code: function FieldTransform(properties) {
                          if (properties)
                              for (var keys = Object.keys(properties), i = 0; i < keys.length; ++i)
                                  if (properties[keys[i]] != null)
                                      this[keys[i]] = properties[keys[i]];
                      }
docs: Properties of a FieldTransform.
  @memberof google.firestore.v1.DocumentTransform
  @interface IFieldTransform
  @property {string|null} [fieldPath] FieldTransform fieldPath
  @property {google.firestore.v1.DocumentTransform.FieldTransform.ServerValue|null} [setToServerValue] FieldTransform setToServerValue
  @property {google.firestore.v1.IValue|null} [increment] FieldTransform increment
  @property {google.firestore.v1.IValue|null} [maximum] FieldTransform maximum
  @property {google.firestore.v1.IValue|null} [minimum] FieldTransform minimum
  @property {google.firestore.v1.IArrayValue|null} [appendMissingElements] FieldTransform appendMissingElements
  @property {google.firestore.v1.IArrayValue|null} [removeAllFromArray] FieldTransform removeAllFromArray
   
  Constructs a new FieldTransform.
  @memberof google.firestore.v1.DocumentTransform
  @classdesc Represents a FieldTransform.
  @implements IFieldTransform
  @constructor
  @param {google.firestore.v1.DocumentTransform.IFieldTransform=} [properties] Properties to set