code: function () {
          var result = {},
              fields = this.fields, 
              key, field;
              
          for (key in fields) {
              if (fields.hasOwnProperty(key)) {
                  field = fields[key];
                  if (field.isValid() && field.getValue() !== null) {
                      result[key] = field.getValue();
                  }
              }
          }
          return result;
      }
docs: Get and return the value of the filter.
  @return {String} The value of this filter