复杂的JavaScript实现侧面下拉菜单的示例代码

下面是一个复杂的JavaScript实现侧面下拉菜单的示例代码，支持多级菜单、鼠标悬停展开子菜单、点击菜单项跳转链接等功能：

HTML代码：

<div class="menu">
  <button class="menu-toggle">菜单</button>
  <ul class="menu-items">
    <li><a href="#">菜单项1</a></li>
    <li>
      <a href="#">菜单项2</a>
      <ul>
        <li><a href="#">子菜单项1</a></li>
        <li>
          <a href="#">子菜单项2</a>
          <ul>
            <li><a href="#">子子菜单项1</a></li>
            <li><a href="#">子子菜单项2</a></li>
            <li><a href="#">子子菜单项3</a></li>
          </ul>
        </li>
        <li><a href="#">子菜单项3</a></li>
      </ul>
    </li>
    <li><a href="#">菜单项3</a></li>
  </ul>
</div>

CSS代码：

.menu {
  position: relative;
  width: 200px;
}

.menu-toggle {
  display: block;
  width: 100%;
  padding: 10px;
  background-color: #eee;
  border: none;
  text-align: left;
  cursor: pointer;
}

.menu-items {
  position: absolute;
  top: 0;
  left: 100%;
  z-index: 1;
  display: none;
  background-color: #fff;
  border: 1px solid #999;
  list-style: none;
  margin: 0;
  padding: 0;
}

.menu-items li {
  margin: 0;
  padding: 0;
}

.menu-items li a {
  display: block;
  padding: 10px;
  text-decoration: none;
  color: #333;
}

.menu-items li a:hover {
  background-color: #f5f5f5;
}

.submenu-items {
  position: absolute;
  top: 0;
  left: 100%;
  z-index: 1;
  display: none;
  background-color: #fff;
  border: 1px solid #999;
  list-style: none;
  margin: 0;
  padding: 0;
}

.submenu-items li {
  margin: 0;
  padding: 0;
}

.submenu-items li a {
  display: block;
  padding: 10px;
  text-decoration: none;
  color: #333;
}

.submenu-items li a:hover {
  background-color: #f5f5f5;
}

JavaScript代码：

var menuToggle = document.querySelector('.menu-toggle');
var menuItems = document.querySelector('.menu-items');

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

document.addEventListener('click', function(event) {
  if (!menuToggle.contains(event.target) && !menuItems.contains(event.target)) {
    menuItems.style.display = 'none';
  }
});

var menuLinks = document.querySelectorAll('.menu-items > li > a');
var submenuLinks = document.querySelectorAll('.menu-items ul li a');

for (var i = 0; i < menuLinks.length; i++) {
  menuLinks[i].addEventListener('click', function(event) {
    if (this.nextElementSibling && this.nextElementSibling.nodeName === 'UL') {
      event.preventDefault();
      var submenu = this.nextElementSibling;
      if (submenu.style.display === 'block') {
        submenu.style.display = 'none';
      } else {
        submenu.style.display = 'block';
      }
    }
  });
}

for (var j = 0; j < submenuLinks.length; j++) {
  submenuLinks[j].addEventListener('click', function(event) {
    if (this.nextElementSibling && this.nextElementSibling.nodeName === 'UL') {
      event.preventDefault();
      var submenu = this.nextElementSibling;
      if (submenu.style.display === 'block') {
        submenu.style.display = 'none';
      } else {
        submenu.style.display = 'block';
      }
    }
  });

  submenuLinks[j].addEventListener('mouseenter', function() {
    if (this.nextElementSibling && this.nextElementSibling.nodeName === 'UL') {
      var submenu = this.nextElementSibling;
      submenu.style.display = 'block';
    }
  });

  submenuLinks[j].addEventListener('mouseleave', function() {
    if (this.nextElementSibling && this.nextElementSibling.nodeName === 'UL') {
      var submenu = this.nextElementSibling;
      submenu.style.display = 'none';
    }
  });
}

以上代码实现了一个复杂的侧面下拉菜单效果，支持多级菜单、鼠标悬停展开子菜单、点击菜单项跳转链接等功能。您可以根据自己的需求进行修改和优化。