server.js

const { App } = require('@slack/bolt');
const async = require('async');

const app = new App({
  signingSecret: process.env.SLACK_SIGNING_SECRET,
  token: process.env.SLACK_BOT_TOKEN,
  userToken: process.env.SLACK_USER_TOKEN
});

//Listen for messages in channels
app.message(async ({ message, context, say }) => {
  var regexString = /(https:\/\/)([^.]+)(\.)(slack.com)(\/archives\/)([^\/]+)(\/p)([^.+-]+)/gm;

  //Regex for link to a Slack message
  //Checks if message is part of a thread, if it contains more than 3 replies and if the parent message contains a link to a Slack message
  if (message.thread_ts !== undefined) {

    try {
      // Call the chat.scheduleMessage method with a token
      var result = await app.client.channels.replies({
        // The token you used to initialize your app is stored in the `context` object
        token: process.env.SLACK_USER_TOKEN,
        channel: message.channel,
        thread_ts: message.thread_ts

      });

      if ((result.messages[0].reply_count == 3) && regexString.test(result.messages[0].attachments[0].from_url)) {


        var eventText = result.messages[0].attachments[0].from_url;
        var regexString = /(https:\/\/)([^.]+)(\.)(slack.com)(\/archives\/)([^\/]+)(\/p)([^.+-]+)/gm;
        var textChannel = "Looks like this discussion is continued in another thread: ";
        var match = regexString.exec(eventText);

        var sourceChannel = match[6];
        var timestamp = match[8];
        var len = timestamp.length;

        var ts = timestamp.substring(0, len - 6) + "." + timestamp.substring(len - 6);




        //Retrieve permalink where the message was cross-posted
        try {
          const permalink = await app.client.chat.getPermalink({
            token: context.botToken,
            channel: message.channel,
            message_ts: result.messages[3].ts
          })

          app.client.chat.postMessage({
            token: context.botToken,
            channel: sourceChannel,
            text: textChannel + permalink.permalink,
            thread_ts: ts,
            unfurl_media: true,
            unfurl_links: true
          })
          console.log(permalink)
        } catch (error) {
          console.error(error);
        }

      };

    }
    catch (error) {
      console.error(error);
    }

  }

});


(async () => {
  // Start the app
  await app.start(process.env.PORT || 3000);

  console.log('⚡️ Bolt app is running!');
})();