yige.org 试一个

<html>
<head>
<script type="text/javascript" src="jquery.js"></script>
<script type="text/javascript">
$(document).ready(function(){
  $("button").click(function(){
    alert($("p:first").hasClass("intro"));
  });
});
</script>
<style type="text/css">
.intro
{
font-size:120%;
color:red;
}
</style>
</head>

<body>
<h1 id="h1">This is a heading</h1>
<p class="intro">This is a paragraph.</p>
<p>This is another paragraph.</p>
<button>检查第一个段落是否拥有类 "intro"</button>
</body>
</html>

结果:

编辑上面的代码，点击查看结果。

yige.org - 试一个