Caesars cipher. (freeCodeCamp JavaScript algorithms and data structures challenge)

Caesars cipher

function rot13(str) {
  newstr = str.split("");
  
  var finalstr = newstr.map(function(letter) {
    lettervalue = letter.charCodeAt(0);
    
    if (lettervalue < 65 || lettervalue > 90) {
      return String.fromCharCode(lettervalue);
    }
    
    else if (lettervalue < 78) {
      return String.fromCharCode(lettervalue + 13);
    } 
    
    else return String.fromCharCode(lettervalue - 13);
    
  }).join("");
  
  return finalstr;
};


rot13("SERR PBQR PNZC"); 
// should decode to "FREE CODE CAMP"

// rot13("SERR CVMMN!"); 
// should decode to "FREE PIZZA!"

// rot13("SERR YBIR?"); 
// should decode to "FREE LOVE?"

// rot13("GUR DHVPX OEBJA QBT WHZCRQ BIRE GUR YNML SBK."); 
// should decode to "THE QUICK BROWN DOG JUMPED OVER THE LAZY FOX." 



//         A B C D E F G H I J K L M
// ROT13
//         N O P Q R S T U V W X Y Z
        
        
//         HELLO
// ROT13
//         URYYB



// ASCII Table



// freeCodeCamp javaScript algorithms and data structures challenge link is given below:

// https://www.freecodecamp.org/learn/javascript-algorithms-and-data-structures/javascript-algorithms-and-data-structures-projects/caesars-cipher