DOM에 추가된 요소를 어떻게 감지하고 대응할 수 있나요?

var observeDOM = (function() {
  var MutationObserver = window.MutationObserver || window.WebKitMutationObserver;

  return function(obj, callback) {
    if (MutationObserver) {
      var mutationObserver = new MutationObserver(callback);
      mutationObserver.observe(obj, {childList: true, subtree: true});
      return mutationObserver;
    }
    // Fallback for legacy browsers
    else if (window.addEventListener) {
      obj.addEventListener('DOMNodeInserted', callback, false);
      obj.addEventListener('DOMNodeRemoved', callback, false);
    }
  }
})();

observeDOM(listEl, function(m) {
  var addedNodes = [], removedNodes = [];

  m.forEach(record => record.addedNodes.length && addedNodes.push(...record.addedNodes));
  m.forEach(record => record.removedNodes.length && removedNodes.push(...record.removedNodes));

  console.clear();
  console.log('Added:', addedNodes, 'Removed:', removedNodes);
});