Google Apps Script for sending Google Calendar event notifications to Mattermost. Inspired by: https://github.com/DjMuffinTops/Google-Calendar-API-to-Discord-Webhook

mattermost.gs

// Inspired by: https://github.com/DjMuffinTops/Google-Calendar-API-to-Discord-Webhook
// This Google Apps Script Will Send a POST to a Mattermost Webhook creating embed messages of any events starting within the next minute of execution.
// Any events that have already started will not appear. 
// This script should be triggered every minute using Google Triggers.
const CHANNEL_POST_URL = "MATTERMOST_WEBHOOK"; 
const CALENDAR_ID = "CALENDAR_ID";
const NO_VALUE_FOUND = "N/A";
const minsInAdvance = 15; // Set the number of minutes in advance you'd like events to be posted. Must be 1 or greater


// Import Luxon
eval(UrlFetchApp.fetch('https://cdn.jsdelivr.net/npm/luxon@2.0.2/build/global/luxon.min.js').getContentText());
let DateTime = luxon.DateTime;
const DTnow = DateTime.now().startOf('minute'); // Will consider 'now' as the beginning the minute to deal with second offsets issues with trigger over time.

function postEventsToChannel() {
  // .list parameters. See https://developers.google.com/calendar/api/v3/reference/events/list?hl=en
  let optionalArgs = {
    timeMin: DTnow.toISO(),
    timeMax: DTnow.plus({minutes: minsInAdvance}).toISO(), // Will only show events starting in the next x minutes
    showDeleted: false,
    singleEvents: true,
    orderBy: 'startTime'
  };
  let response = Calendar.Events.list(CALENDAR_ID, optionalArgs);
  let events = response.items;
  if (events.length > 0) {
    for (i = 0; i < events.length; i++) {
      let event = events[i];
      let ISOStartDate = event.start.dateTime || event.start.date;
      let ISOEndDate = event.end.dateTime || event.end.date;

      // The Calendar API's .list function will continously return events whose endDate has not been reached yet (timeMin is based on the event's end time)
      // Since this script is meant to run every minute, we have to skip these events ourselves
      if (DateTime.fromISO(ISOStartDate) < DTnow.plus({minutes: minsInAdvance - 1})) {
        Logger.log(`Event ${event.summary} [${event.id}] has already started. Skipping`);
        continue;
      }

      let StartDate = DateTime.fromISO(ISOStartDate).toLocaleString(DateTime.TIME_SIMPLE);
      let EndDate = DateTime.fromISO(ISOEndDate).toLocaleString(DateTime.TIME_SIMPLE);

      // Build the POST request
      let options = {
          "method": "post",
          "headers": {
              "Content-Type": "application/json",
          },
          "payload": JSON.stringify({
            "text": `[Google Calendar Event](${event.htmlLink}) \n **Start:** ${StartDate} \n **End:** ${EndDate} \n **Details:** ${event.summary}`
          })
      };
      Logger.log(options, null, 2);
      UrlFetchApp.fetch(CHANNEL_POST_URL, options);
    }
  } else {
    Logger.log(`No events starting within ${minsInAdvance} minute(s) found.`);
  }
}