To check whether Google Maps is fully loaded with JavaScript, we call addListenerOnce to listen for the first idle event triggered.
For instance, we write
google.maps.event.addListenerOnce(map, "idle", () => {
// ...
});
to call addListenerOnce to listen for the first idle event triggered by calling it with the map, 'idle', and the idle event handler callback.
If the callback is called, that means the Google Map is fully loaded.
