App.js

 // state
  const [offline, setOffline] = React.useState(false);
  // effects
  React.useEffect(() => {
    window.addEventListener("offline", offlineListener);
    return () => {
      window.removeEventListener("offline", offlineListener);
    };
  }, []);
  
  {/* add to jsx */}
  {offline ? (
    <div className="banner-offline">The app is currently offline</div>
  ) : null}