var bounds = new google.maps.LatLngBounds();
cities.forEach(function (value, index) {
     var myLatLng = new google.maps.LatLng(value.lat, value.lng);
     bounds.extend(myLatLng);

     var marker = new google.maps.Marker({
          position: myLatLng,
          map: map,
          title: "My Town"
     });
});
map.fitBounds(bounds);