navigator.geolocation.getCurrentPosition(successCallback);

    function successCallback(position){
        var lat = position.coords.latitude;
        var long = position.coords.longitude;
    }

    var map;
    function initMap() {
      map = new google.maps.Map(document.getElementById('map'), {
        center: {lat: lat, lng: long},
        zoom: 8
      });
    }
    </script>
    <script src="https://maps.googleapis.com/maps/api/js?key=xxx&callback=initMap"
    async defer></script>

更新代碼～
這樣要怎麼把lat跟long 丟到initMap？