main.js

// Create a class for the element
class ExpandingList extends HTMLUListElement {
  constructor() {
    // Always call super first in constructor
    // Return value from super() is a reference to this element
    self = super();

    // Get ul and li elements that are a child of this custom ul element
    // li elements can be containers if they have uls within them
    const uls = Array.from(self.querySelectorAll('ul'));
    const lis = Array.from(self.querySelectorAll('li'));

    // Hide all child uls
    // These lists will be shown when the user clicks a higher level container
    uls.forEach(ul => {
      ul.style.display = 'none';
    });

        // Look through each li element in the ul
    lis.forEach(li => {
      // If this li has a ul as a child, decorate it and add a click handler
      if (li.querySelectorAll('ul').length > 0) {
        // Add an attribute which can be used  by the style
        // to show an open or closed icon
        li.setAttribute('class', 'closed');

        // Wrap the li element's text in a new span element
        // so we can assign style and event handlers to the span
        const childText = li.childNodes[0];
        const newSpan = document.createElement('span');

        // Copy text from li to span, set cursor style
        newSpan.textContent = childText.textContent;
        newSpan.style.cursor = 'pointer';
        
        // Add click handler to this span
        newSpan.onclick = self.showul;
        
        // Add the span and remove the bare text node from the li
        childText.parentNode.insertBefore(newSpan, childText);
        childText.parentNode.removeChild(childText);
      }
    });
  }

  // li click handler
  showul = function (e) {
    // next sibling to the span should be the ul
    const nextul = e.target.nextElementSibling;

    // Toggle visible state and update class attribute on ul
    if (nextul.style.display == 'block') {
      nextul.style.display = 'none';
      nextul.parentNode.setAttribute('class', 'closed');
    } else {
      nextul.style.display = 'block';
      nextul.parentNode.setAttribute('class', 'open');
    }
  };
}

// Define the new element
customElements.define('expanding-list', ExpandingList, { extends: 'ul' });