js 点击按钮显示下拉菜单

<li>
    <a id = "rank" onclick="showGroup()"></a>
</li>
<li id = "buttonGroup" style="display: none">
    <a href="#">

    </a>
    <a href="#">

    </a>

</li>

<li>
    <a id = "rank" onclick="showGroup()"><i class="fa fa-edit fa-fw"></i> 关键词排名</a>
</li>
<li id = "buttonGroup" style="display: none">
    <a href="/addMession">
        添加任务
    </a>
    <a href="#">
        任务管理
    </a>

</li>