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); }); };

Answer 1

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

Answer 2

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 is
• 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 user's data.tokens produces an array of arrays. Flatten it and you're done.
• a catch that only throws, is just like having no catch