如何创建dropdwon侧导航栏？

<ul class="navbar">
  <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>

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

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

.dropdown:hover .dropdown-content {
  display: block;
}

var dropdowns = document.getElementsByClassName("dropdown");
var i;

for (i = 0; i < dropdowns.length; i++) {
  dropdowns[i].addEventListener("mouseover", function() {
    this.getElementsByClassName("dropdown-content")[0].style.display = "block";
  });

  dropdowns[i].addEventListener("mouseout", function() {
    this.getElementsByClassName("dropdown-content")[0].style.display = "none";
  });
}