仅当悬停在父菜单项上时强制显示子菜单

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

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

.main-menu li {
  display: inline-block;
  position: relative;
}

.main-menu li a {
  display: block;
  padding: 10px 20px;
  text-decoration: none;
  color: #333;
}

.sub-menu {
  display: none;
  position: absolute;
  top: 100%;
  left: 0;
  background: #fff;
  padding: 10px;
  border: 1px solid #ccc;
}

.main-menu li:hover .sub-menu {
  display: block;
}

var menuItems = document.querySelectorAll('.main-menu li');

for (var i = 0; i < menuItems.length; i++) {
  menuItems[i].addEventListener('mouseover', function() {
    var subMenu = this.querySelector('.sub-menu');
    if (subMenu) {
      subMenu.style.display = 'block';
    }
  });

  menuItems[i].addEventListener('mouseout', function() {
    var subMenu = this.querySelector('.sub-menu');
    if (subMenu) {
      subMenu.style.display = 'none';
    }
  });
}