Longest possible string built from two strings in JavaScript

Problem

We are required to write a JavaScript function that takes in two strings s1 and s2 including only letters from ato z.

Our function should return a new sorted string, the longest possible, containing distinct letters - each taken only once - coming from s1 or s2.

Example

Following is the code −

const str1 = "xyaabbbccccdefww";
const str2 = "xxxxyyyyabklmopq";
const longestPossible = (str1 = '', str2 = '') => {
   const combined = str1.concat(str2);
   const lower = combined.toLowerCase();
   const split =lower.split('');
   const sorted = split.sort();
   const res = [];
   for(const el of sorted){
      if(!res.includes(el)){
         res.push(el)
      }
   }
   return (res.join(''));
};
console.log(longestPossible(str1, str2));

Output

Following is the console output −

abcdefklmopqwxy