<html><head><script type="text/javascript" src="/jquery/jquery.js"></script><script type="text/javascript">$(document).ready(function(){ $("button").click(function(){ alert($("li").size()); });});</script></head><body><button>输出 li 元素的数目</button><ul><li>Coffee</li><li>Milk</li><li>Soda</li></ul></body></html>