NodeJS script to delete messages in a Slack channel.

slack_delete_messages.js

#!/usr/bin/env node

// Channel ID is on the the browser URL.: https://mycompany.slack.com/messages/MYCHANNELID/
// Pass it as a parameter: node ./delete-slack-messages.js CHANNEL_ID

// CONFIGURATION #######################################################################################################

const token = ''; // You can learn it from: https://api.slack.com/custom-integrations/legacy-tokens 

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

let channel = '';

if (process.argv[0].indexOf('node') !== -1 && process.argv.length > 2) {
    channel = process.argv[2];
} else if (process.argv[0].indexOf('delete') !== -1 && process.argv.length > 1) {
    channel = process.argv[1];
} else {
    console.log('Usage: node ./delete-slack-messages.js CHANNEL_ID');
    process.exit(1);
}

const https         = require('https');
const baseApiUrl    = 'https://slack.com/api/';
const messages      = [];
const historyApiUrl = baseApiUrl + 'conversations.history?token=' + token + '&count=1000&channel=' + channel + '&cursor=';
const deleteApiUrl  = baseApiUrl + 'chat.delete?token=' + token + '&channel=' + channel + '&ts='
let   delay         = 300; // Delay between delete operations in milliseconds
let   nextCursor    = '';

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

function deleteMessage() {

    if (messages.length == 0) {

        if (nextCursor) {
            processHistory();
        }

        return;
    }

    const ts = messages.shift();

    https.get(deleteApiUrl + ts, function (res) {

        let body = '';

        res.on('data', function (chunk) {
            body += chunk;
        });

        res.on('end', function(){
            const response = JSON.parse(body);

            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') {
                    delay += 100; // If rate limited error caught then we need to increase delay.
                    messages.push(ts);
                }
            }

            setTimeout(deleteMessage, delay);
        });
    }).on('error', function (e) {
        console.error("Got an error: ", e);
    });
}

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

function processHistory() {

    https.get(historyApiUrl + nextCursor, function(res) {

        let body = '';
    
        res.on('data', function (chunk) {
            body += chunk;
        });
    
        res.on('end', function () {

            nextCursor = null;
    
            const response = JSON.parse(body);

            if (response.messages && response.messages.length > 0) {

                if (response.has_more) {
                    nextCursor = response.response_metadata.next_cursor;
                }

                for (let i = 0; i < response.messages.length; i++) {
                    messages.push(response.messages[i].ts);
                }
        
                deleteMessage();
            }
        });
    }).on('error', function (e) {
          console.error("Got an error: ", e);
    });
}

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

processHistory();

Credits to someone on the Internet.

Edit:

Original script by @firatkucuk