Firestore return after promise

Question

I'm using Firestore, Cloud Functions and Dialogflow to create a Google Home app. My cloud function should return a calculation based on values in a collection, but I can only return a Promise.

// query firestore based on user
var transactions = db.collection('accounts')
               .doc(request.body.result.parameters.accountowner)
               .collection('transactions');
var accountbalance = await transactions.get()
    .then(snapshot => {
        var workingbalance = 0
        snapshot.forEach(doc => {
            workingbalance = workingbalance + doc.data().amount;
        });
        return workingbalance
    })
    .catch(err => {
        console.log('Error getting transactions', err);
    });

console.log(accountbalance)
balanceresponse = {
    "speech": request.body.result.parameters.accountowner + " has a balance of $" + accountbalance,
    "displayText": request.body.result.parameters.accountowner + " has an account balance of $" + accountbalance,
    "data": {},
    "contextOut": [],
    "source": "system"
}

response.send(balanceresponse);    

How can I wait for the promise to return the actual value before calling response.send? I have also tried to put the response.send inside a then, but the function completes and sends headers before the operation completes.

Answer 1

If you're working with promises, you need to return values as part of the Promise.then chain. This is usually done by chaining a bunch of .then() calls/blocks together. Remember, the then() portion will only be called when a Promise resolves.

In your case, it looks like you can just generate balanceresponse and call the response.send() after you calculate workingbalance , inside the same block of code. The call to response.send() doesn't need to be at the end of the function. So you might have that block look something like:

.then(snapshot => {
    var workingbalance = 0
    snapshot.forEach(doc => {
        workingbalance = workingbalance + doc.data().amount;
    });

    balanceresponse = {
      "speech": request.body.result.parameters.accountowner + " has a balance of $" + workingbalance,
      "displayText": request.body.result.parameters.accountowner + " has an account balance of $" + workingbalance,
      "data": {},
      "contextOut": [],
      "source": "system"
    }

    response.send(balanceresponse); 
})

If, for some reason, you wanted to separate the calculation and compute the balanceresponse in a different block, you could do so. I don't necessarily recommend this, but it has some merits (for example, you could put multiple catch blocks after each part, which I've shown here):

// query firestore based on user
var transactions = db.collection('accounts')
               .doc(request.body.result.parameters.accountowner)
               .collection('transactions');
await transactions.get()

    .then(snapshot => {
        var workingbalance = 0
        snapshot.forEach(doc => {
            workingbalance = workingbalance + doc.data().amount;
        });
        return workingbalance
    })

    .catch(err => {
        console.log('Error getting transactions', err);
    });

    .then( accountbalance => {
        balanceresponse = {
            "speech": request.body.result.parameters.accountowner + " has a balance of $" + accountbalance,
            "displayText": request.body.result.parameters.accountowner + " has an account balance of $" + accountbalance,
            "data": {},
            "contextOut": [],
            "source": "system"
        }
        response.send(balanceresponse);    
    })

    .catch(err => {
        console.log('Error sending response', err);
    });