Javascript Find unique letters

Question

(function () {
      var findUniqueLetters;

      findUniqueLetters = function (inputString) {
         var uniqueLetters, result;

         inputString = inputString.toUpperCase();

         uniqueLetters = {};

         inputString.split('').forEach(function (charValue) {

             if (uniqueLetters.hasOwnProperty(charValue)) 

             Object.keys(uniqueLetters).forEach(function (prop) {
               result.value += uniqueLetters[prop];
               });

            });
         });         
         return result;

      }; 

This is my code so far, but it doesn't work and have no idea what I have to revise.

For example, 'Mississippi' should give a result like 'MISP'.