code: function ExistenceFilter(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 an ExistenceFilter.
  @memberof google.firestore.v1
  @interface IExistenceFilter
  @property {number|null} [targetId] ExistenceFilter targetId
  @property {number|null} [count] ExistenceFilter count
   
  Constructs a new ExistenceFilter.
  @memberof google.firestore.v1
  @classdesc Represents an ExistenceFilter.
  @implements IExistenceFilter
  @constructor
  @param {google.firestore.v1.IExistenceFilter=} [properties] Properties to set