Caesars Cipher - How to concatenate strings inside loop?

How can I concatenate the strings inside a for loop to solve the Cipher?

I tried to do as it follows, but my result var just show the first letter. What am I doing wrong?

Your code so far

function rot13(str) { // LBH QVQ VG!
  var result = "";
  for (i = 0; i<str.length; i++){
    if (str[i].charCodeAt(0)>=65 && str[i].charCodeAt(0)<78){
    result += String.fromCharCode(str[i].charCodeAt(0)+13);
    else if (str[i].charCodeAt(0)>=78 && str[i]<=90){
    result += String.fromCharCode(str[i].charCodeAt(0)-13);
      result += str[i];
  return result;

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

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Link to the challenge:

Your return is inside your for loop.