支持异步（async）请求是ajax的重要特性。
示例的文件结构如下：
client.html源码：
<!DOCTYPE html>
<head>
<title>ajax_async_demo_by_xqoffice</title>
<style>
p,div {border: 1px solid gray;width: 100px;}
button {margin-top: 18px;}
</style>
<script type="text/javascript" src="jquery.min.js"></script>
<script type="text/javascript">
$(document).ready(function(){
$('button').click(function(){
$.ajax({
"type":"POST",
"url":"server.php",
"data":"id=1&var2=val2",
"beforeSend":function(){
$('#p1').html('<img src="icon/running.gif" />');
},
"success":function(backdata){
$('#p1').html('<img src="icon/done.png" />');
$('#div1').html(backdata);
},
"error":function(){
$('#p1').html('<img src="icon/error.png" />');
}
});
$.ajax({
"type":"POST",
"url":"server.php",
"data":"id=2&var2=val2",
"beforeSend":function(){
$('#p2').html('<img src="icon/running.gif" />');
},
"success":function(backdata){
$('#p2').html('<img src="icon/done.png" />');
$('#div2').html(backdata);
},
"error":function(){
$('#p2').html('<img src="icon/error.png" />');
}
});
});
});
</script>
</head>
<body>
<p id='p1'>p1</p>
<div id='div1'>div1</div>
<p id='p2'>p2</p>
<div id='div2'>div2</div>
<button>Submit</button>
</body>
</html>server.php代码：
<?php
$id=$_POST['id'];
if ($id==1){
sleep(5);
echo 'one';
} else {
sleep(2);
echo 'two';
}
?>效果：发起两个请求之后，请求2的响应反倒比请求1的先完成，和响应1完成与否无关。
