Firebase Cloud Function .onWrite always logs null or undefined - firebase

Question

I'm at a loss about why this won't log anything but "null" or "undefined" to the console. I'm testing this from the Google Cloud Platform testing browser interface. I've also tried logging EVENT (instead of CHANGE and CONTEXT) with the same result. I have also tried opening the security rules, but that also didn't help. Any advice is highly appreciated.
const functions = require('firebase-functions');
const admin = require('firebase-admin');
admin.initializeApp(functions.config().firebase);
exports.fanOutLink = functions.database.ref('/userLink/BLAH584H5BLAH30BLA/link').onWrite((change, context) => {
console.log('value is:'+change.before.val());
});
And here is the JSON I'm using to test the code above:
{
"userLink": {
"BLAH584H5BLAH30BLA": {
"link": "https://blabla.com"
}
}
}

Answer 1

A Cloud Function must always return a Promise (or if you cannot, at least a value).
Your function should work as is but with a delay and an error in the log like "Function returned undefined, expected Promise or value". It may happen that the Cloud Functions instance running your function shuts down before your function successfully write the message in the log.
If you change your code as follow you will get a (quasi) instant reply:
exports.fanOutLink = functions.database.ref('/userLink/BLAH584H5BLAH30BLA/link').onWrite((change, context) => {
console.log('value is:'+change.before.val());
return true;
});
I would suggest you have a look at those 2 videos from the Firebase team: https://www.youtube.com/watch?v=7IkUgCLr5oA&t=511s and https://www.youtube.com/watch?v=652XeeKNHSk&t=37s
Following our "discussion" in the comments below, it appears that you use the new Cloud Functions syntax but with an old version of the library. Look at this documentation item: https://firebase.google.com/docs/functions/beta-v1-diff, and do as indicated, before redeploying:
Run the following in the functions folder:
npm install firebase-functions#latest --save
npm installfirebase-admin#5.11.0 --save