Populating One Select Box Based on the Selection in Another Select Box – JQuery?

as you mentioned that : towns to be dynamic, and able to be read from a database you can get dynamic data by passing it with ajax.

Ajax Get from controller to read from database based Country which user has selected:

 <script>
        $(document).ready(function () {
           $("#counties").on('change', function () {
                var selectedItem = $(this).val();
                var ddlStates = $("#towns"); // will be update after success ajax call
                var statesProgress = $("#states-loading-progress");
                statesProgress.show();
                $.ajax({     // get states/towns from db from controller
                    cache: false,
                    type: "GET",
                    url: "@(Url.RouteUrl("GetStatesByCountryId"))", 

                    data: { "countryId": selectedItem, "addSelectStateItem": "true" },
                    success: function (data) { 
                        ddlStates.html('');
                        $.each(data, function (id, option) {
                            ddlStates.append($('<option></option>').val(option.id).html(option.name)); 
                        });  // populating result 
                        statesProgress.hide(); // hide loader
                    },
                    error: function (xhr, ajaxOptions, thrownError) {
                        alert('Failed to retrieve states.');
                        statesProgress.hide();
                    }
                });
            });
        });
    </script>

And on controller :

  public virtual IActionResult GetStatesByCountryId(string countryId, bool addSelectStateItem)
    {
        var model = _countryModelFactory.GetStatesByCountryId(countryId, addSelectStateItem);
        return Json(model);
    }

Then on Data access layer _countryModelFactory.GetStatesByCountryId()i get towns/states from db based the country/town user has selected.

Update :
It’s the way i retrieving states/towns from db (dynamic country/towns) and populating them to selectboxin my code.