Hello Friend,

Try the following code:

<script>
var st  = window.prompt("Enter string: ");
var arr=st.split("");
          var resulted = new Array();
          for (var j=0; j<arr.length; j++) {
              var key = arr[j].toString();
              if (!resulted[key]) {
                  resulted[key] = 1
              } else {
                  resulted[key] = resulted[key] + 1;
              }
          }
          var str = "";
          for (var j in results) {
          if(resulted[j]==1){
          str += j;
           }
          }
          alert(str.length);
</script>

Thanks