activitycheck.js

const Discord = require('discord.js');

const client = new Discord.Client();
const prefix = '!';

client.on('message', async msg => {
  if (msg.author.bot || !msg.content.startsWith(prefix)) return;
  const args = msg.content.slice(prefix.length).split(' ');

  if (args[0] === 'activitycheck') {
    // Usage: `!activitycheck {message_id} {TextChannel} {emoji}`

    const targetChannel = msg.mentions.channels.first();
    if (typeof targetChannel === 'undefined') {
      msg.channel.send('No channels mentioned');
      return;
    }

    const emoji = resolveEmoji(args[3]);
    
    try {
      const tMsg = await targetChannel.fetchMessage(args[1]);
      if (typeof tMsg === 'undefined') throw 'MESSAGE_MISSING'; // it would be good practice putting this in a variable, but too much work right now
      
      const reaction = tMsg.reactions.get(emoji);
      if (typeof reaction === 'undefined') throw 'REACTION_MISSING';

      const reactionUsers = await reaction.fetchUsers(); // TODO: make function fetchAllUsers() for when we need more than 100
      const members = targetChannel.members.filter(member => { // TODO: add role filter
        if (member.user.bot) return false;
        if (reactionUsers.has(member.id)) return false;
        if (tMsg.createdTimestamp < member.joinedTimestamp) return false; 
        return true;
      });

      const memberNames = members.reduce((text, member, snowflake) => {
        return text + `\n${member.displayName}`;
      }, '');
      
      const reply = `**${members.size} did not react**\n\`\`\`${memberNames}\`\`\``;
      msg.channel.send(reply);
    }
    catch(error) {
      switch(error) {
        case 'MESSAGE_MISSING':
          msg.channel.send('Couldn\'t fetch message.');
          break;
        case 'REACTION_MISSING':
          msg.channel.send('Couldn\'t fetch reaction.');
          break;
        default:
          console.log(error);
          msg.channel.send('Encountered an unknown error while fetching the message and reactions.');
          break;
      }
    }
  }

  if (args[0] === 'stop') {
    client.destroy();
  }
});

/* 
 * Returns the emoji id for custom emojis.
 * Returns the emoji itself for Unicode emojis.
 * Returns the given input if no emoji id is found.
 */
function resolveEmoji(str) {
  const emoji = str.trim();
  const match = /<:([\d\w_]+:\d{18})>/m.exec(emoji);

  if (match) return match[1];
  else return emoji;
}