Convert Numbers to Roman Numerals in JavaScript (Exercise)

The challenge

Write toRoman(n) for whole numbers from 1 to 3999. The symbols are M=1000, D=500, C=100, L=50, X=10, V=5, I=1, and the special pairs CM=900, CD=400, XC=90, XL=40, IX=9, IV=4.

Example

toRoman(4)     // "IV"
toRoman(1969)  // "MCMLXIX"

Try it

Write your answer below, then press Check my answer. Your code runs right here in your browser, and the tests tell you what works.

Your solution

It prints

What the tests check

toRoman(3) gives "III"
toRoman(4) gives "IV"
toRoman(1969) gives "MCMLXIX"
toRoman(2024) gives "MMXXIV"
toRoman(3999) gives "MMMCMXCIX"

Hints

Stuck? Open one hint at a time. Try again after each one.

Hint 1

Make a list of [value, symbol] pairs from biggest to smallest, including the special pairs.

Hint 2

For each pair: while n is at least the value, add the symbol and subtract the value.

A solution

There’s always more than one right way. Read it, make sure you understand it, then try writing it yourself without looking.

Show one possible solution

Solution

function toRoman(n) {
  const table = [[1000, "M"], [900, "CM"], [500, "D"], [400, "CD"], [100, "C"], [90, "XC"], [50, "L"], [40, "XL"], [10, "X"], [9, "IX"], [5, "V"], [4, "IV"], [1, "I"]];
  let result = "";
  for (const [value, symbol] of table) {
    while (n >= value) {
      result += symbol;
      n -= value;
    }
  }
  return result;
}

Learn what you need

This exercise uses what Grade 10: Array Power Tools: map, filter & reduce teaches. Read those notes first if any of it is new.

Convert numbers to Roman numerals