function onMessage(event) {
  const message = event.message;

  if (message.slashCommand) {
    switch (message.slashCommand.commandId) {
      case 1: // Help command
        return createHelpCard();
      case 2: // Block out day command
        return blockDayOut();
      case 3: // Cancel all meetings command
        return cancelAllMeetings();
      case 4: // Set auto reply command
        return setAutoReply();
    }
  }
}
Responds to a MESSAGE event triggered in Chat.
@param {object} event the event object from Chat
@return {function} call the respective function