离子菜单不做动画，如何添加动画

<ul class="menu">
  <li>菜单项1</li>
  <li>菜单项2</li>
  <li>菜单项3</li>
</ul>

@keyframes slideIn {
  0% {
    transform: translateX(-100%);
  }
  100% {
    transform: translateX(0);
  }
}

@keyframes fadeIn {
  0% {
    opacity: 0;
  }
  100% {
    opacity: 1;
  }
}

.menu li {
  animation: slideIn 0.5s ease-in-out forwards;
  animation-delay: 0.2s;
  opacity: 0;
}

.menu li:nth-child(1) {
  animation-delay: 0.2s;
}

.menu li:nth-child(2) {
  animation-delay: 0.4s;
}

.menu li:nth-child(3) {
  animation-delay: 0.6s;
}

document.addEventListener('DOMContentLoaded', function() {
  var menuItems = document.querySelectorAll('.menu li');
  menuItems.forEach(function(item) {
    item.style.opacity = 1;
  });
});