freeCodeCamp Intermediate Algorithm Scripting: Missing letters

fearNotLetter.js

// Find the missing letter in the passed letter range and return it.
// If all letters are present in the range, return undefined.

function fearNotLetter(str) {
  let lastLetter=str.charCodeAt(0);
  let missingLetter;
  for (let i=1; i<str.length; i++) {
    let currentLetter=str.charCodeAt(i);
    console.log(lastLetter, currentLetter);
    if ((lastLetter+1) < currentLetter) {
      missingLetter = String.fromCharCode(currentLetter-1);
      break;
    }
    else {
      lastLetter=currentLetter;
    }
  }
  return missingLetter;
}

fearNotLetter("abce");

const fearNotLetter=(s,x="charCodeAt")=>s.split("").reduce((a,c,i)=>a?a:i&&c[x](0)-s[i-1][x](0)>1?String.fromCharCode(s[i-1][x](0)+1):undefined,0,);