Ceasar's Cipher Project

[LeKoels27]

I just completed the Ceasar’s Cipher Project of the JavaScript curriculum. Please comment and provide suggestions on how I could improve/refactor my code.

function rot13(str) {
  let alphabet = "abcdefghijklmnopqrstuvwxyz".toUpperCase();
  let rot13Index = [];
  let decodedStr = [];

  for (let i = 0; i < str.length; i++) {
    if (alphabet.indexOf(str[i]) !== -1) {
      rot13Index.push(alphabet.indexOf(str[i]) + 13);
    } else {
      rot13Index.push(str[i]);
    }
  } 

  for (let j = 0; j < rot13Index.length; j++) {
    if (typeof rot13Index[j] === "number" && rot13Index[j] < 26) {
      decodedStr.push(alphabet[rot13Index[j]])
    } else if (typeof rot13Index[j] === "number" && rot13Index[j] >= 26) {
      decodedStr.push(alphabet[rot13Index[j] - 26]);
    } else {
      decodedStr.push(rot13Index[j]);
    }
  }

  return decodedStr.join("");
}

console.log(rot13("SERR PBQR PNZC"));