Working mutation observer

function waitForElm(selector) {
  return new Promise(resolve => {
    if (document.querySelector(selector)) {
      return resolve(document.querySelector(selector));
    }

    const observer = new MutationObserver(mutations => {
      if (document.querySelector(selector)) {
        resolve(document.querySelector(selector));
        observer.disconnect();
      }
    });

    observer.observe(document.body, {
      childList: true,
      subtree: true
    });
  });
}

waitForElm('.ief-entity-submit').then((elm) => {
  console.log('Element is ready');
  console.log(elm.textContent);
});