Convert any number between 1 and 3999 to roman using JS

roman.js

var mapa =[
	[1000, { capital : 'M', condition : gt, transform : repeat }],
	[900, { capital :'CM', condition : gt, transform : difference }],
	[500, { capital : 'D', condition : gt, transform : difference }],
	[400, { capital : 'CD', condition : gt, transform : difference }],
	[100, { capital : 'C', condition : gt, transform : repeat }],
	[90, { capital : 'XC', condition : gt, transform : difference }],
	[50, { capital : 'L', condition : gt, transform : difference }],
	[40, { capital : 'XL', condition : gt, transform : difference }],
	[10, { capital : 'X', condition : gt, transform : repeat }],
	[9, { capital : 'IX', condition : eq }],
	[5, { capital :'V', condition : gt, transform : difference }],
	[4, { capital : 'IV', condition : eq }],
	[1, { capital : 'I', condition : gt, transform : repeat }]
];

function gt (a, b) { return a >= b }
function eq (a, b) { return a === b }

function repeat (letter, base, number) {
	return letter.repeat(Math.floor(number / base)) + to_roman(number % base);
}

function difference (letter, base, number) {
	return letter + to_roman(number - base);
}

function to_roman(number) {

	if (number < 0 || number > 3999) throw new Error('Não convertemos esse número')

	for (var [x, obj] of mapa)
		if (obj.condition(number, x))
			return obj.transform ? obj.transform(obj.capital, x, number) : obj.capital

    return ''
}

exports.to_roman = to_roman;

roman_spec.js

var assert = require ('assert'),
    to_roman = require('./roman').to_roman;

describe('Números romanos', function() {
    it('transforma 1', function () {
        assert.equal(to_roman(1), 'I')
    })

    it('transforma 2', function () {
        assert.equal(to_roman(2), 'II')
    })

    it('transforma 3', function () {
        assert.equal(to_roman(3), 'III')
    })

	it('transforma 4', function () {
        assert.equal(to_roman(4), 'IV')
    })

    it('transforma 5', function () {
        assert.equal(to_roman(5), 'V')
    })

    it('transforma 10', function () {
        assert.equal(to_roman(10), 'X')
    })

    it('transforma 20', function () {
        assert.equal(to_roman(20), 'XX')
    })

    it('transforma 30', function () {
        assert.equal(to_roman(30), 'XXX')
    })

    it('transforma 9', function () {
        assert.equal(to_roman(9), 'IX')
    })

	it('transforma 11', function () {
        assert.equal(to_roman(11), 'XI')
    })

    it('transforma 6', function () {
        assert.equal(to_roman(6), 'VI')
    })

    it('transforma 7', function () {
        assert.equal(to_roman(7), 'VII')
    })

    it('transforma 8', function () {
        assert.equal(to_roman(7), 'VII')
    })

    it('transforma 39', function () {
        assert.equal(to_roman(39), 'XXXIX')
    })


    it('transforma 34', function () {
        assert.equal(to_roman(34), 'XXXIV')
    })


    it('transforma 40', function () {
        assert.equal(to_roman(40), 'XL')
    })


    it('transforma 50', function () {
        assert.equal(to_roman(50), 'L')
    })

    it('transforma 51', function () {
        assert.equal(to_roman(51), 'LI')
    })

    it('transforma 41', function () {
        assert.equal(to_roman(41), 'XLI')
    })

    it('transforma 100', function () {
        assert.equal(to_roman(100), 'C')
    })

    it('transforma 500', function () {
        assert.equal(to_roman(500), 'D')
    })

    it('transforma 148', function () {
        assert.equal(to_roman(148), 'CXLVIII')
    })

    it('transforma 90', function () {
        assert.equal(to_roman(90), 'XC')
    })

    it('transforma 91', function () {
        assert.equal(to_roman(91), 'XCI')
    })

    it('transforma 550', function () {
        assert.equal(to_roman(550), 'DL')
    })

    it('transforma 55', function () {
        assert.equal(to_roman(55), 'LV')
    })

    it('transforma 445', function () {
        assert.equal(to_roman(445), 'CDXLV')
    })

    it('transforma 1001', function () {
        assert.equal(to_roman(1001), 'MI')
    })
    
    it('transforma 3001', function () {
        assert.equal(to_roman(3001), 'MMMI')
    })

    it('transforma 3999', function () {
        assert.equal(to_roman(3999), 'MMMCMXCIX')
    })


    

})