how to call function of javascript in dropdown selected index change in asp.net

script type="text/javascript">
        $(document).ready(function () {
            $("#ddlpostoffice").onchange(function () {
                var geocoder = new google.maps.Geocoder();
                var strSearch = document.getElementById('txtSearch').value;
                geocoder.geocode({ 'address': strSearch }, function (results, status) {
                
                        map = new google.maps.Map(document.getElementById("Map_Div"), myOptions);
                        var marker = new google.maps.Marker({
                            position: new google.maps.LatLng(x, y),
                            map: map,
                            title: strSearch
                        });
                        s
                        var infowindow = new google.maps.InfoWindow({
                            content: strSearch
                        });
                        infowindow.open(map, marker);
                        google.maps.event.addDomListener(window, 'load');
                    } else {
                        document.getElementById('txtSearch').value = "This Location Map Not Found";
                    }
                });
            });
        });
    </script



 asp:DropDownList CssClass="select" ID="ddlpostoffice" runat="server" 
              AutoPostBack="True"    onselectedindexchanged="ddlpostoffice_SelectedIndexChanged" onchange="return false;"