避免下拉子菜单元素在单击时关闭主下拉菜单

<div id="dropdown-menu">
  <button id="main-menu">主下拉菜单</button>
  <ul id="sub-menu">
    <li>子菜单1</li>
    <li>子菜单2</li>
    <li>子菜单3</li>
  </ul>
</div>

var dropdownMenu = document.getElementById('dropdown-menu');
var mainMenu = document.getElementById('main-menu');
var subMenu = document.getElementById('sub-menu');

dropdownMenu.addEventListener('click', function(event) {
  if (event.target !== mainMenu) {
    event.stopPropagation(); // 阻止事件冒泡
  }
});

subMenu.addEventListener('click', function(event) {
  event.stopPropagation(); // 阻止事件冒泡
});

// 其他相关操作代码...