// Created on savesnippets.com · https://savesnippets.com/UlRiyqAxoZErA9
function domReady(fn) {
  if (document.readyState !== 'loading') {
    fn();
  } else {
    document.addEventListener('DOMContentLoaded', fn, { once: true });
  }
}

// Usage
domReady(() => {
  console.log('DOM is ready');
  document.querySelector('#app').textContent = 'Hello!';
});