observe undefined dom text

observe-undefined-dom-text.js

(function () {
  function observeUndefinedValue(selector, cb) {
    const targetNode = document.querySelector(selector);
    if (!targetNode) {
      return;
    }
    cb(targetNode);

    const config = { attributes: false, childList: true, subtree: false };

    const callback = (mutationList) => {
      mutationList.forEach((mutation) => {
        if (mutation.type === 'childList') {
          cb(targetNode);
        }
      });
    };
    const observer = new MutationObserver(callback);

    observer.observe(targetNode, config);
  }

  observeUndefinedValue('#zfu .red', (target) => {
    if (target.innerText.startsWith('undefined')) {
      target.style.display = 'none';
    } else {
      target.style.display = 'inline';
    }
  });
}());