如何在外部获取当前A标签的ID值

<div class="diskmain">

<ul id="folder">

<li><span class='documents'><a href='javascript:void(0)' id='1' onclick='getid(this);'>分类A</a></span></li>

<li><span class='lists'><a href='javascript:void(0)' id='2' onclick='getid(this);'>分类B</a></span></li>

<li><span class='pictures'><a href='javascript:void(0)' id='3' onclick='getid(this);'>分类C</a></span></li>

<li><span class='share'><a href='javascript:void(0)' id='4' onclick='getid(this);'>分类D</a></span></li>

</ul>

</div>

<script>

function getid(a){

alert(a.id)

};

</script>