Roman dates

function toRoman(date)
{
    const thousands = Math.floor(date / 1000);
    date %= 1000;
    const hundreds = Math.floor(date / 100);
    date %= 100;
    const tenths = Math.floor(date / 10);
    const singles = date % 10;
    const convert = function (digit, single, five, next) {
        if (digit == 0) {
            return '';
        }
        return digit == 9 ? single + next : digit == 4 ? single + five :  ((digit > 4 ? five : '') + single.repeat(digit % 5));
    }

    let result = '';
    if (thousands > 0) {
        result += 'M'.repeat(thousands);
    }
    result += convert(hundreds, 'C', 'D', 'M');
    result += convert(tenths, 'X', 'L', 'C');
    result += convert(singles, 'I', 'V', 'X');

    return result;
}

function fromRoman(date)
{
    const cost = {
        'I': 1,
        'V': 5,
        'X': 10,
        'L': 50,
        'C': 100,
        'D': 500,
        'M': 1000
    };
    return date.toUpperCase().split('').reduce(function (accumulator, el, i, letters) {
        if (typeof cost[el] == "undefined") {
            return accumulator;
        }
        if (
            el == 'I' && 'VX'.indexOf(letters[i+1]) != - 1
            || el == 'X' && 'LC'.indexOf(letters[i+1]) != - 1
            || el == 'C' && 'DM'.indexOf(letters[i+1]) != - 1
        ) {
            return accumulator - cost[el];
        }
        return accumulator + cost[el];
    }, 0);
}