Increment counter with Cloud Functions for Firebase

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.

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