transform the crazy firebase output in a generic type, the way it should be since the init

parseFirebaseSnapshot.ts

import { DocumentData, DocumentSnapshot, QuerySnapshot } from "firebase-admin/firestore";

type withDocId<T> = T & { docId: string }
type SingleDocument = DocumentSnapshot<DocumentData>;
type QueryDocuments = QuerySnapshot<DocumentData>;

type Snapshot = SingleDocument | QueryDocuments;
type SnapshotNullable = Snapshot | null;

const checkSingle = (snapshot: Snapshot): snapshot is SingleDocument => {
  return "id" in snapshot || !Array.isArray(snapshot.docs);
};

const toObject = <T extends object>(data: DocumentSnapshot<DocumentData>): withDocId<T> => {
  return { ...data.data() as T, docId: data.id };
};

type getReturn<T, U> = U extends SingleDocument ? T : T[];

export const parseFirebaseSnapshot = <R extends object, S extends SnapshotNullable = SingleDocument>(
  snapshot: S
): getReturn<withDocId<R>, S> => {
  if (!snapshot) {
    return {} as getReturn<withDocId<R>, S>;
  }
  const isSingle = checkSingle(snapshot);
  if (isSingle) {
    return toObject(snapshot);
  }
  return snapshot.docs.map(toObject<R>) as getReturn<withDocId<R>, S>;
};