test.js

const fearNotLetter = (str) => {
  const letters = str.split('');
  const charCodes = letters.map((letter) => letter.charCodeAt(0)).sort((a, b) => a - b);
  
  for (let i = 0; i < charCodes.length; i++) {
    if ((charCodes[i + 1] - charCodes[i]) > 1) {
      return String.fromCharCode(charCodes[i] + 1);
    }
  }
  
  return undefined;
}