
   
}

//callback
function callback(){
     //request server success
     if(xmlHttp.readyState == 4){
         //response success
         if(xmlHttp.status == 200){
              var result = xmlHttp.responseText;
              //handler response result
         }else{
             alert("response failed");
         }
     }
}