JavaScript 如何实现下拉菜单功能？

<div class="dropdown">
  <button class="dropbtn">下拉菜单</button>
  <div class="dropdown-content">
    <a href="#">菜单项1</a>
    <a href="#">菜单项2</a>
    <a href="#">菜单项3</a>
  </div>
</div>

.dropdown-content {
  display: none;
  position: absolute;
  background-color: #f9f9f9;
  min-width: 160px;
  box-shadow: 0px 8px 16px 0px rgba(0,0,0,0.2);
  z-index: 1;
}

.dropdown-content a {
  color: black;
  padding: 12px 16px;
  text-decoration: none;
  display: block;
}

.dropdown:hover .dropdown-content { /*当鼠标悬浮于.dropdown元素上时*/
  display: block;
}

// 获取下拉菜单按钮和下拉菜单内容的元素
var dropdownBtn = document.querySelector('.dropbtn');
var dropdownContent = document.querySelector('.dropdown-content');

// 点击下拉菜单按钮时触发
dropdownBtn.addEventListener('click', function() {
  // 切换下拉菜单内容的显示状态
  dropdownContent.style.display = (dropdownContent.style.display === 'block') ? 'none' : 'block';
});