Caesars Cipher algorithm

JavaScript
1

Tell us what’s happening:
Hi Everyone,

I have been stuck on this for a while now. I am able to return the proper numbers but I cannot convert them back to letters. What am I doing wrong?

Thank you in Advance for your help!

Your code so far

function rot13(str) { // LBH QVQ VG!
  var result = [];

  for(var i = 0; i<str.length; i++){
   result.push(str[i].charCodeAt());
    if(result[i] < 78 && result[i] > 64){
     result[i]+= 13;
    }else if(result[i] > 77 && result[i] < 91){
      result[i]-=13;
    }
  }
  String.fromCharCode(result);
}


// Change the inputs below to test
rot13("SERR PBQR PNZC");

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Link to the challenge:
https://www.freecodecamp.org/challenges/caesars-cipher