XMLHttpRequest

1）说明
  XmlHttp提供了网页加载后与服务器进行通信的方法。XmlHttp最大的用处是可以更新网页的部分内容而不需要刷新整个页面。

2）创建
   IE:    httpRequest = new ActiveXObject("Microsoft.XMLHTTP");     
   非IE:  httpRequest = new XMLHttpRequest();

3）get请求例子

var xmlhttp;

    function loadXMLDoc(url) {

        xmlhttp = null;

if (window.XMLHttpRequest) {// code for all new browsers

            xmlhttp = new XMLHttpRequest();

        }

else if(window.ActiveXObject)

        {// code for IE5 and IE6

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

        }

        if (xmlhttp != null) {

  xmlhttp.onreadystatechange = state_Change;

  xmlhttp.open("GET", url, true);

            xmlhttp.send(null);

        } else {

            alert("Your browser does not support XMLHTTP.");

        }

        console.log("the xml has been loaded");

    }

    function state_Change() {

if (xmlhttp.readyState == 4) {// 4 = "loaded"

            if (xmlhttp.status == 200) {// 200 = OK

   var data = xmlhttp.responseXML.getElementsByTagName_r("welcome-file")[0].firstChild.data;

                processData(data);

            // ...our code here...

            } else {

                alert("Problem retrieving XML data");

            }

        }

    }

    function processData(data){

        console.log("Data is : ",data);

        var text= document_createTextNode(data);

        var p = document.getElementByIdx_x("p1");

        p.a(text);

    }