$(document).ready(function(){
    
    function requestAjax(){
        $.ajax({
		  type: "POST",
          url: '/api/json/my_get_fields/',
          dataType: "json",
          success: function(data) {
            alert(data);
          }
        });
    }
    
    $("#form_add").on('change','#type_item', function() {
        var valThis = $(this).val();
        requestAjax();
    });
});

[{"name":"Lada(Ваз)","name_other":"vaz","category":"cars"},{"name":"BMW","name_other":"bmw","category":"cars"}]

console.log(data);
console.log(data[0].category);