Caesars Cipher, how to modify a string but leaving some characters as they are

Tell us what’s happening:
When I do calculate the UTF-16 code - 13 some letters do not match, for example the R becomes an 8 I also can’t figure out how to modify only the A-Z characters from the string

Your code so far

function rot13(str) {
  let i = 0
  for (let l of str){
    let x = str.charCodeAt([i])
    console.log(String.fromCharCode(x - 13))
  return str;

rot13("SERR PBQR PNZC");

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Challenge: Caesars Cipher

Link to the challenge:

consider that if you substract 13 from the code for A, you don’t get the code for N

Hello there,
I would recommend a different approach to this challenge:

-control your input

convert all the characters in the string to lowercase and make sure you don’t have any unwanted symbols.
If needed you can always change the back to uppercase at the end.

-use your own array of valid characters

It’s the best way to make sure you get all the correct characters.

Here’s a great example of caesars cipher function:

function caesarCipther(str, num) {

num = num % 26;    

const lowerCaseString = str.toLowerCase();    

const charArr = 'abcdefghijklmnopqrstuvwxyz';

let newStr = '';

for (var i = 0; i < lowerCaseString.length; i++) {

    const char = lowerCaseString[i];

    if (char === " ") {

        newStr += char;



    const currIdx = charArr.indexOf(char);

    let newIdx = currIdx + num;

    if (newIdx > 25) newIdx -= 26;

    if (newIdx < 0) newIdx += 26;

    if (str[i] === str[i].toUpperCase()) {

        newStr += charArr[newIdx].toUpperCase()


    else newStr += charArr[newIdx];