Increment counter with Cloud Functions for Firebase

Question

I have seen an increment counter with Cloud Functions referencing Realtime Database, but not Firebase Firestore yet.

I have a cloud function that listens for new documents:

exports.addToChainCount = functions.firestore
    .document('chains/{name}')
    .onCreate((snap, context) => {

    // Initialize document
    var chainCounterRef = db.collection('counters').doc('chains');

    var transaction = db.runTransaction(t => {
        return t.get(chainCounterRef).then(doc => {
            // Add to the chain count
            var newCount = doc.data().count + 1;
            t.update(chainCounterRef, { count: newCount });
        });
    }).then(result => {
        console.log('Transaction success!');
    }).catch(err => {
        console.log('Transaction failure:', err);
    });
    return true;
});

I'm attempting the above transaction, but when I run firebase deploy in terminal I get this error:

error Each then() should return a value or throw promise/always-return functions predeploy error: Command terminated with non-zero exit code1

This is my first attempt at anything node.js, and I'm not sure I've written this right.

Answer 1

There is now a much simpler way to increment/decrement a field in a document: FieldValue.increment(). Your sample would be something like this:

const FieldValue = require('firebase-admin').firestore.FieldValue;
var chainCounterRef = db.collection('counters').doc('chains');
chainCounterRef.update({ count: FieldValue.increment(1) });

See:

• Incrementing Values Atomically with Cloud Firestore

Answer 2

If you want to safely increment a number in a document, you can use a transaction. The following code is taken directly from the linked page. It adds one to a field called population in a document /cities/SF after giving it some initial values:

// Initialize document
var cityRef = db.collection('cities').doc('SF');
var setCity = cityRef.set({
  name: 'San Francisco',
  state: 'CA',
  country: 'USA',
  capital: false,
  population: 860000
});

var transaction = db.runTransaction(t => {
  return t.get(cityRef)
      .then(doc => {
        // Add one person to the city population
        var newPopulation = doc.data().population + 1;
        t.update(cityRef, { population: newPopulation });
      });
}).then(result => {
  console.log('Transaction success!');
}).catch(err => {
  console.log('Transaction failure:', err);
});

Bear in mind that Firestore is limited to one write per second under sustained load, so if you're going to be writing a lot, you will need to use a sharded counter instead.