slack-dump.js

const token = process.env.SLACK_TOKEN
const Slack = require('slack')
const bot = new Slack({token})

const CHANNEL = process.argv[2];
const SINCE = process.argv[3];
const UNTIL = process.argv[4];

function toSlackTime(t) {
  return new Date(t).getTime() / 1000.0;
}

async function getConversation(channelId) {
  let more = true;
  let messages = [];
  let since = toSlackTime(SINCE);
  let until = toSlackTime(UNTIL);

  while(more) {
     const response = await bot.conversations.history({
      channel: channelId,
      oldest: since,
      latest: until,
    });
    messages = messages.concat(response.messages);
    more = response.has_more;
    until = messages[messages.length - 1].ts;
  }
  return messages;
}

(async function main() {
  const users = (await bot.users.list({}))
    .members
    .reduce((us, u) => { us[u.id] = u.real_name; return us }, {});

  const channels = (await bot.channels.list({exclude_archived: false}))
    .channels
    .filter(c => !c.is_private && (!CHANNEL || c.name == CHANNEL));

  const conversations = channels.map(
    c => ({name: c.name, messages: getConversation(c.id)}));

  conversations.map(({name, messages}) => {
    messages.then(messages => {
      messages
        .filter(m => m.type == 'message')
        .reverse()
        .map(m => {
          const time = new Date(parseFloat(m.ts) * 1000)
            .toLocaleTimeString('en-US', {hour12: false})

          if(m.reactions) {
            const reactions = m.reactions
              .map(r => `:${r.name}:(${r.users.map(u => users[u]).join(', ')})`)
              .join(', ');
            console.log(`[${time}] ${users[m.user]}: ${m.text} [${reactions}]`);
          } else {
            console.log(`[${time}] ${users[m.user]}: ${m.text}`);
          }
        });
    });
  });
})();