How to properly use Google's Spanner nodejs client library to do a transactional update in a Promise workflow

exampleTransaction.js

import spanner from '@google-cloud/spanner';
 
const spannerOptions = {
  min: 10,
  max: 100,
};
// Pretend projectId is a variable with your project ID string in it
// and instanceName is a variable with your db instance name in it
// and databaseName is a variable with your db database name in it
const db = spanner({ projectId }).instance(instanceName).database(databaseName, spannerOptions);

new Promise((resolve, reject) => {
  db.runTransaction(async (err, dbTrx) => {
    if (err) {
      // Error getting session, creating transactional locks, etc
      return reject(err);
    }
      
    // do stuff, like:
    dbTrx.insert(tableName, rowData);
    
    // If you'd like to bail out on the txn and have only done reads, do the following:
    return dbTrx.end().then(resolve).catch(reject);
    
    // If you'd like to bail but you've staged writes already, do:
    return dbTrx.rollback().catch((rollbackErr) => {
      return dbTrx.end().then(() => reject(rollbackErr)).catch((endErr) => reject(endErr));
    });
    // Ending may be changed to determine if promise should resolve or reject 
    // when a rollback was done vs an error performing the rollback, up to you
    
    
    // When done staging writes and would like to commit, do the following:
    // When passed no callback commit() returns a promise
    return dbTrx.commit()
    .then(resolve)
    .catch((commitErr) => {
      return dbTrx.end().then(() => reject(commitErr));
    }); // Release the session
  });
})
.then((resValue) => {
  console.log('resolved');
})
.catch((err) => {
  console.error('rejected');
});