Missing letters Intermediate Algorithms

I just completely my Missing Letters but I just can’t get the last case. Can you see a bug?

function fearNotLetter(str) {
  let alpha = 'abcdefghijklmnopqrstuvwxyz';
  let range = alpha.slice(alpha.indexOf(str[0]), str.length + 1)
                   .split('')
                   .filter(value => !str.includes(value));
  return range[0];
}

I understand I didn’t do the indented solution of comparing the ASCII code but this example works for all but ‘stvwx’ for some reason.
Thank You.

Challenge Link

Try to temporarily split it more into steps and see if at each step everything looks like it should.

I broke up your code into smaller pieces to let you see what is happening.

function fearNotLetter(str) {
  let alpha = 'abcdefghijklmnopqrstuvwxyz';
  console.log('alpha.indexOf(str[0]) = ' + alpha.indexOf(str[0]));
  console.log('str.length + 1 = ' + (str.length + 1));
  let slice = alpha.slice(alpha.indexOf(str[0]), str.length + 1);
  console.log('slice = ' + slice);
  let rangeArr = slice.split('');
  console.log(rangeArr);
  let range = rangeArr.filter(value => !str.includes(value));
  console.log('range = ' + range);
  return range[0];
}
console.log(fearNotLetter("stvwx")) // undefined
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