js实现ul二级菜单

<ul class="menu">
  <li>
    <a href="#">菜单项1</a>
    <ul class="submenu">
      <li><a href="#">子菜单项1-1</a></li>
      <li><a href="#">子菜单项1-2</a></li>
    </ul>
  </li>
  <li>
    <a href="#">菜单项2</a>
    <ul class="submenu">
      <li><a href="#">子菜单项2-1</a></li>
      <li><a href="#">子菜单项2-2</a></li>
    </ul>
  </li>
</ul>

.menu {
  list-style-type: none;
  padding: 0;
}

.menu > li {
  position: relative;
}

.submenu {
  display: none;
  position: absolute;
  top: 100%;
  left: 0;
  background-color: #fff;
  border: 1px solid #ccc;
  list-style-type: none;
  padding: 0;
  min-width: 150px;
}

.menu > li:hover .submenu {
  display: block;
}

document.addEventListener('DOMContentLoaded', function() {
  var menuItems = document.querySelectorAll('.menu > li');

  menuItems.forEach(function(item) {
    item.addEventListener('mouseenter', function() {
      var submenu = this.querySelector('.submenu');
      if (submenu) {
        submenu.style.display = 'block';
      }
    });

    item.addEventListener('mouseleave', function() {
      var submenu = this.querySelector('.submenu');
      if (submenu) {
        submenu.style.display = 'none';
      }
    });
  });
});