Function that waits for the DOM to be ready to be manipulated.

waitForReadyDOM.js

/**
 * Function that waits for the DOM to be ready to be manipulated.
 * @returns {Promise<undefined>}
 *   A promise that will be finished after the DOM is ready to be manipulated.
 */
function waitForReadyDOM() {
  return new Promise(async resolve => {
    if (document.readyState === 'loading') document.addEventListener('DOMContentLoaded', () => resolve());
    else resolve();
  });
}