How to recursively list subcollections in Firestore - node.js

Question

We back up all our firestore collections daily, using an empty array [] in:
const client = new firestore.v1.FirestoreAdminClient();
return client
.exportDocuments({
name: databaseName,
outputUriPrefix: storageName,
collectionIds:[], // empty array backs up all collections and subcollections
})
However, according to the docs, were we ever to want to import data, we would need to import ALL collections and subcollections, which is substantial. In order to import more granularly (import only necessary collections), we need to provide exportDocuments.collectionIds an array of all our collections' and subcollections' names. ie: ['Coll1', 'Coll2', 'SubCollX', etc.]. Since our collections are subject to change, we need a way to get an array of the names of all collections and subcollections programatically.
This code gets the names of only the root collections, not any subcollections:
const functions = require('firebase-functions');
const admin = require('firebase-admin');
admin.initializeApp();
exports.getCollections = functions.https.onRequest(async (req, res) => {
var rootCollectionIds = [];
var subCollectionIds = [];
const rootCollections = await admin.firestore().listCollections();
rootCollections.map(async rootColl => {
rootCollectionIds.push(rootColl.id);
// without named doc, this returns empty:
const subCollections = await rootColl.doc().listCollections();
subCollectionIds = subCollections.map(subColl => subColl.id);
});
res.status(200).send({"rootCollectionIds": rootCollectionIds, "subCollectionIds": subCollectionIds});
});
This must be doable. In the console (https://console.cloud.google.com/firestore/import-export?project={myproject}), Google lists all collections and subcollections when using
Export one or more collection groups
Best I can tell, listCollections() onlys works at the root, or on a specific document. Since we do not know which specific documents contain subcollections, we need a way to find the subcollections globally. Is there some kind of allDescendents param that could be used?
Running node.js in a Cloud Function.
Any help greatly appreciated.

Answer 1

No, no such option exists. The only way you can find out about subcollections is to build a DocumentReference and call listCollections on it to find which subcollections are available under that one document.