Flutter Cloud Funtion: Update fieds in subcollection

I have a Cloud Function in my Flutter app with the following code:

const functions = require("firebase-functions");
const admin = require("firebase-admin");
admin.initializeApp();
const database = admin.firestore();
const min = "* * * * *";

exports.scheduled = functions.pubsub.schedule(min).onRun(async (context) => {
  const batch = database.batch();
  const {docs} = await database.collection("ADSGHANA").get();
  docs.map((doc) => {
    const {
      TimeToDelete,           //this is a Timestamp in a main collection
      TimestampArrival,      //this is a Timestamp in a main collection
      TimestampDeparture,    //this is a Timestamp in a main collection
      PostStatus,            //This is a string in a main collection
    } = doc.data();
    const now = admin.firestore.Timestamp.now();
    const starting = now > TimestampDeparture;    //If this time is reached an update in its corresponding subCollection should be performed.
    const ending = now > TimestampArrival;
    let val = PostStatus;
    if (starting) val = "Started";
    if (ending) val = "Completed";

    batch.set(
        database.doc(`ADSGHANA/${doc.id}`), {
          Timestamp: TimeToDelete? TimestampArrival : now,
          PostStatus: val,
          //the subCollection update should be here
        }, {
          merge: true,
        },
    );
  });

  await batch.commit();
});

I want that if const starting = now > TimestampDeparture; is reached it should update a particular update field in its subcollection as in the flutter code below. Except that I don’t know how to write this in JS.

FirebaseFirestore.instance.collection("collectionName")
                          .doc(theDocumentID)
                          .collection("subCollectionName")
                          .where("AdStatus", whereNotIn: ["Passenger cancelled",
                           "Rejected"])
                          .get()
                          .then((value) =>
                              value.docs.forEach((element) {
                                  FirebaseFirestore.instance
                                  .collection(collectionName)
                                  .doc(uid)
                                  .collection(subCollectionName)
                                  .doc(element.id)
                                  .update({"AdStatus": "Started"});
                                    }));

Any help please?