<div class="dropdown">
  <button onclick="myFunction()" class="dropbtn">下拉菜單</button>
  <div id="myDropdown" class="dropdown-content">
    <input type="text" placeholder="Search.." id="myInput" onkeyup="filterFunction()">
    <a href="#about" rel="external nofollow" >Google</a>
    <a href="#base" rel="external nofollow" >Runoob</a>
    <a href="#blog" rel="external nofollow" >Taobao</a>
    <a href="#contact" rel="external nofollow" >Wiki</a>
    <a href="#custom" rel="external nofollow" >Zhihu</a>
    <a href="#support" rel="external nofollow" >Tmall</a>
    <a href="#tools" rel="external nofollow" >Weibo</a>
  </div>
</div>

/* 下拉菜單按鈕 */
.dropbtn {
  background-color: #04AA6D;
  color: white;
  padding: 16px;
  font-size: 16px;
  border: none;
  cursor: pointer;
}
 
/* 鼠標移動到下拉菜單按鈕到樣式*/
.dropbtn:hover, .dropbtn:focus {
  background-color: #3e8e41;
}
 
/* 搜索框 */
#myInput {
  box-sizing: border-box;
  background-image: url('searchicon.png');
  background-position: 14px 12px;
  background-repeat: no-repeat;
  font-size: 16px;
  padding: 14px 20px 12px 45px;
  border: none;
  border-bottom: 1px solid #ddd;
}
 
/* 搜索框獲取焦點的樣式 */
#myInput:focus {outline: 3px solid #ddd;}
 
/* 容器 <div> - 定位下拉菜單 */
.dropdown {
  position: relative;
  display: inline-block;
}
 
/* 下拉菜單內容 (默認隱藏) */
.dropdown-content {
  display: none;
  position: absolute;
  background-color: #f6f6f6;
  min-width: 230px;
  border: 1px solid #ddd;
  z-index: 1;
}
 
/* 下拉菜單鏈接樣式 */
.dropdown-content a {
  color: black;
  padding: 12px 16px;
  text-decoration: none;
  display: block;
}
 
/* 鼠標移動到鏈接上的樣式 */
.dropdown-content a:hover {background-color: #f1f1f1}
 
/* 顯示下拉菜單 (使用 JS 添加 .dropdown-content 類) */
.show {display:block;}

/* 點擊按鈕設置下拉菜單的顯示與隱藏 */
function myFunction() {
  document.getElementById("myDropdown").classList.toggle("show");
}
 
/* 搜索功能 */
function filterFunction() {
  var input, filter, ul, li, a, i;
  input = document.getElementById("myInput");
  filter = input.value.toUpperCase();
  div = document.getElementById("myDropdown");
  a = div.getElementsByTagName("a");
  for (i = 0; i < a.length; i++) {
    txtValue = a[i].textContent || a[i].innerText;
    if (txtValue.toUpperCase().indexOf(filter) > -1) {
      a[i].style.display = "";
    } else {
      a[i].style.display = "none";
    }
  }
}