底层Ajax实现原理

<script>

var xhr=false;

function createXHR(){

try{

xhr=new ActiveXObject("MSXML.XMLHTTP");

}catch(e){

try{

xhr=new ActiveXObject("Microsoft.XMLHTTP");

}catch(e2){

xhr=new XMLHttpRequest();

}

}

}

function tt(){

var loginname=document.getElementById("loginname").value;

createXHR();

xhr.open("post","dozhuce.jsp",true); //请求处理页面

xhr.setRequestHeader("Content-Type","application/x-www-form-urlencoded;charset=UTF-8");//异步传输需要设置编码

xhr.send("loginname="+loginname);//发送的参数

xhr.onreadystatechange=changg;//回调的函数

}

function changg(){

if(xhr.readyState==4){

if(xhr.status==200){

var data=xhr.responseText;

alert(data);

}

}

}

</script>