roman.ex

defmodule Roman do
  @roman_symbols [
    ["M", 1000],
    ["CM", 900],
    ["D",  500],
    ["CD", 400],
    ["C",  100],
    ["XC",  90],
    ["L",   50],
    ["XL",  40],
    ["X",   10],
    ["IX",   9],
    ["V",    5],
    ["IV",   4],
    ["I",    1],
  ];

  def numeral(0), do: ""
  def numeral(n) do
    [symbol, value] = Enum.filter(@roman_symbols, fn x -> n >= Enum.at(x, 1) end) |> List.first
    symbol <> numeral(n - value)
  end
end

romanNumeral.js

const romanSymbols = [
  ['M', 1000],
  ['CM', 900],
  ['D',  500],
  ['CD', 400],
  ['C',  100],
  ['XC',  90],
  ['L',   50],
  ['XL',  40],
  ['X',   10],
  ['IX',   9],
  ['V',    5],
  ['IV',   4],
  ['I',    1],
];

function romanNumeral(n) {
  if (n === 0) {
    return '';
  }
  
  const targetSymbol = romanSymbols.filter(symbol => n >= symbol[1])[0];
  return targetSymbol[0] + romanNumeral(n - targetSymbol[1]);
}