RomanNumeralsConverter.js

function convert( num ) {
  var x = '';
  [1000, 900, 500, 400, 100, 90, 50, 40, 10, 9, 5, 4, 1].map(
    function( obj, i ) {
      while ( num >= obj ) {
        x += ['M', 'CM', 'D', 'CD', 'C', 'XC', 'L', 'XL', 'X', 'IX', 'V', 'IV', 'I'][i];
        num -= obj;
      }
    }
  );
  return x;
}

console.log( convert( 21 ) );