Add counter for Firestore Collection

counter.function.js

// functions/income/counter.function.js

'use strict'

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

// Prevent firebase from initializing twice
try { admin.initializeApp(functions.config().firebase) } catch (e) {}

// My collection is called "incomes"
const incomeRef = functions.firestore.document('incomes/{incomeId}')
// My counter is allocated in `/counters/incomes`, maybe you wanna follow this structure instead: https://firebase.google.com/docs/firestore/solutions/counters
const counterRef = functions.firestore.document('counters/incomes')
// These references are not the same as Firestore DocumentReferences
// check https://firebase.google.com/docs/reference/functions/functions.firestore.DocumentBuilder

// Perform an increment when income is added
module.exports.incrementIncomesCounter = incomeRef.onCreate(event => {
  const counterRef = event.data.ref.firestore.doc('counters/incomes')

  counterRef.get()
  .then(documentSnapshot => {
    const currentCount = documentSnapshot.exists ? documentSnapshot.data().count : 0

    counterRef.set({
      count: Number(currentCount) + 1
    })
    .then(() => {
      console.log('Incomers counter increased!')
    })
  })
})

// Perform an decrement when income is deleted
module.exports.decrementIncomesCounter = incomeRef.onDelete(event => {
  const counterRef = event.data.ref.firestore.doc('counters/incomes')

  counterRef.get()
  .then(documentSnapshot => {
    const currentCount = documentSnapshot.exists ? documentSnapshot.data().count : 0

    counterRef.set({
      count: Number(currentCount) - 1
    })
    .then(() => {
      console.log('Incomers counter decreased!')
    })
  })
})

// Perform a fresh recount(this is expensive) when the counter is deleted (This is optional as well)
module.exports.recountIncomesCount = counterRef.onDelete(event => {
  const incomesRef = event.data.ref.firestore.collection('incomes')

  return incomesRef.get()
    .then(querySnapshot => {
      counterRef.set({
        count: querySnapshot.docs.length
      })
    })
})

index.js

// functions/index.js

/** EXPORT ALL FUNCTIONS
 *
 *   - Loads all `.function.js` files
 *   - Supports multiple exports from a single `.function.js` file
 *   - It is optimized with `FUNCTION_NAME` env and omiting `node_modules` as well
 *   - Every function from any file must have unique name
 *   - Default export is not supported (`module.exports = ...`), instead use: `module.exports.functionName = ...` 
 *
 *   Based on this thread:
 *     https://github.com/firebase/functions-samples/issues/170
 */
const glob = require('glob') // npm i -S glob
const files = glob.sync('./**/*.function.js', { cwd: __dirname, ignore: './node_modules/**' })

files.forEach(file => {
  const functionModule = require(file)
  const functionNames = Object.keys(functionModule)

  functionNames.forEach(functionName => {
    if (!process.env.FUNCTION_NAME || process.env.FUNCTION_NAME === functionName) {
      exports[functionName] = functionModule[functionName]
    }
  })
})

The index.js works great for me, thanks!

@AntonKL I haven't tested the example function, but I'm using admin.firestore() to get a firestore reference.