Reduce - exercise two solution

reduce_exercise_two.js

function longest(s1, s2) { 
  const allLetters = s1 + s2; 
  const distinctLetters = allLetters.split('').reduce((distinctLetters, currentLetter) => { 
    distinctLetters[currentLetter] = true; 
    return distinctLetters; 
}, {});
  const letters = Object.keys(distinctLetters); 
  return letters.sort().join('');
}