For Loop - Help Understanding / Caesars Cipher Basic Algorithm

Hi,

I am solving the last algorithm (Caesars Cipher) for the Basic Java course and need some help understanding where my code has gone wrong. I was able to convert the value of the first character 13 places back but I am having trouble getting the for loop to return the whole converted string. My loop only returns the first letter converted.

Can anyone help with an explanation on what I’m doing wrong and what I should take a look at - or if the way i’m going about is completely wrong.

CODE BELOW

function rot13(str) { // LBH QVQ VG!
   
  str.split("");
  var letsCount = "";
  for (var i = 0, len = str.length; i < len; i++) {
    letsCount = str[i];
    var ourChar = letsCount.charCodeAt();
    ourChar -= 13;
    var letters = String.fromCharCode(ourChar);
    return letters;
  } 
}

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