You never know when you might need to convert digits to roman numerals

digit-to-romans.js

var table = ['i', 'v', 'x', 'l', 'c', 'd', 'm'];

var map = {
  4: "01",
  5: "1",
  6: "10",
  7: "100",
  8: "1000",
  9: "02"
};

function convert(num) {
  if (num > 9999) {
    // invalid roman numeral
    // since roman numerals are based on dates, Romans clearly weren't visionaries
    return 'eat a bag of dicks';
  }
  n = num.toString();
  var value = [];
  for (var i = n.length - 1, j = 0; i >= 0; i--, j = j + 2) {
    var c = Array(parseInt(n[i], 10) + 1).join(table[j]);
    value.unshift(c);
  }
  value = value.join('');
  for (var i=0; i < table.length; i = i + 2) {
    var letter = table[i];
    var reg = new RegExp(letter + "{4,9}");
    var found = value.match(reg);
    if (found && i !== table.length - 1) {
      var key = map[found[0].length];
      var newKey = [table[i], table[i + 1], table[i + 2]];
      var replacer = key.split('').map(function(el) {
        return newKey[parseInt(el, 10)];
      }).join('');
      value = value.replace(reg, replacer);
    }
  }
  return value.toUpperCase();
}


// all cases should be true
console.log(convert(1) == 'I');
console.log(convert(3) == 'III');
console.log(convert(4) == 'IV');
console.log(convert(5) == 'V');
console.log(convert(9) == 'IX');
console.log(convert(10) == 'X');
console.log(convert(19) == 'XIX');
console.log(convert(50) == 'L');
console.log(convert(100) == 'C');
console.log(convert(1000) == 'M');
console.log(convert(1944) == 'MCMXLIV');
console.log(convert(1999) == 'MCMXCIX');
console.log(convert(888) == 'DCCCLXXXVIII');
console.log(convert(9999) == 'MMMMMMMMMCMXCIX');
console.log(convert(51) == 'LI');
console.log(convert(56) == 'LVI');
console.log(convert(118) == 'CXVIII');
console.log(convert(119) == 'CXIX');

Line 16 is pure gold.