TL;DR
Use Function.prototype as a noop callback to quickly get rid of the error message.
https://maps.googleapis.com/maps/api/js?key=YOUR_API_KEY&callback=Function.prototype
Full Explanation
According to the Google Maps API documentation, the callback parameter has been required for a very long time. In practice, however, Google has only recently (within the past few days) begun enforcing this requirement.
While it doesn’t seem to break anything, it now shows an ugly error message in the console…
Loading the Google Maps JavaScript API without a callback is not supported.
How can I fix it?
The short answer is to specify a callback value. Set the name of the JavaScript function that you would like to trigger once the external API library has finished loading.
https://maps.googleapis.com/maps/api/js?key=YOUR_API_KEY&callback=FUNCTION_NAME
Be warned, you must specify a real function name! Otherwise you’ll trigger an “is not a function” error message, and simply be trading one error message for another.
But I don’t have/need a callback function!
If you don’t actually have a callback function to run immediately after the library loads, you can use Function.prototype as a noop stand-in.
https://maps.googleapis.com/maps/api/js?key=YOUR_API_KEY&callback=Function.prototype
The Function.prototype method is native to JavaScript, and does absolutely nothing… It’s exactly what you need in a situation like this!
For more information about Function.prototype, see this unrelated SO thread…