Wait for an element be on DOM

waitForElement.ts

function waitForElm <T extends Element> (selector: string): Promise<T | null> {
  return new Promise(resolve => {
      if (document.querySelector(selector)) {
          return resolve(document.querySelector(selector) as T | null);
      }

      const observer = new MutationObserver(() => {
          if (document.querySelector(selector)) {
              resolve(document.querySelector(selector) as T | null);
              observer.disconnect();
          }
      });

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