Convert an integer to a roman numeral in JavaScript

Roman numerals originated in ancient Rome and were used for many centuries. They are still used today in some contexts, mainly for stylistic reasons.

In order to convert an integer to a roman numeral, we can use a lookup table containing roman numeral values and their integer counterparts. We can then use a simple algorithm to repeatedly divide the input number by the values in the lookup table and add the corresponding roman numeral to the result.

Having the lookup table, we can use Array.prototype.reduce() to loop over the values and repeatedly divide num by the value. We can then use String.prototype.repeat() to add the roman numeral representation to the result.

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],
];

const toRomanNumeral = num =>
  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'