Ajax实例 用Ajax进行一次HEAD请求 用Ajax进行一次指定的HEAD请求 【检索资源（文件）的头信息】
<!DOCTYPE html>
<html>
<head>
<script>
function loadXMLDoc(url)
{
var xmlhttp;
if (window.XMLHttpRequest)
{// code for IE7+, Firefox, Chrome, Opera, Safari
xmlhttp=new XMLHttpRequest();
}
else
{// code for IE6, IE5
xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");
}
xmlhttp.onreadystatechange=function()
{
if (xmlhttp.readyState==4 && xmlhttp.status==200)
{
document.getElementById('p1').innerHTML=xmlhttp.getAllResponseHeaders();
}
}
xmlhttp.open("GET",url,true);
xmlhttp.send();
}
</script>
</head>
<body>
<p id="p1">The getAllResponseHeaders() function returns the header information of a resource, like length, server-type, content-type, last-modified, etc.</p>
<button onclick="loadXMLDoc('/try/ajax/ajax_info.txt')">Get header information</button>
</body>
</html>
<!DOCTYPE html>
<html>
<head>
<script>
function loadXMLDoc(url)
{
var xmlhttp;
if (window.XMLHttpRequest)
{// code for IE7+, Firefox, Chrome, Opera, Safari
xmlhttp=new XMLHttpRequest();
}
else
{// code for IE6, IE5
xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");
}
xmlhttp.onreadystatechange=function()
{
if (xmlhttp.readyState==4 && xmlhttp.status==200)
{
document.getElementById('p1').innerHTML="Last modified: " + xmlhttp.getResponseHeader('Last-Modified');
}
}
xmlhttp.open("GET",url,true);
xmlhttp.send();
}
</script>
</head>
<body>
<p id="p1">The getResponseHeader() function is used to return specific header information from a resource, like length, server-type, content-type, last-modified, etc.</p>
<button onclick="loadXMLDoc('/try/ajax/ajax_info.txt')">Get "Last-Modified" information</button>
</body>
</html>