test.js

function fearNotLetter(str) {
  let letters = str.split('');
  let startingCharCode = letters[0].charCodeAt(0);
  let endingCharCode = letters[letters.length-1].charCodeAt(0);

  // Find the current sum of char codes
  let currentTotalCharCodes = letters.reduce((total, letter) => total += letter.charCodeAt(0), 0);

  // Find the expected sum of char codes
  let expectedTotalCharCodes = 0;
  for(let i = startingCharCode; i <= endingCharCode; i++) {
    expectedTotalCharCodes += i;
  }

  // Calculate the difference to find the missing letter
  let missingLetterCharCode = expectedTotalCharCodes - currentTotalCharCodes;
  return missingLetterCharCode === 0 ? undefined : String.fromCharCode(missingLetterCharCode);
}