Is there any solution to wait for nested forEach until return result using async/await in node.js

Question

I'm working on FCM and need device tokens for all member in channel/room to send push notifications and every member have multiple devices, for that i need two for loop.

I'm using async/await with firestore queries but it not wait for result, process it in background and move to next statement which need result data.

const notification = async (channelId) => {
    let tokens = []
    const members = await db.collection('channels/' + channelId + '/members').get();
    await members.forEach(async (member) => {
        const deviceTokens = await db.collection('users/' + member.id + '/devices').get();
        await deviceTokens.forEach(async (token) => {
            console.log(token.id);
            await tokens.push(token.data().token);
        })
    })
    console.log(tokens);
    return await sendPush(tokens); // calling other functions
}

I expect the output is tokens = ['token1', 'token2', 'token3'], but the actual output is tokens = []

Answer 1

forEach cannot be efficiently used together with async..await. Since a query returns query snapshot, an array should be iterated instead. Promise chains can be executed in series with for..of or in parallel with Promise.all and array map, as explained in related question, e.g.:

const notification = async (channelId) => {
    let tokens = [];
    const members = await db.collection('channels/' + channelId + '/members').get();
    for (const member of members.docs) {
      const deviceTokens = await db.collection('users/' + member.id + '/devices').get();
      for (const deviceToken of deviceTokens.docs) {
        tokens.push(deviceToken.data().token);
      }
    }

    return await sendPush(tokens);
}

await sendPush(...) will work correctly only if sendPush returns a promise.

Answer 2

I guess you missunderstood the usage of async/await.

You can use await for functions that are async. Array.forEach is not an async function so you dont need to use it for your inner loop.

I would recommend to use promises here:

    async doStuff(){
        let tokens = [];
        const members = await db.collection('channels/' + channelId + '/members').get();

        let promises = members.map(member => {
            return db.collection('users/' + member.id + '/devices').get();
        })
        // or the forEach
        let promises = [];

        members.forEach(member => {
            promises.push(db.collection('users/' + member.id + '/devices').get());
        });

        Promise
            .all(promises)
            .then(values => {
                values.map(token => {
                    console.log(token.id);
                    tokens.push(token.data().token);
                });

                sendPush(tokens);
            })
    }

Now you gather all the members then start for all members the token requests and when all() of them finished you can push your tokens in that array and send them.