roman.js

const template = (i, v, x) => ({
  0:  '',
  1:  `${i}`,
  2:  `${i}${i}`,
  3:  `${i}${i}${i}`,
  4:  `${i}${v}`,
  5:  `${v}`,
  6:  `${v}${i}`,
  7:  `${v}${i}${i}`,
  8:  `${v}${i}${i}${i}`,
  9:  `${i}${x}`,
  10: `${x}`,
})

const ranges = {
  1: ['I', 'V', 'X'],
  2: ['X', 'L', 'C'],
  3: ['C', 'D', 'M'],
}

const to = (n) => {
  const $ = (r, n, c) => {
    const cc = Math.pow(10, c - 1)
    const m = n % cc
    const x = (n - m) / cc
    const res = r + (c === 4 ? Array.from({ length: x }, _ => 'M').join('') : template.apply(null, ranges[c])[x])
    return c === 1 ? res : $(res, m, c - 1)
  }
  return $('', n, Math.min((n + '').length, 4))
}

const dictFrom = {
  'I': 1,
  'V': 5,
  'X': 10,
  'L': 50,
  'C': 100,
  'D': 500,
  'M': 1000,
}

const from = (n) => {
  return n.split('').reduce(([res, p1, p2], char) => {
    const m = dictFrom[char]
    if (!m) throw new Error('Invalid input.')
    if (p1 < m && p2 < m) throw new Error('Invalid input.')
    return [res + (p1 < m ? m - 2 * p1 : m), m, p1]
  }, [0, 1000, 1000])[0]
}

// console.log(from('MMMIXF'))

// console.log(g(2))
// console.log(g(58))
// console.log(g(346))
// console.log(g(1666))
// console.log(g(2008))
// console.log(g(9000))

const { forAll, generators } = require('rapid-check')

const prop = (n) => console.log(n, to(n), from(to(n))) || from(to(n)) === n

console.log(forAll(generators.fmap((n) => n + 1, generators.uint), prop, 10000))