dice_roll.js

const commando = require('discord.js-commando');

class DiceRollCommand extends commando.Command {
    constructor(client) {
        super(client, {
            name: 'roll',
            group: 'random',
            memberName: 'roll',
            description: 'Rolls a die'
        });
    }

    async run(message, args) {
        var roll = Math.floor(Math.random() * 6) + 1;
        message.reply("You rolled a " + roll);
    }
}

module.exports = DiceRollCommand;

index.js

const commando = require('discord.js-commando');
const bot = new commando.Client();

bot.registry.registerGroup('random', 'Random');
bot.registry.registerDefaults();
bot.registry.registerCommandsIn(__dirname + "/commands");

bot.login('MzU1MTI2MzcyMTg1ODAwNzE2.DJJTLg.CfjBgPTZ3Uh9PiCRNhLzgvonba4');