i have a dropdown menu that is not closing after opening,

when i click on it , it opens , but after the second click it doesn't close

 function menuFunction(){
    var menu = document.getElementById('ani-menu');

   if(menu.style.zIndex = "-1"){
     menu.style.visibility = "visible"
     menu.style.transform = "translateY(0)"
     menu.style.opacity = "1"
     menu.style.zIndex = "1"
     
   }
   else{
    menu.style.visibility = "hidden"
     menu.style.transform = "translateY(-3em)"
     menu.style.opacity = "0"
     menu.style.zIndex = "-1"
   }

  }  


 <li class="menu-btn1" onclick="menuFunction()"><a class="services" href="#ICT">Services<i class="fas fa-arrow-down"></i></a>
<ul class="ani-menu" id="ani-menu">
  <li><a href="#ICT">ICT Solutions</a></li>
  <li><a href="#One-On-One">Trainings</a></li>
  <li><a href="#materials">Materials</a></li>
  <li><a href="#packs">Exclusive Offers</a></li>
</ul>
</li>