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>