dropdown.js

   /* Utility Functions */

const elementHasClass = function(element, theClass) {
   return element.className.includes(theClass);
};

const elementAddsClass = function (element, theClass) {
   var classArray = element.className.split(' ');
   var newClassArray = classArray.concat(theClass);
   element.className = newClassArray.join(' ');
};

const elementRemovesClass = function(element, theClass) {
   var classArray = element.className.split(' ');
   var newClassArray = classArray.filter((oneClass) => (oneClass != theClass));
   element.className = newClassArray.join(' ');
};

const elementTogglesClass = function(element, theClass) {
   if (elementHasClass(element, theClass))
      elementRemovesClass(element, theClass);
   else
      elementAddsClass(element, theClass);
};

   /* Subsequent Tasks */

const findTheList = function(event) {
   var theButton = event.target;
   var theList = theButton.nextElementSibling;
   // ---
   return theList
};

const findOtherShownLists = function(event) {
   var theList = findTheList(event);
   // ---
   var allShownLists = document.querySelectorAll('.dropdown-list.shown');
   var allShownListArray = Array.from(allShownLists);
   var otherShownListArray = allShownListArray.filter((oneList) => (oneList != theList));
   // ---
   return otherShownListArray
};

const toggleVisibility = function(arrayOfList) {
   // ---
   var toggleVisibilityClasses = function(oneList) {
      elementTogglesClass(oneList, 'shown');
      elementTogglesClass(oneList, 'hidden');
   };
   // ---
   arrayOfList.forEach((oneList) => toggleVisibilityClasses(oneList));
};

   /* Event Listeners */

const toggleTheList = function(event) {
   var theList = findTheList(event);
   toggleVisibility( [theList] );
};

const hideOtherShownLists = function(event) {
   var otherShownListArray = findOtherShownLists(event);
   toggleVisibility(otherShownListArray);
};

   /* Binding Lísteners */

const buttonList = document.getElementsByClassName('dropdown-button');
const buttonArray = Array.from(buttonList);

const bindListeners = function(button) {
   button.addEventListener('click', toggleTheList);
   button.addEventListener('click', hideOtherShownLists);
};

buttonArray.forEach(bindListeners);