dropdown.js

var toggleTheList = function(event) {
   var theBtn = event.target;
   var theList = theBtn.nextElementSibling;

   var theListIsHidden = theList.className.includes('hidden');
   if (theListIsHidden)
      theList.className = 'dropdown-list shown';
   else
      theList.className = 'dropdown-list hidden';
};

var hideOtherLists = function(event) {
   var theBtn = event.target;
   var theList = theBtn.nextElementSibling;

   var allList = document.getElementsByClassName('dropdown-list');
   var allListArray = Array.from(allList);
   var otherLists = allListArray.filter(function(oneList) {
      return oneList != theList
   });
   
   otherLists.forEach(function(oneList) {
      oneList.className = 'dropdown-list hidden';
   });
};

/* Main function */

var toggleList = function(event) {
   toggleTheList(event);
   hideOtherLists(event);
};

/* Binding to buttons */

var btnList = document.getElementsByClassName('dropdown-btn');
var btnArray = Array.from(btnList);

btnArray.forEach(function (btn) {
   btn.onclick = toggleList;
});