Javascript String mySlice(start, end)

String.prototype.mySlice = function(start, end) {
 
 var S = this; /*  w  w w.j  a  v  a  2  s  .com*/
 var len = S.length;
 var intStart = start - 0;
 var intEnd = end ? (end - 0) : len;

 if ( intStart != intStart)
  intStart = 0;

 var from = intStart < 0
    ? Math.max(len + intStart, 0)
    : Math.min(intStart, len);

 var to = intEnd < 0
    ? Math.max(len + intEnd, 0)
    : Math.min(intEnd, len);

 var span = Math.max(to - from, 0);

 var str='';
 for ( var i = 0; i < span; i++ ){
  str += S[from + i];
 }

 return str;

}