Firestore cloud function that maintains a document field that contains number of items in a collection

firestore-collection-count-cloud-function.js

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

const db = admin.firestore();

exports.updateMyCollectionCount = functions.firestore
  .document(`users/{userId}/my-collection/{itemId}`)
  .onWrite(event => {

    const isUpdate = event.data.exists && event.data.previous.exists;

    if (isUpdate) {
      console.log('Skipping because it is an update operation.');
      return null;
    }

    const userRef = db
      .collection('users')
      .doc(event.params.userId);

    return db.runTransaction(t => {
      return t.get(userRef).then(userDoc => {
        return userDoc.ref.collection('my-collection').get().then(collectionSnapshot => {
          const newCount = collectionSnapshot.size;

          const updateData = {
            'my-collection-count': newCount
          };

          // If the `user` document alread exists, we need to use `update` operation, 
          // otherwise, if we use `set`, then all the data on the `user` document 
          // will be overriden. On the other hand, if the `user` document doesn't 
          // exist, we cannot use `update` because it requires the document to exist. 
          if (userDoc.exists) {
            t.update(userRef, updateData);
          }
          else {
            t.set(userRef, updateData);
          }

          console.log('Count updated successfully. New count is: ' + newCount);
        });
      });
    });
  });