Lovely and informative article. I would like to point out that functions can now be written in other languages like go and python.

"The Cloud Functions for Firebase SDK exports a functions.firestore object that allows you to create handlers tied to specific Cloud Firestore events..."

The firestore object functions seem to map very neatly to RESTful operations PUT, POST and DELETE (or CREATE, UPDATE and DELETE if you prefer the crud nomenclature). Any reason why there isn't a corresponding onRead() function? For example I might want to trigger an action on my backend based on unique visitors etc. Or is 5here a neater way to achieve this?

Good write-up.