code: function networkIdleCallback(fn, options = {timeout: 0}) {
    // Call the function immediately if required features are absent
    if (
      !('MessageChannel' in window) ||
      !('serviceWorker' in navigator) ||
      !navigator.serviceWorker.controller
    ) {
      DOMContentLoad.then(() => fn({didTimeout: false}));
      return;
    }
  
    const messageChannel = new MessageChannel();
    navigator.serviceWorker.controller
        .postMessage(
            'NETWORK_IDLE_ENQUIRY',
            [messageChannel.port2],
        );
  
    const timeoutId = setTimeout(() => {
      const cbToPop = networkIdleCallback.__callbacks__
          .find(cb => cb.id === timeoutId);
  
      networkIdleCallback.__popCallback__(cbToPop, true);
    }, options.timeout);
  
    networkIdleCallback.__callbacks__.push({
      id: timeoutId,
      fn,
      timeout: options.timeout,
    });
  
    messageChannel.port1.addEventListener('message', handleMessage);
    messageChannel.port1.start();
  }
docs: networkIdleCallback works similar to requestIdleCallback,
  detecting and notifying you when network activity goes idle
  in your current tab.
  @param {*} fn - A valid function
  @param {*} options - An options object