Deletes slack public/private channel and chat messages.

delete-slack-messages.js

#!/usr/bin/env node

// Channel ID is on the the browser URL.: https://mycompany.slack.com/messages/MYCHANNELID/
// Token can get from: https://api.slack.com/custom-integrations/legacy-tokens
// Pass it as a parameter: node ./delete-slack-messages.js TOKEN CHANNEL_ID
// Or use it without download the script: curl -Ls RAW_GIST_URL | node - TOKEN CHANNEL_ID

// GLOBALS #############################################################################################################

const [, , token, channel, _delay = 300] = process.argv
let delay = _delay // Allow inscrease delay later - thanks `colinricardo` for pointing it out

if (!token || !channel) {
  console.log('Usage: node ./delete-slack-messages.js TOKEN CHANNEL_ID')
  console.log('Usage: curl -Ls RAW_GIST_URL | node - TOKEN CHANNEL_ID')
  process.exit(1)
}

const https = require('https')
const baseApiUrl = 'https://slack.com/api'
const historyApiUrl = `${baseApiUrl}/conversations.history?token=${token}&count=1000&channel=${channel}&cursor=`
const deleteApiUrl = `${baseApiUrl}/chat.delete?token=${token}&channel=${channel}&ts=`

// ---------------------------------------------------------------------------------------------------------------------
const sleep = ms => new Promise(r => setTimeout(r, +ms))

const getJsonAsync = url =>
  new Promise((resolve, reject) =>
    https
      .get(url, res => {
        let body = ''

        res.on('data', chunk => (body += chunk))
        res.on('end', () => resolve(JSON.parse(body)))
      })
      .on('error', reject)
  )

async function deleteMessage(messages) {
  console.log(`Deleting ${messages.length} messages`)

  while (messages.length > 0) {
    const ts = messages.shift()
    const response = await getJsonAsync(deleteApiUrl + ts)

    if (response.ok === true) {
      console.log(ts + ' deleted!')
    } else if (response.ok === false) {
      console.log(ts + ' could not be deleted! (' + response.error + ')')

      if (response.error === 'ratelimited') {
        await sleep(1000)
        delay += 100 // If rate limited error caught then we need to increase delay.
        messages.unshift(ts)
      }
    }

    await sleep(delay)
  }
}

// ---------------------------------------------------------------------------------------------------------------------

async function processHistory() {
  let nextCursor = ''
  while (true) {
    const { ok, error, messages, has_more, response_metadata } = await getJsonAsync(historyApiUrl + nextCursor)
    if (!ok) throw new Error(error)

    if (Array.isArray(messages)) await deleteMessage(messages.map(z => z.ts))
    else console.log('No message found')

    if (!has_more) break
    nextCursor = response_metadata.next_cursor
  }
}

// ---------------------------------------------------------------------------------------------------------------------

processHistory().catch(console.log)

Update:
You can now use the gist without downloading it (using cURL and make node execute script from stdin)

curl -Ls RAW_GIST_URL | node - TOKEN CHANNEL_ID

One thing here: if you get rate limited, then the script tries to reassign the delay variable, which is a const, so the script fails.

Something like the following will fix it:

Change:

const [, , token, channel, delay = 300] = process.argv

to:

const [, , token, channel] = process.argv
let delay = 300;

Yeah you're right, thanks for pointing it out @colinricardo 👍

Gist updated 😄