a

 jQuery("#billing_city").on('change', function(e){
        //console.log(this.value);
        jQuery("input[name='billing[city]']").val(this.value);

        var UnavailableCity = ["Bangued", "Boliney", "Bucay", "Bucloc" ,"Daguioman","Danglas","Dolores"
            ,"La Paz","Lacub","Lagangilan","Lagayan","Langiden","Licuan","Luba","Malibcong"
            ,"Manabo","Peñarrubia","Pidigan","Pilar","Sal-Lapadan","San Isidro","San Juan"
            ,"San Quintin","Tayum","Tineg","Tubo","Villaviciosa"];

        if (this.value == UnavailableCity){
            alert('City Unavailable');
            }

        else{
            alert('City Available');
        }

    });