simple discord bot to react to every message in a list/array of channel IDs

bot.js

const Discord = require('discord.js');
const bot = new Discord.Client({ intents: [Discord.Intents.FLAGS.GUILDS] });

// config
let botToken = 'Insert-Bot-Token-Here';
let messageLimit = 2000; //number of messages to look back by not limited by discord.js default of 100
let emoji = '✅'; //emoji to react with
let channelsList = ['952594922194739220','952587451728285746']; //list of channels to react in


let reactionTotal = 0;

async function fetchMore(channel, limit) {
  if (!channel) {
    throw new Error(`Expected channel, got ${typeof channel}.`);
  }
  if (limit <= 100) {
    return channel.messages.fetch({ limit });
  }

  let collection = new Discord.Collection();
  let lastId = null;
  let options = {};
  let remaining = limit;

  while (remaining > 0) {
    options.limit = remaining > 100 ? 100 : remaining;
    remaining = remaining > 100 ? remaining - 100 : 0;

    if (lastId) {
      options.before = lastId;
    }

    let messages = await channel.messages.fetch(options);

    if (!messages.last()) {
      break;
    }

    collection = collection.concat(messages);
    lastId = messages.last().id;
  }

  return collection;
}
// Listener Event: Runs whenever the bot sends a ready event (when it first starts for example)
bot.on('ready', async () => {
console.log('connected')
  try {
    for (channelID in channelsList) {
      console.log('running in channel ${channelsList[channelID]}')
      const channel = bot.channels.cache.get(channelsList[channelID]);
      if (!channel) return console.error('Invalid ID or missing channel.');
      const messages = await fetchMore(channel, messageLimit)
      for (const [id, message] of messages) {
        reactionTotal++
        await message.react(emoji);
      }
      console.log('Reactions: ${reactionTotal}');
    }
    console.log('Reactions total: ${reactionTotal}');
    console.log('you may now kill the bot.')
  } catch(err) {
    console.error(err);
  }

});

bot.login(botToken);