原生js提交ajax请求

<script type="text/javascript">
        window.onload = function(){
            var btn = document.getElementById("btn");
            var pcontent = document.getElementById("content");
            //获取XMLHttpRequest 兼容各种浏览器 包括IE6,IE5
            function getXmlHttp(){
                if(window.XMLHttpRequest){
                    return  new XMLHttpRequest();
                }else if( window.ActiveXObject){
                    var version = [
                        "MSXML2.XMLHttp.6.0",
                        "MSXML2.XMLHttp.3.0",
                        "MSXML2.XMLHttp"
                    ];
                    for(var i=0;i<version.length;i++){
                        try { return  new ActiveXObject(version[i]);}catch (e) {}
                    }
                }else{
                    alert("您的浏览器不支持XMLHttp");
                }
            }
            btn.onclick = function(){
                var xmlHttp = getXmlHttp();
                xmlHttp.onreadystatechange = function(){
                    if(xmlHttp.readyState == 4  ){
                        if(xmlHttp.status == 200){
                            pcontent.innerHTML =  xmlHttp.responseText;
                        }else{
                            alert("错误信息:"+ xmlHttp.statusText);
                        }
                    }
                }
                xmlHttp.open("GET","data.txt",true);
                xmlHttp.send(null);
            }

        }