​x
 
<html>
<head>
<script type="text/javascript" src="/jquery/jquery.js"></script>
<script type="text/javascript">
$(document).ready(function(){
  $("button").click(function(){
    $('p').addClass(function(n){
      return 'par_' + n;
    });
  });
});
</script>
<style type="text/css">
.par_0
{
color:blue;
}
.par_1
{
color:red;
}
</style>
</head>
​
<body>
<h1>This is a heading</h1>
<p>This is a paragraph.</p>
<p>This is another paragraph.</p>
<button>向 p 元素添加类</button>
</body>
</html>
​