Firebase Cloud Functions - snapshot.forEach(...).then is not a function

Question

My db layout is:

@posts
   @postId_abc
     -total_score: ... // will update from cloud function
     -score_count: ... // will update from cloud function

@scores
   @postId_abc
      -uid_1: 10
      -uid_2: 20
      _uid_3: 50

Whenever a user sets a score I want to use a cloud function to add up all of the scores from the scores ref and set them to the total_score property on that particular post. When I try the following code below I get errors:

FIREBASE WARNING: Exception was thrown by user callback. TypeError: snapshot.forEach(...).then is not a functio and Exception from a finished function: TypeError: snapshot.forEach(...).then is not a function

It seems my snapshot.forEach((child) => { ... }).then(() => { isn't working but the scoreCountProperty.set(...) does increment.

const functions = require('firebase-functions');
const admin = require('firebase-admin');
admin.initializeApp();

exports.updateScore = functions.https.onCall((data, response) => {

    const postId = data.postId;

    const postsRef = admin.database().ref('/posts/' + postId);
    const scoreCountProperty = admin.database().ref('/posts/' + postId + '/' + 'score_count');

    var totalScore = 0.0;

    admin.database().ref('scores').child(postId).once('value', snapshot => {

    if (snapshot.exists()) {

        snapshot.forEach((child) => {

            totalScore += child.val()

        })
        .then(() => { 
                
            console.log('totalScore: ', totalScore);

            return postsRef.set({ "total_score": totalScore });       
        })
        .then(() => { 
                
            return scoreCountProperty.set(admin.database.ServerValue.increment(1));                
        })
        .catch((error) => {
            console.log('ERROR - updateScore Failed: ', error);
        });
    });
});

score 0 · Accepted Answer · answered Feb 15 '21 at 04:33

As the error message says, Snapshot.forEach() doesn't return an object on which you can call then(). In fact, I'm pretty sure it returns nothing at all.

But you don't need a then() anyway, since Snapshot.forEach() is not an synchronous operation.

So this should be what you want:

snapshot.forEach((child) => {
    totalScore += child.val()
})
console.log('totalScore: ', totalScore);

return postsRef.set({ "total_score": totalScore }).then(() => { 
    return scoreCountProperty.set(admin.database.ServerValue.increment(1));                
})
.catch((error) => {
    console.log('ERROR - updateScore Failed: ', error);
});

Firebase Cloud Functions - snapshot.forEach(...).then is not a function

1 Answers1

Linked