Slack Appsにメンションすると反応するだけのGAS

slackbot.js

function doPost(e) {
    var postData = JSON.parse(e.postData.getDataAsString());
    if(postData.type == 'url_verification') {
    return ContentService.createTextOutput(postData.challenge);
  }

  // Slackからのメッセージを取得
  // app_mention
  // https://api.slack.com/events/app_mention
  var payload   = JSON.parse(e.postData.contents);
  var user      = payload.event.user;
  var text      = payload.event.text;
  var ts        = payload.event.ts;
  var channel   = payload.event.channel;
  var event_ts  = payload.event.event_ts;
  var botUserId = "XXXX"

  if (text.includes(`<@${botUserId}>`)) {
    postMessage("こんにちは", channel,ts);
  }

}

function postMessage(message, channelId, ts) {
  // Slackにメッセージを投稿
  var slackToken = "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx";
  var slackApiUrl = "https://slack.com/api/chat.postMessage";
  
  var payload = {
    method: "post",
    headers: { "Authorization": "Bearer " + slackToken },
    contentType: "application/json",
    payload: JSON.stringify({
      text: message,
      channel: channelId,
      thread_ts: ts
    })
  };
  
  UrlFetchApp.fetch(slackApiUrl, payload);
}