export const waitForElement = (selector: string): Promise<Element> =>
  new Promise((resolve) => {
    const existing = document.querySelector(selector);
    if (existing) {
      return resolve(existing);
    }
    const observer = new MutationObserver(() => {
      const el = document.querySelector(selector);
      if (el) {
        observer.disconnect();
        resolve(el);
      }
    });
    observer.observe(document.documentElement, {
      childList: true,
      subtree: true,
    });
  });

// Usage
const modal = await waitForElement('.modal');
modal.classList.add('visible');