processEvent.ts

 async processEvent(event: GoogleCalendarEvent) {
  // Test event for a summary in the format of '[#channel] ...'
  const regex = /^\[(#[a-z0-9-_]+)\]/;
  const elements = regex.exec(event.summary);

  // This event does not match format (could have been processed before)
  if (elements === null) {
    return;
  }

  // Channel is extracted from regex, message is in event description
  const channel = elements[1]; // first captured group
  const message = event.description;

  if (message) {
    await this.postSlackMessage(channel, message);
  }

  // Set new summary to '[!channel] ...' (or '[?channel] ...' if no message was found)
  const newSymbol = message ? '!' : '?';
  const newSummary = '[' + newSymbol + event.summary.substring(2);

  // Patch calendar event with new summary
  await this.calendar.events.patch({
    calendarId: process.env.EVENT_CALENDAR_ID,
    eventId: event.id,
    requestBody: {
      summary: newSummary
    }
  });
}

async postSlackMessage(channel: string, text: string) {
  const data = { channel, text, link_names: true };
  const config = { headers: { Authorization: 'Bearer ' + process.env.SLACK_TOKEN }};
  const endpoint = 'https://slack.com/api/chat.postMessage';

  await this.httpService.post(endpoint, data, config).toPromise();
}