toRomanNumeral javascript Code Snippet in 2025

Converts an integer to its roman numeral representation. Accepts value between 1 and 3999 (both inclusive).

• Create a lookup table containing 2-value arrays in the form of (roman value, integer).
• Use Array.prototype.reduce() to loop over the values in lookup and repeatedly divide num by the value.
• Use String.prototype.repeat() to add the roman numeral representation to the accumulator.

const toRomanNumeral = num => {
  const lookup = [
    ['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],
  ];
  return lookup.reduce((acc, [k, v]) => {
    acc += k.repeat(Math.floor(num / v));
    num = num % v;
    return acc;
  }, '');
};

toRomanNumeral(3); // 'III'
toRomanNumeral(11); // 'XI'
toRomanNumeral(1998); // 'MCMXCVIII'