Return Duplicates from Array

gistfile1.js

/* Find & return only duplicates from an Array
 * also returned is an object with the # of duplicates found
 * myArray = ["ccc", "aaa", "bbb", "aaa", "aaa", "aaa", "aaa", "bbb"];
 * x = myArray.getDuplicates();
 * // x = [ array of duplicates, associative object with # found]
 * // x = [ ['aaa','bbb'] , { 'aaa' : 5, 'bbb' : 2 } ]
 * alert(x[0]) // x[0] = ['aaa','bbb'] & alerts aaa,bbb
 * alert(x[1]['aaa']) // alerts 5;
 */
Array.prototype.getDuplicates = function(sort) {
 var u={},a=[],b={},c,i,l=this.length;
 for(i=0;i<l;++i){
  c=this[i];
  if (c in u) {
   if (c in b) { b[c]+=1; } else { a.push(c); b[c]=2; }
  }
  u[c] = 1;
 }
 // return array and associative array with # found
 return (sort) ? [a.sort(), b] : [a, b];
}