Caesars Cipher

const alphabet = ['A','B','C','D','E','F','G','H','I','J','K','L','M','N','O','P','Q','R','S','T','U','V','W','X','Y','Z'];

function rot13(str) {

  //create accumulator
  let accumulator = '';
  //loop through the string
  for(let i=0; i < str.length; i++){

    //if character is not letter then simply add to accumulator
    const char = str[i];
    const isALetter = alphabet.includes(char);
    if (isALetter===false){
      accumulator += char;
    }
    // else substract or add 13 indices and add to accumulator
    else{
      const charIndex = alphabet.findIndex((c) => c === char);
      accumulator+= alphabet [charIndex+13] || alphabet[charIndex-13];
    }
  }
    //return accumulator
  return accumulator;
}
rot13("SERR PBQR PNZC");