onCreate handler's DocumentSnapshot undefined - node.js

Question

I'm trying to set a trigger for an onCreate event in a subcollection. I can get the event to trigger fine, but for some reason the DocumentSnapshot is undefined when I try to access it.
My function is based on the sample here: https://github.com/firebase/functions-samples/blob/master/child-count/functions/index.js
exports.onCreateReview = functions.firestore
.document('spots/{spotId}/reviews/{reviewId}')
.onCreate(
(snapshot, _) => {
console.log(snapshot.exists())
const collectionRef = snapshot.ref.parent
const countRef = collectionRef.parent.child('num_reviews');
const promise = countRef.transaction((current) => {
return current + 1;
});
console.log('num_reviews incremented');
return promise;
});
When I check the logs in my Firebase project, I get a TypeError: Cannot read property of undefined

Answer 1

A snapshot is not a promise as it can fire multiple times whereas a promise resolves once.
Here's a snippet of one of my .onCreate functions that you might find helpful as a guide.
exports.newFeedback = functions.firestore.document("feedback/{feedback}").onCreate(function (snap, context) {
var feedbackObj = snap.data();
var msg = feedbackObj.message;
var subject = "New Feedback: " + msg.substring(0,36);
var feedbackMsg = {
to: "ron#hightechtele.com",
from: "noreply#hightechtele.com",
subject: subject,
templateId: "----------------------------",
substitutionWrappers: ["{{", "}}"],
substitutions: {
message: msg,
senderDisplayName: feedbackObj.sender,
senderEmail: feedbackObj.senderEmail,
senderUid: feedbackObj.senderUid
}
};
return sendgrid.send(feedbackMsg);
});