Is it possible to test a Firebase trigger locally?

This in now possible. I finally find this clear medium post which explain how do it : https://medium.com/@moki298/test-your-firebase-cloud-functions-locally-using-cloud-functions-shell-32c821f8a5ce

To run a trigger locally, use the "functions shell" tool :

1 - My code for firestore (the medium post use RealTime Database) :

exports.testTrigger = functions.firestore
.document('messages/{messageId}')
.onCreate((snapshot, context) => {
    let message = snapshot.data()
    return snapshot.ref.set({
        ...message,
        status : 'sent' 
    })
})

2 - I run the firebase shell functions in the terminal

firebase functions:shell

3 - I call my "testTrigger" cloud function trigger with data in parameter

testTrigger({text:"hello"})

4- My firestore database has a new "message" updated by my trigger

Since May 21st 2020 it's now possible to test triggers locally!

See release blog post https://firebase.googleblog.com/2020/05/local-firebase-emulator-ui.html

Always good to also take a look at the release notes https://firebase.google.com/support/releases#may_21_2020

And note that 8.4 requires Node 10, as discussed in question Update of Firebase CLI to 8.4.0 gives errors about "Unsupported engine" saying '{"node":">=10"}'

Very recently, the Firebase team published an update to the Firebase CLI the added the ability to invoke other types of triggers locally. It's documented here. Be sure you've updated your CLI to the latest version to get this new functionality: npm install -g firebase-tools@latest

Doug answered it in the comment. While invoking a trigger is probably useful in some cases (thanks Doug), it isn't the same as a deployed trigger, which can't be done yet.

So I'll stick with deploying remotely and watching the remote log ... good 'ole Marco Polo debugging.