Conversions for Integers <-> Roman Numerals

romans.js

function int2roman(n) {
    var map = [
        { factor: 1000, letter: 'M' },
        { factor: 900, letter: 'CM' },
        { factor: 500, letter: 'D' },
        { factor: 400, letter: 'CD' },
        { factor: 100, letter: 'C' },
        { factor: 90, letter: 'XC' },
        { factor: 50, letter: 'L' },
        { factor: 40, letter: 'XL' },
        { factor: 10, letter: 'X' },
        { factor: 9, letter: 'IX' },
        { factor: 5, letter: 'V' },
        { factor: 4, letter: 'IV' },
        { factor: 1, letter: 'I' },
    ];    
    
    return map.reduce(function(roman, m) {
        var rem = parseInt(n / m.factor);
        if (rem >= 1) {
            roman += Array(rem+1).join(m.letter);
            n = parseInt(n % m.factor);
        }
        return roman;
    }, '');
}

function roman2int(r) {
    var map = {
            'I': 1,
            'IV': 4,
            'V': 5,
            'IX': 9,
            'X': 10,
            'XL': 40,
            'L': 50,
            'XC': 90,
            'C': 100,
            'CD': 400,
            'D': 500,
            'CM': 900,
            'M': 1000
        },
        sum = 0,
        letters = r.toUpperCase().split('');

    for (var i=0, l=letters.length; i<l; i++) {
        var key2 = letters[i] + letters[i+1],
            key = letters[i];
        
        if (map[key2]) {
            sum += map[key2];
            i += 1;
        }
        else {
            sum += map[key];
        }
    }
    return sum;
}