Write a number in roman numerals efficiently in javascript

fromroman.js

function fromRoman(str) {
  // Parse str as a roman numeral
  var values = {I:1, V:5, X:10, L:50, C:100, D:500, M:1000};
  str = str.toUpperCase();
  var n = 0, v1 = 0, v2 = values[str[0]];
  for (var k = 0; k < str.length; k++) {
    var v1 = v2, v2 = k+1 < str.length ? values[str[k+1]] : 0;
    n += (v2 > v1) ? -v1 : v1;
  }
  return n;
}

toroman.js

function toRoman (n, lowercase) {
 var letters = lowercase ? "mdclxvi" : "MDCLXVI";
 var k = 1000, l=0, r=[], letter, prevletter;
 while(n > 0) {
   prevletter = letter;
   letter = letters[l];
   if(n>=k) {
     var len = n/k;
     r.push(letter);
     if (len >= 4 && l > 0) r.push(prevletter);
     else for(var i=2; i<=len; i++) r.push(letter);
     n %= k;
  }
  l++;
  k /= (l%2) ? 2 : 5;
 }
 return r.join('');
}