JSFiddle - React, Tailwind, and code Playground

HTML

<ul id="list1">
<li>
  <span class="label">List 1 Item 1</span>
  <ul id="list2">
  <li>
    <span class="label">List 2 Item 1</span>
  </li>
  <li>
    <span class="label">List 2 Item 2</span>
  </li>
  </ul>
</li>
<li>
  <span class="label">List 1 Item 2</span>
</li>
</ul>

<div>Output:</div>
<div id="output"></div>

JavaScript

(function( $ ) {

  $.fn.closestDown = function(selector) {

    var selection = [];

    var query = function () {
      this.children().each(function() {
        if ($(this).is(selector)) {
          selection.push(this);
        }
        else {
          query.call(this);
        }
      });
    };
    query.call(this);

    return this.pushStack(selection, "closestDown", selector);
  };

})(jQuery);

$(function () {

  var selection = $("#list1").closestDown("li");

  selection.each(function() {
    $("#output").append($(".label", this).html() + "<br />");
  });

});