example-3.js

function getElementsByClassName2(classNameStr) {

   function checkChildren(child) {

      // check if the child has a matching class. If so, push the the elements array
      if (child.classList.contains(classNameStr)) {
         elements.push(child)
      }

      // check if that child has children of its own. If so, call checkChildren one each child
      if (child.children) {
         const children = child.children
         children.forEach(checkChildren)
     }
   }

}