How to use the @google-cloud/firestore.FieldValue.arrayUnion function in @google-cloud/firestore

public toPartialUpdate(uuidv4: (() => string)): { [key: string]: any } {
    if (this.isNoop()) return {};
    return {
      _updates_: FieldValue.arrayUnion({
        _id_: uuidv4(),
        _data_: this.data,
      }),
    };
  }

public toPartialUpdate(): { [key: string]: any } {
    return {
      _updates_: FieldValue.arrayUnion({
        _timestamp_: FieldValue.serverTimestamp(),
        _data_: this._data,
      }),
    };
  }