Letter.js

// You are given an array of characters letters that is sorted in non-decreasing order,
// and a character target. There are at least two different characters in letters.

// Return the smallest character in letters that is lexicographically greater than target. 
//If such a character does not exist, return the first character in letters.

var nextGreatestLetter = function(letters, target) {
    return letters.find((letter)=>letter >target)|| letters[0]
};