code: function forEls(parent, fn) {
      let i = 0;
      let child = parent.firstElementChild;
      while (child) {
          fn(child, i++);
          child = child.nextElementSibling;
      }
  }
docs: Calls a function on each element within a parent element.
  
  @param {HTMLElement} parent The parent to walk through.
  @param {(el: HTMLElement, i: number) => any} fn The function to call.