go.js

var dbg = false;
var obj = {
    I: 1,
    V: 5,
    X: 10,
    L: 50,
    C: 100,
    D: 500,
    M: 1000
};
var one, two = '';
var num = 0;
function output(number) {
    if (dbg) console.log(number);
    number = number.split('');
    number.map(function (val, i) {
        number[i] = obj[val];
    });

    if (dbg) console.log(number);

    number.forEach(function (item, i) {
        one = number[i];
        two = number[i + 1];
        if (typeof two !== 'undefined') {
            if (dbg) console.log(one, two);
            if ((one > two) || (one === two)) {
                if (i === 0) {
                    num = num + one + two;
                    if (dbg) console.log('сложение ONE', num);
                } else {
                    num = num + two;
                    if (dbg) console.log('сложение TWO', num);
                }
            }
            if (one < two) {
                num = num + (-one) + two;
                if (dbg) console.log('вычитание', num);
            }
        }
    });

    console.log(num);
    num = 0;
    oneStep = true;
    if (dbg) console.log('\n================\n');
}

output('KI');
output('IV');
output('VIII');
output('IX');
output('XXXI');
output('XLVI');
output('XCIX');