firebase function on db trigger throws Function returned undefined, expected Promise or value - firebase

Question

my firebase function based on realtime database trigger looks like below
exports.on_user_created = functions.database.ref("/users/{id}")
.onCreate((change, context) => {
console.log("start of on_user_created ")
const user = change.val();
console.log("New user:::" + JSON.stringify(user))
const uid = user._uid
const referralCode = user._referralCode
console.log("creating referral node for uid:" + uid + " with code:" + referralCode)
if(referralCode === undefined){
console.error("No referral code created for the user while sign up. Referral node cannot be created.")
return true
}
var db = admin.database();
var ref = db.ref('referrals')
ref.child(referralCode).set({"uid": uid}).then(
(resp) => {
console.log("referral node created")
return true
}
).catch(
(err) => {
console.error("unable to create referral node on user create:" + err)
return true
}
)
})
it on run throws
5:47:02.035 AM on_user_created Function returned undefined, expected Promise or value
I am failing to understand why

Answer 1

Adapted following Doug's comment below: "If you have no async work to be done, it's typical to return null"
This is because you don't return the Promise returned by the set() asynchronous operation.
You should do something like:
exports.on_user_created = functions.database.ref("/users/{id}")
.onCreate((change, context) => {
console.log("start of on_user_created ")
const user = change.val();
console.log("New user:::" + JSON.stringify(user))
const uid = user._uid
const referralCode = user._referralCode
console.log("creating referral node for uid:" + uid + " with code:" + referralCode)
if(referralCode === undefined){
console.error("No referral code created for the user while sign up. Referral node cannot be created.")
return null // <-- See Doug's comment below.
}
var db = admin.database();
var ref = db.ref('referrals')
return ref.child(referralCode).set({"uid": uid}).then( // <-- !! Here we return
(resp) => {
console.log("referral node created")
return null
}
).catch(
(err) => {
console.error("unable to create referral node on user create:" + err)
return null
}
)
})
Note that you could streamline your code as follows if you don't need the console.log()s, e.g. for production.
exports.on_user_created = functions.database.ref("/users/{id}")
.onCreate((change, context) => {
const user = change.val();
const uid = user._uid
const referralCode = user._referralCode
if (referralCode === undefined) {
return null;
} else {
var db = admin.database();
var ref = db.ref('referrals')
return ref.child(referralCode).set({"uid": uid});
}
});
For more detail on the importance of returning the promises in a Cloud Function, I would suggest you watch the official video series and in particular the ones titled "Learn JavaScript Promises": https://firebase.google.com/docs/functions/video-series/