Roman Numerals Converter

roman_numerals.js

let RomanNumerals = {
  toRoman: (n) => {
    let m = [
      ['I', 'V'],
      ['X', 'L'],
      ['C', 'D'],
      ['M']
    ];
    let c = ' 0 00 000 01 1 10 100 1000 02'.split(' ');
    let res = [];
    let i = 0;
    while (n > 0) {
      let t = n % 10;
      res.unshift(c[t].split('').map((e) => {
        if (+e === 2) return m[i+1][0];
        return m[i][e];
      }).join(''));
      ++i;
      n = (n - t) * 0.1;;
    }
    return res.join('');
  },
  fromRoman: (s) => {
    let m = {
      'I':   1, 'V':   5,
      'X':  10, 'L':  50,
      'C': 100, 'D': 500,
      'M':1000
    };
    return s.split('').reduce((a, c, i) => {
      let curr = m[c];
      let last = a.slice(-1)[0];
      if (last > curr || i === 0) {
        a.push(curr);
      } else if (last < curr) {
        a.push(-a.pop());
        a.push(curr);
      } else {
        a.push(a.pop() + curr);
      }
      return a;
    }, []).reduce((acc, n) => acc + n);
  }
};