Caesars Cipher - Combine multiple alphabet strings into one single sentence

1

I am getting the correct output but currently, each alphabet is a single string. Is there a way to combine all the individual alphabets into one single sentence?

function rot13(str) {
  var original = str.split('')
  var alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZ".split('')
  let i = 0;
  while(i<original.length){
  let indexVal = alphabet.indexOf(original[i])
  if(indexVal>= 13){
    indexVal -=13
  }else if(indexVal===-1){
    indexVal = 33
  }
  else{
    indexVal += 13
  }
 var final = alphabet[indexVal]
    i++;
    console.log(final)
  }
}
rot13("SERR CVMMN");//should log FREE PIZZA
/* 
currently logs:
F
R
E
E
undefined
P
I
Z
Z
A
undefined
*/
2

Came up with this solution in the end

function rot13(str) {
  var original = str.split('')
  var alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZ".split('')
  let i = 0;
  let arr = "";
  while(i<original.length){
  let indexVal = alphabet.indexOf(original[i])
  if(indexVal>= 13){
    indexVal -=13
  }else if(indexVal===-1){
    indexVal = 33
  }
  else{
    indexVal += 13
  }
  arr+=alphabet[indexVal];
  arr = arr.replace(undefined, original[i])
  
  i++;
  }
  return arr
}
rot13("GUR DHVPX OEBJA SBK WHZCF BIRE GUR YNML QBT.");