Select cascade Province italiane e comuni jquery

cascade-select-comuni.html

<-- Esempio parte in HTML -->
 
                    
<div style="margin-top:3rem;" class="row">
   <label for="billing_address">(È richiesto indirizzo completo)*</label>

  <!-- select delle province -->
      <div class="col-md-4 ">
         <div class="form-group">
             <select class="select form-control user-success" placeholder="Provincia" required="required" id="address_state">
                                    <option value="Agrigento">Agrigento</option>
                                        <option value="Alessandria">Alessandria</option>
                                        <option value="Ancona">Ancona</option>
                                        <option value="Arezzo">Arezzo</option>
                                        <option value="Ascoli Piceno">Ascoli Piceno</option>
                                        <option value="Asti">Asti</option>
                                        <option value="Avellino">Avellino</option>
                                        <option value="Bari">Bari</option>
                                        <option value="Barletta-Andria-Trani">Barletta-Andria-Trani</option>
                                        <option value="Belluno">Belluno</option>
                                        <option value="Benevento">Benevento</option>
                                        <option value="Bergamo">Bergamo</option>
                                        <option value="Biella">Biella</option>
                                        <option value="Bologna">Bologna</option>
                                        <option value="Bolzano">Bolzano</option>
                                        <option value="Brescia">Brescia</option>
                                        <option value="Brindisi">Brindisi</option>
                                        <option value="Cagliari">Cagliari</option>
                                        <option value="Caltanissetta">Caltanissetta</option>
                                        <option value="Campobasso">Campobasso</option>
                                        <option value="Caserta">Caserta</option>
                                        <option value="Catania">Catania</option>
                                        <option value="Catanzaro">Catanzaro</option>
                                        <option value="Chieti">Chieti</option>
                                        <option value="Como">Como</option>
                                        <option value="Cosenza">Cosenza</option>
                                        <option value="Cremona">Cremona</option>
                                        <option value="Crotone">Crotone</option>
                                        <option value="Cuneo">Cuneo</option>
                                        <option value="Enna">Enna</option>
                                        <option value="Fermo">Fermo</option>
                                        <option value="Ferrara">Ferrara</option>
                                        <option value="Firenze">Firenze</option>
                                        <option value="Foggia">Foggia</option>
                                        <option value="Forlì">Forl&igrave;-Cesena</option>
                                        <option value="Frosinone">Frosinone</option>
                                        <option value="Genova">Genova</option>
                                        <option value="Gorizia">Gorizia</option>
                                        <option value="Grosseto">Grosseto</option>
                                        <option value="Imperia">Imperia</option>
                                        <option value="Isernia">Isernia</option>
                                        <option value="La Spezia">La Spezia</option>
                                        <option value="L'Aquila">L'Aquila</option>
                                        <option value="Latina">Latina</option>
                                        <option value="Lecce">Lecce</option>
                                        <option value="Lecco">Lecco</option>
                                        <option value="Livorno">Livorno</option>
                                        <option value="Lodi">Lodi</option>
                                        <option value="Lucca">Lucca</option>
                                        <option value="Macerata">Macerata</option>
                                        <option value="Mantova">Mantova</option>
                                        <option value="Massa-Carrara">Massa-Carrara</option>
                                        <option value="Matera">Matera</option>
                                        <option value="Messina">Messina</option>
                                        <option value="Milano">Milano</option>
                                        <option value="Modena">Modena</option>
                                        <option value="Monza e della Brianza">Monza e della Brianza</option>
                                        <option value="Napoli">Napoli</option>
                                        <option value="Novara">Novara</option>
                                        <option value="Nuoro">Nuoro</option>
                                        <option value="Ogliastra">Ogliastra</option>
                                        <option value="Oristano">Oristano</option>
                                        <option value="Padova">Padova</option>
                                        <option value="Palermo">Palermo</option>
                                        <option value="Parma">Parma</option>
                                        <option value="Pavia">Pavia</option>
                                        <option value="Perugia">Perugia</option>
                                        <option value="Pesaro e Urbino">Pesaro e Urbino</option>
                                        <option value="Pescara">Pescara</option>
                                        <option value="Piacenza">Piacenza</option>
                                        <option value="Pisa">Pisa</option>
                                        <option value="Pistoia">Pistoia</option>
                                        <option value="Pordenone">Pordenone</option>
                                        <option value="Potenza">Potenza</option>
                                        <option value="Prato">Prato</option>
                                        <option value="Ragusa">Ragusa</option>
                                        <option value="Ravenna">Ravenna</option>
                                        <option value="Reggio Calabria">Reggio Calabria</option>
                                        <option value="Reggio nell'Emilia">Reggio nell'Emilia</option>
                                        <option value="Rieti">Rieti</option>
                                        <option value="Rimini">Rimini</option>
                                        <option value="Roma">Roma</option>
                                        <option value="Rovigo">Rovigo</option>
                                        <option value="Salerno">Salerno</option>
                                        <option value="Sassari">Sassari</option>
                                        <option value="Savona">Savona</option>
                                        <option value="Siena">Siena</option>
                                        <option value="Siracusa">Siracusa</option>
                                        <option value="Sondrio">Sondrio</option>
                                        <option value="Sud Sardegna">Sud Sardegna</option>
                                        <option value="Taranto">Taranto</option>
                                        <option value="Teramo">Teramo</option>
                                        <option value="Terni">Terni</option>
                                        <option value="Torino">Torino</option>
                                        <option value="Trapani">Trapani</option>
                                        <option value="Trento">Trento</option>
                                        <option value="Treviso">Treviso</option>
                                        <option value="Trieste">Trieste</option>
                                        <option value="Udine">Udine</option>
                                        <option value="Valle d'Aosta">Valle d'Aosta</option>
                                        <option value="Varese">Varese</option>
                                        <option value="Venezia">Venezia</option>
                                        <option value="Verbano-Cusio-Ossola">Verbano-Cusio-Ossola</option>
                                        <option value="Vercelli">Vercelli</option>
                                        <option value="Verona">Verona</option>
                                        <option value="Vibo Valentia">Vibo Valentia</option>
                                        <option value="Vicenza">Vicenza</option>
                                        <option value="Viterbo">Viterbo</option>
          </select>
       </div>
     </div>
        
