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function onReadyPromise() {
  return new Promise(function(resolve, reject) {
    let readyState = document.readyState;
    if(readyState === 'interactive' || readyState === 'complete') {
      resolve();
    } else {
      window.addEventListener('DOMContentLoaded', resolve);
    }
  });
}

onReadyPromise().then(function onFulfilled() {
  console.log('DOM loaded');
});