js导航栏下拉悬浮

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

.navbar {
  background-color: #333;
  overflow: hidden;
}

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

.nav-item {
  position: relative;
}

.nav-item a, .dropbtn {
  display: inline-block;
  color: white;
  text-align: center;
  padding: 14px 16px;
  text-decoration: none;
}

.nav-item a:hover, .dropbtn:hover {
  background-color: #575757;
}

.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;
}

.show {
  display: block;
}

document.addEventListener('DOMContentLoaded', function() {
  var dropdowns = document.getElementsByClassName('dropdown');
  for (var i = 0; i < dropdowns.length; i++) {
    dropdowns[i].addEventListener('mouseover', function() {
      this.querySelector('.dropdown-content').classList.add('show');
    });
    dropdowns[i].addEventListener('mouseout', function() {
      this.querySelector('.dropdown-content').classList.remove('show');
    });
  }
});