Can't authenticate google service account using Node.js

My goal is to allow people to add events to a calendar through my website (which uses Firebase currently.) I'm attempting to allow them to add the events to a service account; however, I'm having issues getting the service account authenticated. When I attempt to deploy my code, I get an 'unexpected token google' error at the await google.auth.getClient part. I have google API tools installed globally.

//ADD CALENDAR EVENT
const {google} = require('googleapis');
const calendar = google.calendar('v3');

//Authenticate Service Acounnt
function getCredentials() {
  const filePath = path.join(__dirname, 'credentials.json';
  if (fs.existsSync(filePath)) {
    return require(filePath)
  }
  if (process.env.CREDENTIALS) {
    return JSON.parse(process.env.CREDENTIALS)
  }
  throw new Error('Unable to load credentials')
}

//Create Calendar Event
function addEvent(event) {
  return new Promise(function(resolve, reject) {
      calendar.events.insert({
          calendarId: 'primary',
          resource: {
              'summary': event.eventName,
              'description': event.description,
              'start': {
                  'dateTime': event.startTime,
                  'timeZone': TIME_ZONE,
              },
              'end': {
                  'dateTime': event.endTime,
                  'timeZone': TIME_ZONE,
              },
          },
      }, (err, res) => {
          if (err) {
              console.log('Rejecting because of error');
              reject(err);
          }
          console.log('Request successful');
          resolve(res.data);
      });
  });
}

//Add Event To Service Acount 
exports.addEventToCalendar = functions.https.onRequest((request, response) => {

  const eventData = {
      eventName: request.body.eventName,
      description: request.body.description,
      startTime: request.body.startTime,
      endTime: request.body.endTime
  }; 

  const credentials = getCredentials();
  const client = await google.auth.getClient({
    credentials,
    scopes: 'https://www.googleapis.com/auth/calendar',
  }) 

  addEvent(eventData, client).then(data => {
      response.status(200).send(data);
      return;
  }).catch(err => {
      console.error('Error adding event: ' + err.message); 
      response.status(500).send(ERROR_RESPONSE); 
      return;
  });

}); 

I've been following a combination of these two tutorials: https://medium.com/zero-equals-false/integrating-firebase-cloud-functions-with-google-calendar-api-9a5ac042e869 https://dev.to/mornir/create-a-service-account-to-authenticate-with-google-5b1k

I've spent a lot of time googling what could be wrong, but to be honest, this google authentication stuff confuses the heck out of me. I'd appreciate whatever help I can get, thanks :)

• You want to create new event to the Google Calendar using the service account.
• credentials.json is the credential file of the service account.
• You want to achieve this using googleapis with Node.js.
• You have already been able to use Calendar API.

If my understanding is correct, how about this modification?

Modification points:

• When the service account is used, you can use google.auth.JWT instead of google.auth.getClient.
• In your script, client is given to addEvent(eventData, client). But at function addEvent(event) {}, client is not used.
• About path.join(__dirname, 'credentials.json';, ) is required to be added. And I think that const path = require("path"); is also required to added.

Modified script:

//ADD CALENDAR EVENT
const { google } = require("googleapis");
const path = require("path");  // Added

// --- I added below script.
const key = require(path.join(__dirname, 'credentials.json'));
const jwtClient = new google.auth.JWT(
  key.client_email,
  null,
  key.private_key,
  ["https://www.googleapis.com/auth/calendar"],
  null
);
const calendar = google.calendar({ version: "v3", auth: jwtClient });
// ---

//Create Calendar Event
function addEvent(event) {
  return new Promise(function(resolve, reject) {
    calendar.events.insert(
      {
        calendarId: "primary",
        resource: {
          summary: event.eventName,
          description: event.description,
          start: {
            dateTime: event.startTime,
            timeZone: TIME_ZONE
          },
          end: {
            dateTime: event.endTime,
            timeZone: TIME_ZONE
          }
        }
      },
      (err, res) => {
        if (err) {
          console.log("Rejecting because of error");
          reject(err);
          return;
        }
        console.log("Request successful");
        resolve(res.data);
      }
    );
  });
}

//Add Event To Service Acount
exports.addEventToCalendar = functions.https.onRequest((request, response) => {
  const eventData = {
    eventName: request.body.eventName,
    description: request.body.description,
    startTime: request.body.startTime,
    endTime: request.body.endTime
  };

  addEvent(eventData) // Modified
    .then(data => {
      response.status(200).send(data);
      return;
    })
    .catch(err => {
      console.error("Error adding event: " + err.message);
      response.status(500).send(ERROR_RESPONSE);
      return;
    });
});

Note:

• This modified script supposes that the object of request.body can be used for resource of calendar.events.insert().
• Is TIME_ZONE declared elsewhere? Please be careful this.
• When the email of service account is not shared with the Google Spreadsheet, the event cannot be created. So please be careful this.

References:

• google-api-nodejs-client
• Events: insert