javascript 原生态ajax

function requestStatus(verifycode){
var req=createRequest();
req.open("POST","getVerifycode.do?isVerify=1&verifycode="+verifycode);
req.onreadystatechange=function(){callback(req);}
req.send(null);
}
function createRequest(){
if(window.XMLHttpRequest){
return new XMLHttpRequest();
}else{
try{
return new ActiveXObject("Msxm12.XMLHTTP");
}catch(e){
return new ActiveXObject("Microsoft.XMLHTTP");
}
}
return null;
}
//回调函数
function callback(req){
if(req.readyState==4){
if(req.status==200){
//获取返回值
var data=req.responseText;
if(data==1){
loginSubmit();
}else{
alert('验证码输入有误');
document.getElementById('verifycode').value='';
getVerifycode();
}
return;
}
}
}