不能获取变量值

我正在编写用于地理定位的javascript代码。我不明白为什么它不拾取全局声明的变量的值。

我如何正确传递"lat和"lng"变量?我错过了什么?

var lat; //Works if I set value manually
var lng;
if (navigator.geolocation) {
    navigator.geolocation.getCurrentPosition(showPosition);
} else {
    x.innerHTML = "Geolocation is not supported by this browser.";
}
function showPosition(position) {
    lat = position.coords.latitude; //Doesn't pick up this value
    lng = position.coords.longitude; //Doesn't pick up this value
    document.getElementById('demo').innerHTML = lat; //Value is correct
    document.getElementById('demo2').innerHTML = lng; //Value is correct
}     
var map;
var marker;
var myLatlng = new google.maps.LatLng(lat,lng); //Works if I add value manually
var geocoder = new google.maps.Geocoder();
var infowindow = new google.maps.InfoWindow();
function initialize(){
    var mapOptions = {
        zoom: 18,
        center: myLatlng,
        mapTypeId: google.maps.MapTypeId.ROADMAP
    };
    map = new google.maps.Map(document.getElementById("myMapSearch"), mapOptions);
    marker = new google.maps.Marker({
        map: map,
        position: myLatlng,
        draggable: true 
    });     
    geocoder.geocode({'latLng': myLatlng }, function(results, status) {
        if (status == google.maps.GeocoderStatus.OK) {
            if (results[0]) {
                $('#address').val(results[0].formatted_address);
                $('#latitude').val(marker.getPosition().lat());
                $('#longitude').val(marker.getPosition().lng());
                infowindow.setContent(results[0].formatted_address);
                infowindow.open(map, marker);
            }
        }
    });

    google.maps.event.addListener(marker, 'dragend', function() {
        geocoder.geocode({'latLng': marker.getPosition()}, function(results, status) {
            if (status == google.maps.GeocoderStatus.OK) {
                if (results[0]) {
                    $('#address').val(results[0].formatted_address);
                    $('#latitude').val(marker.getPosition().lat());
                    $('#longitude').val(marker.getPosition().lng());
                    infowindow.setContent(results[0].formatted_address);
                    infowindow.open(map, marker);
                }
            }
        });
    });
}
google.maps.event.addDomListener(window, 'load', initialize);