How to loop through an array of Firebase Firestore documents and push values into array

Question

I am writing a Firebase Cloud Function in node.JS that reads user data from Firebase Firestore. I am unable to push the token values into the token array and return all tokens at the end of the function. My code is as follows:

function getTokens(subscribers) {
    return new Promise(async function (resolve, reject) {
        const tokenArray = [];
        subscribers.forEach(async (subscriber) => {
            await firestore
                .collection('users')
                .doc(subscriber)
                .get()
                .then((user) => {
                    console.log("Getting user data for user: ", user);
                    const tokens = user.data().tokens;
                    if (tokens) {
                        tokens.forEach((token) => {
                            console.log("Adding token to token array"); // data is available here but does not push into tokenArray
                            tokenArray.push(token);
                        });
                    }
                }).catch((error) => { console.error(error); reject(error); });
        });
        console.log("Token Array -- Final: ", tokenArray);
        resolve(tokenArray);
    });
};

danh · Accepted Answer · 2021-12-03 21:29:07Z

3

The OP code can be corrected and made more concise as follows:

async function getTokens(subscribers) {
  const getUser = subscriber => firestore.collection('users').doc(subscriber).get();
  const promises = subscribers.map(getUser);
  const users = await Promise.all(promises);
  return users.map(user => user.data().tokens).flat()
}

Some notes:

decorate the function as async, since it includes an await
don't create any extra promise with Promise.new(), Firestore's get() returns a promise
collect get promises in an array with map, and run them with Promise.all()
mapping the resulting users' data.tokens produces an array of arrays. Flatten it and you're done.
a catch that only throws, is just like having no catch

edited Dec 3, 2021 at 21:29

answered Dec 3, 2021 at 18:12

danh

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1 Comment

LeadDreamer Over a year ago

I like the .map() with .flat(). Nice!

Dharmaraj · Accepted Answer · 2021-12-03 18:04:15Z

2

You cannot use async-await in a forEach loop. Try mapping an array of promises and then using Promise.all() as shown below:

function getTokens(subscribers) {
  return new Promise(async function (resolve, reject) {
    
    const subscribersDocs = await Promise.all(
      subscribers.map((subscriber) => {
        return firestore.collection("users").doc(subscriber).get();
      })
    );


    const tokenArray = subscribersDocs.reduce((acc, curr) => {
      const tokens = curr.data().tokens;
      if (tokens) {
        acc = acc.concat(tokens);
      }
      return acc;
    }, []);

    console.log("Token Array -- Final: ", tokenArray);
    resolve(tokenArray);
  });
}

Also checkout: Using async/await with a forEach loop

edited Dec 3, 2021 at 18:04

answered Dec 3, 2021 at 17:54

Dharmaraj

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2 Comments

LeadDreamer Over a year ago

A couple performance points: if your subscribers at the start is not an totally arbitrary list, it would be better to use a query to select them. Same cost in reads, but only one round trip. Similarly, rather than looping over tokens and using push(), you should be able to use .concat()

Dharmaraj Over a year ago

Thanks @LeadDreamer. I should have mentioned the in operator though it's useful only if the array has upto 10 elements.

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