纯js实现二级导航栏

<nav class="navbar">
  <ul>
    <li><a href="#">首页</a></li>
    <li class="dropdown">
      <a href="#" class="dropbtn">产品</a>
      <div class="dropdown-content">
        <a href="#">产品1</a>
        <a href="#">产品2</a>
        <a href="#">产品3</a>
      </div>
    </li>
    <li><a href="#">关于我们</a></li>
  </ul>
</nav>

.navbar ul {
  list-style-type: none;
  margin: 0;
  padding: 0;
  overflow: hidden;
  background-color: #333;
}

.navbar li {
  float: left;
}

.navbar li a {
  display: block;
  color: white;
  text-align: center;
  padding: 14px 16px;
  text-decoration: none;
}

.dropdown {
  position: relative;
  display: inline-block;
}

.dropdown-content {
  display: none;
  position: absolute;
  background-color: #f9f9f9;
  min-width: 160px;
  box-shadow: 0px 8px 16px 0px rgba(0,0,0,0.2);
  z-index: 1;
}

.dropdown-content a {
  color: black;
  padding: 12px 16px;
  text-decoration: none;
  display: block;
  text-align: left;
}

.dropdown-content a:hover {
  background-color: #f1f1f1;
}

document.addEventListener('DOMContentLoaded', function() {
  var dropdowns = document.querySelectorAll('.dropdown');

  dropdowns.forEach(function(dropdown) {
    dropdown.addEventListener('mouseover', function() {
      this.querySelector('.dropdown-content').style.display = 'block';
    });

    dropdown.addEventListener('mouseout', function() {
      this.querySelector('.dropdown-content').style.display = 'none';
    });
  });
});

dropdown.addEventListener('mouseleave', function() {
  this.querySelector('.dropdown-content').style.display = 'none';
});