Promise returning empty value from Firestore when data is present

I have an asynchronous function that is called and a list of user ids are passed as an argument. The function queries firestore to get the token for the user id within that iteration.

async function getTokensForUsers(ids) {
  const promises = ids.map(async (id) => {
    console.log("Querying for user with id: " + id);
    const querySnapshot = await db.collection('users').where('deviceUid', '==', id).get();
    return querySnapshot.docs.map(doc => doc.data().deviceToken);
  });
  let tokens = await Promise.all(promises);
  return tokens.flat();
}

This method is called as follows:

const ids = await getIdsOfUsersWithName(name);
console.log("Getting tokens for users" + ids);
const tokens = await getTokensForUsers(ids);
console.log("Tokens" + tokens)

When the final console.log() prints, it prints as "Tokens,,,,," suggesting that it hasn't returned the data from the Promise, when I can see the data within the collection.

I have recreated this issue from the code you have provided and got the intended result. I have tried with both firebase V9(Modular) and with firebase V8(namespaced).

This issue seems to have the deviceToken field being empty in the firestore database collection. I recommend you to change this line

return querySnapshot.docs.map(doc => doc.data().deviceToken);

to

const data = querySnapshot.docs.map((doc) => doc.data().deviceToken);
    console.log(data);
    return data;

This will show you what is being returned from the doc.data().deviceToken of each document and as you mentioned you are getting "Tokens,,,,," which means you will probably will get something like this :

[ '' ]
[ '' ]
[ '' ]
[ '' ]
[ '' ]
Tokens,,,,

Which seems like your deviceToken field is empty.
Here’s my code which successfully worked.:

// Initialize Firebase
firebase.initializeApp(firebaseConfig);
const db = firebase.firestore();

async function getTokensForUsers(ids) {
  const promises = ids.map(async (id) => {
    console.log("Querying for user with id: " + id);
    const querySnapshot = await db
      .collection("users")
      .where("deviceUid", "==", id)
      .get();
    const data = querySnapshot.docs.map((doc) => doc.data().deviceToken);
    console.log(data);
    return data;
  });
  let tokens = await Promise.all(promises);
  return tokens.flat();
}

const name = "rohit";
async function fire() {
  const ids = await getIdsOfUsersWithName(name);
  console.log("Getting tokens for users: " + ids);
  const tokens = await getTokensForUsers(ids);
  console.log("Tokens" + tokens);
}

async function getIdsOfUsersWithName(name) { //assumed
  const querySnapshot = await db
    .collection("users")
    .where("name", "==", name)
    .get();
  return querySnapshot.docs.map((doc) => doc.data().deviceUid);
}
fire();