如何创建下拉汉堡菜单

<nav id="menu">
  <div class="hamburger"></div>
  <ul>
    <li><a href="#">菜单项1</a></li>
    <li><a href="#">菜单项2</a></li>
    <li><a href="#">菜单项3</a></li>
  </ul>
</nav>

#menu {
  position: relative;
  display: none;
}

.hamburger {
  width: 30px;
  height: 3px;
  background-color: #000;
  position: absolute;
  top: 50%;
  left: 50%;
  transform: translate(-50%, -50%);
  cursor: pointer;
}

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

li {
  margin-bottom: 10px;
}

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

var menu = document.getElementById('menu');
var hamburger = document.querySelector('.hamburger');

hamburger.addEventListener('click', function() {
  if (menu.style.display === 'none') {
    menu.style.display = 'block';
  } else {
    menu.style.display = 'none';
  }
});