how to make an action triggered only if a field is changed using onUpdate trigger Firestore cloud function?

Question

There is a field in my event document called title which contains a boolean value. I have set onUpdate firestore trigger in my document. I want if my title is updated then I will do some action. but if other fields is updated then I will not perform any action at all. how to do that ?

I am okay if the function below is invoked every time there is an update on the document, but I want only do some further action only if the title is updated

exports.dbEventsOnUpdate = functions.firestore
.document('events/{eventId}').onUpdate(async (change,context) => {

        try {        
            const eventID = context.params.eventId

            if (titleIsUpdated) {
               return db.doc(``).set(newData)
            } else {
               // do nothing here
               return null
            }

        } catch(error) {
            console.log(error)
            return null
        }

    })

Answer 1

Currently, Firestore Cloud Functions cannot be triggered based on a field update. It's only triggered when a document is updated.

You can actually check if the title was updated using the following code:

const newValue = change.after.data();
const previousValue = change.before.data();

const titleIsUpdated = newValue.title !== previousValue.title;

But keep in mind that your function will always be triggered when a field is changed in that document. And this might incur more costs, since Cloud Functions charge based on functions invocations. (See pricing)