Convert numbers to their roman numeral equivalents, and vice-versa.

function Level(i, v, x) {
  this.i = i;
  this.v = v;
  this.x = x;
}

levels = [];

levels[0] = new Level('I', 'V', 'X');
levels[1] = new Level('X', 'L', 'C');
levels[2] = new Level('C', 'D', 'M');

function calcDigit(d, l) {
  if (l > 2) {
    str = '';
    for (var m = 1; m <= d * Math.pow(10, l - 3); m++)
    str += 'M';
    return str;
  } else if (d == 1) return levels[l].i
  else if (d == 2) return levels[l].i + levels[l].i
  else if (d == 3) return levels[l].i + levels[l].i + levels[l].i
  else if (d == 4) return levels[l].i + levels[l].v
  else if (d == 5) return levels[l].v
  else if (d == 6) return levels[l].v + levels[l].i
  else if (d == 7) return levels[l].v + levels[l].i + levels[l].i
  else if (d == 8) return levels[l].v + levels[l].i + levels[l].i + levels[l].i
  else if (d == 9) return levels[l].i + levels[l].x
  else return ''
}

function toRoman(n) {
  var r = ''
  for (var c = 0; c < n.length; c++)
  r += calcDigit(eval(n.charAt(c)), n.length - c - 1);
  return r
}

function fromRoman(n) {
  var r = 0

  for (var c = 0; c < n.length; c++) {
    var chr = n.charAt(c).toLowerCase();
    if (c < n.length - 1) var next = n.charAt(c + 1).toLowerCase()
    else var next = '';

    if (c > 0) var prev = n.charAt(c - 1).toLowerCase()
    else var prev = '';

    if (chr == 'i') {
      if (next == 'v') r += 4
      else if (next == 'x') r += 9
      else r += 1;
      continue
    }

    if (chr == 'v') {
      if (prev != 'i') r += 5;
      continue
    }

    if (chr == 'x') {
      if (prev != 'i') if (next == 'l') r += 40
      else if (next == 'c') r += 90
      else r += 10;
      continue
    }

    if (chr == 'l') {
      if (prev != 'x') r += 50;
      continue
    }

    if (chr == 'c') {
      if (prev != 'x') if (next == 'd') r += 400
      else if (next == 'm') r += 900
      else r += 100;
      continue
    }

    if (chr == 'd') {
      if (prev != 'c') r += 500;
      continue
    }

    if (chr == 'm') {
      if (prev != 'c') r += 1000;
      continue

    }
  }

  return r

}

function isNumeric(str) {
  for (var c = 0; c < str.length; c++) {
    var chr = str.charAt(c);
    if (chr != '0' & chr != '1' & chr != '2' & chr != '3' & chr != '4' & chr != '5' & chr != '6' & chr != '7' & chr != '8' & chr != '9') return false
  }
  return true;
}

//----------------------------------------
// Example Usage
//----------------------------------------
// fromRomain("4"); => "4"
// toRoman("4");    => "IV"