Converts decimal numbers to roman numerals.

rom.js

function convert(num) {
  var map = [
    [1, 1, 'I'],
    [1, 5, 'V'],
    [2, 10, 'X'],
    [2, 50, 'L'],
    [3, 100, 'C'],
    [3, 500, 'D'],
    [4, 1000, 'M'],
    [4, 5000, ''],
    [5, 10000, '']
  ];
  function get_rom(digit, n) {
    var unit = map.filter(function(a) { return a[0] == n;   })[0][2];
    var five = map.filter(function(a) { return a[0] == n;   })[1][2];
    var ten  = map.filter(function(a) { return a[0] == n+1; })[0][2];
    var rom = '';
    switch (digit) {
      case 9: return unit+ten;
      case 8: rom += unit;
      case 7: rom += unit;
      case 6: rom += unit;
      case 5: return five+rom;
      case 4: return unit+five;
      case 3: rom += unit;
      case 2: rom += unit;
      case 1: rom += unit;
      case 0: return rom;
    }
    return '';
  }
  
  var dec = '' + num;
  var str = '';
  for (var i = dec.length; i > 0; i--) {
    str += get_rom(parseInt(dec[dec.length-i]), i);
  }
  
  return str;
}

convert(123);