hugo/docs/themes/gohugoioTheme/assets/js/menutoggle.js

// Grab any element that has the 'js-toggle' class and add an event listner for the toggleClass function
var toggleBtns = document.getElementsByClassName('js-toggle')
  for (var i = 0; i < toggleBtns.length; i++) {
    toggleBtns[i].addEventListener('click', toggleClass, false)
  }

function toggleClass() {
  // Define the data target via the dataset "target" (e.g. data-target=".docsmenu")
  var content = this.dataset.target.split(' ')
  // Find any menu items that are open
  var mobileCurrentlyOpen = document.querySelector('.mobilemenu:not(.dn)')
  var desktopCurrentlyOpen = document.querySelector('.desktopmenu:not(.dn)')
  var desktopActive = document.querySelector('.desktopmenu:not(.dn)')

  // Loop through the targets' divs
  for (var i = 0; i < content.length; i++) {
    var matches = document.querySelectorAll(content[i]);
    //for each, if the div has the 'dn' class (which is "display:none;"), remove it, otherwise, add that class
    [].forEach.call(matches, function(dom) {
        dom.classList.contains('dn') ?
        dom.classList.remove('dn') :
        dom.classList.add('dn');
         return false;
       });
        // close the currently open menu items
      if (mobileCurrentlyOpen) mobileCurrentlyOpen.classList.add('dn')
      if (desktopCurrentlyOpen) desktopCurrentlyOpen.classList.add('dn')
      if (desktopActive) desktopActive.classList.remove('db')

    }
  }