Haha-Yes/commands/thing/avatar.js

const { Command } = require('discord.js-commando');
module.exports = class AvatarCommand extends Command {
    constructor(client) {
        super(client, {
            name: 'avatar',
            group: 'thing',
            memberName: 'avatar',
            description: 'Send the avatar of the mentionned user.',
        });
    }

    run(message) {
        if (!message.mentions.users.size) {
            return message.channel.send(`Your avatar: ${message.author.displayAvatarURL}`);
        }

        const avatarList = message.mentions.users.map(user => {
            return `${user.username}'s avatar: ${user.displayAvatarURL}`;
        });
        message.channel.send(avatarList);
    }
};