  <!-- select dei comuni -->
     <div class="col-md-4 ">
        <div class="form-group">
           <select class="select form-control" placeholder="Città" required="required" id="address_city" disabled>
               <option value="">Seleziona comune</option>
                              
            </select>
         </div>
      </div>
        
                      

cascade-select-comuni.js

$( document ).ready(function() {

// Carico il json dei comuni 

var comuni_api='comuni.json';

  $.getJSON( comuni_api,{dataType: 'jsonp'})

  .done(function( data ) {

    // intercetto evento select province change

    $('select#address_state').on('change', function (e) {
        var optionSelected = $("option:selected", this);
        var provincia = this.value;
        var comune_obj = data[provincia];
        seleziono_comuni_per_province(provincia, comune_obj)
    });

    

})
  .fail(function( jqxhr, textStatus, error ) {
    var err = textStatus + ", " + error;
    console.log( "Request Failed: " + err + "-" + comuni_api + "--" + jqxhr);

})
});



// preno dcomini jsono e li appendo alla select

function prendo_comuni(){
  $.getJSON( "comuni.json", function( data ) {
  $.each( data, function( key, val ) { 
    $( "#address_city" ).append("<option data-province='" + val.provincia.nome + "' value='" + val.nome + "'>" + val.nome + "</option>");
  });

});
};

 // seleziona i comuni della provincia selezionata

  function seleziono_comuni_per_province(provincia, comune_obj){

   // var province = $("#donation_billing_address_state").find('option:selected').text(); // stores province
    //console.log(provincia + ' -> ');
    var $newoptions ='';
    Object.getOwnPropertyNames(comune_obj).forEach(
      function (val, idx, array) {
    
    //console.log(comune_obj[val].nome);
     
     // creao le option dei comuni
     if (comune_obj[val].nome != undefined){
        $newoptions +="<option data-province='" + escape(provincia) + "' value='" + comune_obj[val].nome + "'>" + comune_obj[val].nome + "</option>";
     }
    
     });

     // appendo le option dei comuni alla selct comuni
     $("#address_city").html($newoptions);

     $("#address_city").removeAttr("disabled"); // enables select
  }

comuni.json

improved algorithm performance