Find new sorted string and remove duplicate char in string

sortedLongestStr.js

//Take 2 strings s1 and s2 including only letters from ato z. Return a new sorted string, the longest possible, containing distinct letters, - each taken only once - coming from s1 or s2.
/*
Examples:

a = "xyaabbbccccdefww"
b = "xxxxyyyyabklmopq"
longest(a, b) -> "abcdefklmopqwxy"

a = "abcdefghijklmnopqrstuvwxyz"
longest(a, a) -> "abcdefghijklmnopqrstuvwxyz"
*/

function longest(s1, s2) {
  var newStr = s1.concat(s2);
  var longestCharArr = unique(newStr).split("").sort();
  
  return longestCharArr.join("");
}

function unique(str) {
  var result = "";
  for(var i = 0; i < str.length; i++) {
    if(result.indexOf(str[i]) == -1) {
      result += str[i];
    }
  }
  return result;
}