JavaScript 获取网页中第一个链接 ID 的方法

<!DOCTYPE html>
<html>
<body>
<h1>sharejs.com</h1>
<img src ="planets.gif" width="145" height="126" 
alt="Planets" usemap ="#planetmap">
<map name="planetmap">
<area id="sun" shape="rect" coords="0,0,82,126" 
href="sun.htm" alt="Sun">
<area id="mercury" shape="circle" coords="90,58,3" 
href="mercur.htm" alt="Mercury">
<area id="venus" shape="circle" coords="124,58,8" 
href="venus.htm" alt="Venus">
</map>
<p><a id="javascript" href="/js/">JavaScript Tutorial</a>
</p>
<p>Id of first area/link:
<script>
document.write(document.links[0].id);
</script></p>
</body>
</html>