js左边下拉导航俩级

<ul class="nav">
  <li><a href="#">菜单1</a></li>
  <li>
    <a href="#">菜单2</a>
    <ul class="sub-nav">
      <li><a href="#">子菜单2-1</a></li>
      <li><a href="#">子菜单2-2</a></li>
    </ul>
  </li>
  <li><a href="#">菜单3</a></li>
</ul>

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

.nav > li {
  position: relative;
}

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

.nav > li:hover .sub-nav {
  display: block;
}

document.querySelectorAll('.nav > li').forEach(item => {
  item.addEventListener('mouseenter', function() {
    this.querySelector('.sub-nav').style.display = 'block';
  });
  item.addEventListener('mouseleave', function() {
    this.querySelector('.sub-nav').style.display = 'none';
  });
});