How to load Google Maps libraries correctly?

Question

I am making a map, using Google api. How to load all needed maps libraries correctly? I get 'google is not defined'

<script src=" https://maps.googleapis.com/maps/api/js?key=key&callback=initMap&libraries=maps" async defer></script>

Must I put v.3exp or something else?

Is this code good?

<script>
        let map;
        function initMap() {


            map = new window.google.maps.Map( document.getElementById( 'map' ), {
                center: {
                    lat: 51.513329,
                    lng: -0.088950
                },
                zoom: 14
            });



        }



google.maps.event.addDomListener(window, 'load', initialize); //error here
google.maps.event.addListener(map, 'click', function(event) {document.write('Hi');});

    </script>

I tried all variants, no work