如何使导航与悬停内联

<nav>
  <ul>
    <li><a href="#">首页</a></li>
    <li><a href="#">产品</a></li>
    <li><a href="#">服务</a></li>
    <li><a href="#">关于我们</a></li>
  </ul>
</nav>

nav ul {
  list-style: none;
  padding: 0;
  margin: 0;
}

nav li {
  display: inline-block;
  margin-right: 10px;
}

nav li a {
  text-decoration: none;
  color: #000;
}

nav ul.sub-menu {
  display: none;
  position: absolute;
  background-color: #fff;
  padding: 10px;
}

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

var navItems = document.querySelectorAll('nav li');

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

  navItems[i].addEventListener('mouseout', function() {
    this.querySelector('ul.sub-menu').style.display = 'none';
  });
}