Javascript String toBin()

String.prototype.toBin = function() {
 var st,i,j,d;/*w  w  w.  j av a 2 s .c o  m*/
 var arr = [];
 var len = this.length;
 for (i = len; i>0; i--) {
   d = this.charCodeAt(i-1);
  for (j = 0; j < 8; j++) {
    arr[arr.length] = d%2;
   d = Math.floor(d/2);
  }
 }
 return arr.reverse().join("");
}