<script type="text/javascript" src="/js/jquery.js"></script>
<script type="text/javascript">
$(document).ready(function(){
$("button").click(function(){
alert($("p:first").hasClass("intro"));
<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>