JavaScript实现点击按钮展开下拉菜单

<!DOCTYPE html>
<html>
  <head>
    <meta charset="utf-8">
    <title>开关按钮设置</title>
  </head>
  <link rel="stylesheet" href="css/Default style sheet.css" rel="external nofollow" />
  <style>
    div{
    width:120px;
    margin: 0px auto;
    border: 2px solid black;}
    ul{
      display: none;
    }
  </style>
  <script>
    window.onload=function(){
      var odiv=document.getElementById("v");
      var oli=odiv.getElementsByTagName("ul")[0];
      var   onf=true
        odiv.onclick=function(){
          if (onf==true) {
            oli.style.display='block';
            onf=false
          } else{
            oli.style.display='none';
            onf=true
          }
        }
    }
  </script>
  <body>
    <div id="v">
      <h4>你好啊，小老弟</h2>
    <ul>
      <li>come</li>
      <li>on</li>
      <li>do</li>
    </ul>
    </div>
     
  </body>
</html><br>