romanNumeral.js

var normalFormat = [1, 4, 5, 9, 10, 40, 50, 90, 100, 400, 500, 900, 1000];

var romanFormat = ['I', 'IV', 'V', 'IX', 'X', 'XL', 'L', 'XC', 'C', 'CD', 'D', 'CM', 'M'];

function convertToRoman(num) {
  if (num === 0) return '';

  var value = '';

  for (var i = 0; i < normalFormat.length; i++) {
    if (num >= normalFormat[i]){
      value = '';
     
      value = romanFormat[i];
    } 
  }

  var index = romanFormat.indexOf(value);
  
  var subValue = normalFormat[index];

  return value + convertToRoman(num - subValue);
}

convertToRoman(6);

Ironically I feel like yours is more sophisticated than mine. I like it though, because it solves the problem in a very different way from me. I'll send you mine via gist too. It's fun to share code on GH :))