multiple markers map async and only when #map exists

map.js

jQuery(window).on('load', function () {
          if(jQuery("#map").length){
              loadMap();
          }
  });


    function loadMap() {
            var script = document.createElement('script');
            script.defer = true;
            script.async = true;
            script.src = 'https://maps.googleapis.com/maps/api/js?key=' + '' + '&callback=initMap';
            document.body.appendChild(script);
    }

    function initMap() {

      var markers = [

        {
            "title": 'Laganas Rank',
            "lat": '37.731103',
            "lng": '20.863153',
            "description": 'Laganas Rank'
        },
        {
            "title": 'Alykes Rank',
            "lat": '37.841162',
            "lng": '20.768494',
            "description": 'Alykes Rank'
        },
        {
            "title": 'Vassilikos Rank',
            "lat": '37.715704',
            "lng": '20.980716',
            "description": 'Vassilikos Rank'
        },
        {
            "title": 'Agios Nikolaos Port Rank',
            "lat": '37.905918',
            "lng": '20.706018',
            "description": 'Agios Nikolaos Port Rank'
        },
        {
            "title": 'Tsilivi Rank',
            "lat": '37.816781',
            "lng": '20.864023',
            "description": 'Tsilivi'
        },
        {
            "title": 'Solomos Square Rank',
            "lat": '37.786384',
            "lng": '20.899399',
            "description": 'Solomos Square Rank'
        },
      {
          "title": 'Zakynthos International Airport "Dionysios Solomos"',
          "lat": '37.754452',
          "lng": '20.887103',
          "description": 'Zakynthos International Airport "Dionysios Solomos"'
      },
      {
          "title": 'Zakynthos Main Port',
          "lat": '37.779211',
          "lng": '20.901796',
          "description": 'Zakynthos Main Port'
      }];

        var image ="images/map-marker.png";

        var mapOptions = {
            zoom: 9,
            minZoom: 10,
            maxZoom: 19,
            mapTypeId: google.maps.MapTypeId.MAP,
            mapTypeControl: true,
            mapTypeControlOptions: {
                style: google.maps.MapTypeControlStyle.DROPDOWN_MENU
            },
            zoomControl: true,
            zoomControlOptions: {
                style: google.maps.ZoomControlStyle.LARGE,
                position: google.maps.ControlPosition.LEFT_CENTER
            },
            scrollwheel: false,
            scaleControl: true,
            panControl: true,
            streetViewControl: false,
            'disableDefaultUI': false,
            'styles': [{
                stylers: [{
                    lightness: 5
   }, {
                    saturation: -30
   }]
  }]
        };

        var map = new google.maps.Map(document.getElementById('map'), mapOptions);

        var bounds = new google.maps.LatLngBounds();
        var infoWindow = new google.maps.InfoWindow();

        for (var i = 0; i < markers.length; i++) {
            var data = markers[i];
            var myLatlng = new google.maps.LatLng(data.lat, data.lng);
            var marker = new google.maps.Marker({
                position: myLatlng,
                map: map,
                icon: image,
                title: data.title
            });
            bounds.extend(marker.position);
            (function (marker, data) {
                google.maps.event.addListener(marker, "click", function (e) {
                    infoWindow.setContent("<div class='map-text'>" + data.description + "</div>");
                    infoWindow.open(map, marker);
                });
            })(marker, data);
        }

        map.fitBounds(bounds);

        google.maps.visualRefresh = true;
        google.maps.event.addDomListener(window, "resize", function() {
         var center = map.getCenter();
         google.maps.event.trigger(map, "resize");
         map.setCenter(center);
        });
    }