Javascript String shrink(size, font)

String.prototype.shrink = function(size, font) {
 if(size > this.width())
  return this;/*w w w .j a  v  a2s .  c  o  m*/
 var count = 0;
 var l = 0;
 var w = 0;
 do {
  l += Math.floor(((size - w) / size) * l) + 1;
  console.log(l);
  w = this.substring(0, l).width(font);
  count += 1;
 } while(w < size && count < 1000);
 return this.substring(0, l);
}