Javascript String isUnique()

String.prototype.isUnique = function() {
  var lettersHash = {};
  var nonUniqueLetters = "";

  Array.prototype.map.call(this, function(x){
    if(lettersHash[x]) {
      nonUniqueLetters += x//from w  w  w  . j  av  a 2s. com
    } else {
      lettersHash[x] = true;
    }
  });

  return nonUniqueLetters;
}

var nonUnique = "The cat is very bored.";
var unique    = "he was red";

console.log(nonUnique.isUnique());