When to commit a mongo transaction? - mongodb

Question

below is a code snippet from https://www.mongodb.com/blog/post/quick-start-nodejs--mongodb--how-to-implement-transactions
It works perfectly but here is what I don't understand:
This function didn't call session.commitTransaction(), how does it commit the transaction?
it aborts the transaction by determining if isListingReservedResults is null or not null, but my implementation is to throw an error in the if block and catches the error then calling the session.abortTransaction() (which result in a "MongoError: Cannot call abortTransaction twice"), I wonder Why would this happen since i only call it onece.
async function createReservation(client, userEmail, nameOfListing, reservationDates, reservationDetails) {
const usersCollection = client.db("sample_airbnb").collection("users");
const listingsAndReviewsCollection = client.db("sample_airbnb").collection("listingsAndReviews");
const reservation = createReservationDocument(nameOfListing, reservationDates, reservationDetails);
const session = client.startSession();
try {
const transactionResults = await session.withTransaction(async () => {
const usersUpdateResults = await usersCollection.updateOne(
{ email: userEmail },
{ $addToSet: { reservations: reservation } },
{ session });
const isListingReservedResults = await listingsAndReviewsCollection.findOne(
{ name: nameOfListing, datesReserved: { $in: reservationDates } },
{ session });
if (isListingReservedResults) {
await session.abortTransaction();
return;
// throw new Error('message'); myi mplementaion, throw error here then abort the transaction in catch block
}
const listingsAndReviewsUpdateResults = await listingsAndReviewsCollection.updateOne(
{ name: nameOfListing },
{ $addToSet: { datesReserved: { $each: reservationDates } } },
{ session });
});
if (transactionResults) {
console.log("The reservation was successfully created.");
} else {
console.log("The transaction was intentionally aborted.");
}
} catch(e){
console.log("The transaction was aborted due to an unexpected error: " + e);
// await session.abortTransaction(); result in a "MongoError: Cannot call abortTransaction twice"
} finally {
await session.endSession();
}
}