Intermediate Algorithm Scripting: Missing letters - freeCodeCamp solution

sol.js

function fearNotLetter(str) {
  let letters = str.split("");

  let alph = 'abcdefghijklmnopqrstuvwxyz'.split("");

  let range = alph.slice(alph.indexOf(letters[0]), alph.indexOf(letters[0])+letters.length+1);

  console.log("Original range: " + range);

  for (let i in str) {
    if (range.includes(str[i])) {
      range.splice(range.indexOf(str[i]), 1)
    }
  }

  console.log(range);

  if (range.length > 0) {
    return range.join();
  } else {
    return undefined;
  }

}

fearNotLetter("abcdefghijklmnopqrstuvwxyz")