js实现伸缩二级菜单

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

.submenu {
  display: none;
}

.submenu.active {
  display: block;
}

document.addEventListener('DOMContentLoaded', function() {
  const menuItems = document.querySelectorAll('.menu-item');

  menuItems.forEach(item => {
    item.addEventListener('click', function(e) {
      e.preventDefault();
      const submenu = this.nextElementSibling;
      submenu.classList.toggle('active');
    });
  });
});