ajax 异步获取数据实现代码 （js创建ajax对象）

<script type="text/javascript"><!--
var request = false;
try {
request = new XMLHttpRequest();
} catch (trymicrosoft) {
try {
request = new ActiveXObject("Msxml2.XMLHTTP");
} catch (othermicrosoft) {
try {
request = new ActiveXObject("Microsoft.XMLHTTP");
} catch (failed) {
request = false;
}
}
}
if (!request) alert("Error initializing XMLHttpRequest!");
//获取信息
function GetInfo() {
var url = "test.aspx?userName=" + document.getElementById("txtName").value;
request.open("GET", url, true);
request.onreadystatechange = updatePageTime;
request.send(null);
}
//更新页面
function updatePageTime() {
if (request.readyState == 4) {
if (request.status == 200) {
var response = request.responseText;
document.getElementById("txtLoginName").value = response;
} else if (request.status == 404) {
alert("Requested URL is not found.");
} else if (request.status == 403) {
alert("Access denied.");
} else
alert("status is " + request.status);
}
}
// --></script>
<form id="form1" runat="server">
<input id="txtName" onchange="GetInfo()" type="text" />
<input id="txtLoginName" type="text" />
</form>