Can I get some advice on my solution for Caesars Cipher?

1 
function rot13(str) { // LBH QVQ VG!
  var arr = str.split('');
  function add13(someStr){
    if (someStr.charCodeAt() > 90 || someStr.charCodeAt() < 65) {
      return String.fromCharCode(someStr.charCodeAt() + 0);
    }
    else if ((someStr.charCodeAt() + 13) > 90) {
      return String.fromCharCode(someStr.charCodeAt() + 13 - 90 + 64);    
    } else {
      return String.fromCharCode(someStr.charCodeAt() + 13);
    } 
  }
  var newArr = arr.map(add13);
  var regExp = /-/gi;
  return newArr.join('').replace(regExp, " ");
}

// Change the inputs below to test
rot13("SERR CVMMN!");

Any advice on how to make it more readable, concise, clean, etc would be appreciated .

4

Whoops! Thanks for blurring that out, I didn’t realize.

5

Thanks for the suggestions, I’ll work on trying to implement them.