code:

const parseArguments = (...args) => {
    var diceCount = args[0],
        faceCount = args[1] || 6,
        modifier = args[2] || 0,
        matches;

    if (args.length === 1) {
        matches = /^(\d+)?(?=d)?(?=\b)?d?(\d+)?((\+|-)\d+)?/.exec(args);

        diceCount = matches[1] || 1,
            faceCount = matches[2] || 6,
            modifier = matches[3] || 0;
    }
    return {
        diceCount,
        faceCount,
        modifier
    }
}

const diceRoll = (sides, mod) => {
    return Math.max((Math.floor(Math.random() * sides) + 1) + mod,0);
}

const castDice = (...args) => {
    var diceCount = args[0],
        faceCount = args[1] || 6,
        modifier = args[2] || 0,
        ret = 0;
    for (var i = diceCount - 1; i >= 0; i--) {
        ret += diceRoll(faceCount, parseInt(modifier));
    }
    return ret;
}

export const getDie = function(...args) {
    var {
        diceCount,
        faceCount,
        modifier
    } = parseArguments(...args);
    const dices = () => {
        return castDice(diceCount, faceCount, modifier);
    };
    dices.diceNotation = () => {
        var ret = '';
        if (diceCount && diceCount > 1) {
            ret += diceCount;
        }
        if (faceCount && faceCount !== 6) {
            ret += `d${faceCount}`;
        }
        if (modifier && modifier !== 0) {
            ret += `${modifier}`;
        }
        return ret
    }
    return dices;
};

export function roll(...args) {
    var {
        diceCount,
        faceCount,
        modifier
    } = parseArguments(...args);
    return castDice(diceCount, faceCount, modifier);
}