forked and edited slack cleaner - should be a command line app - or lambda function

slack-cleaner.js

var https = require('https');
// CONFIGURATION 
var token           = 'YOUR TOKEN';
var channel         = 'THE CHANNEL ID (not name) YOU WANT TO CLEAN';
var privateChannel  = false;
var isDirectMessage = true
var delay           = 300; // delay between delete operations in millisecond
// GLOBALS 
var channelApi    = privateChannel ? 'groups' : 'channels';
var channelApi    = isDirectMessage ? 'conversations' : privateChannel
var baseApiUrl    = 'https://slack.com/api/';
var historyApiUrl = baseApiUrl + channelApi + '.history?token=' + token + '&count=1000&channel=' + channel;
var deleteApiUrl  = baseApiUrl + 'chat.delete?token=' + token + '&channel=' + channel + '&ts='
var messages      = [];

function deleteMessage() {
    if (messages.length == 0) {
        return;
    }
    var ts = messages.shift();
    https.get(deleteApiUrl + ts, function (res) {
        var body = '';
        res.on('data', function (chunk) {
            body += chunk;
        });
        res.on('end', function(){
            var response = JSON.parse(body);

            if (response.ok === true) {
                console.log(ts + ' ' +  response.channel + ' deleted!');
            } else if (response.ok === false) {
                messages.push(ts);
            }
            setTimeout(deleteMessage, delay);
        });
    }).on('error', function (e) {
        console.log("Got an error: ", e);
    });
}
// RUNTIME
https.get(historyApiUrl, function(res) {
    var body = '';
    res.on('data', function (chunk) {
        body += chunk;
    });
    res.on('end', function () {
        var response = JSON.parse(body);
        console.log(response)
        for (var i = 0; i < response.messages.length; i++) {
            messages.push(response.messages[i].ts);
        }
        deleteMessage();
    });
}).on('error', function (e) {
      console.log("Got an error: ", e);
});