Javascript Array getDuplicity()

Array.prototype.getDuplicity = function() {
 var obj = {};/*from   w  ww .  ja  v  a  2 s .  com*/
 var returnObj = {};
 this.forEach(function(value, index) {
  if (!obj[value]) {
   returnObj[value] = 1;
   obj[value] = true;
  } else {
   returnObj[value]++;
  }
 })
 return returnObj;
}