handler.js

'use strict'
const admin = require('firebase-admin')
const serviceAccount = require('your_firebase_key')
admin.initializeApp({
   credential: admin.credential.cert(serviceAccount)
})
const firestore = admin.firestore()
const settings = { timestampsInSnapshots: true }

module.exports.hello = async (event) => {
   const data = firebase.collection('my_nice_collection')
    .get()
    .then(s => {
      // do whatever you need with your data
    })
}