caesarsCipher.js

/* One of the simplest and most widely known ciphers is a Caesar cipher, also known as a shift cipher. In a shift cipher the meanings of the letters are shifted by some set amount.
A common modern use is the ROT13 cipher, where the values of the letters are shifted by 13 places. Thus 'A' ↔ 'N', 'B' ↔ 'O' and so on.
Write a function which takes a ROT13 encoded string as input and returns a decoded string.
All letters will be uppercase. Do not transform any non-alphabetic character (i.e. spaces, punctuation), but do pass them on. */

function rot13(str) {
    var arr = str.split('');
    for(i = 0; i < arr.length; i++) {
      if (str.charCodeAt(i) > 64 && str.charCodeAt(i) < 78) {
        arr[i] = String.fromCharCode(str.charCodeAt(i)+13);
      }
      else if (str.charCodeAt(i) >= 78 && str.charCodeAt(i) < 91) {
        arr[i] = String.fromCharCode(str.charCodeAt(i)-13);
      }
      else {
        //keep same char
      }
    }
    str = arr.join('');
    return str;
  }
  
  // Change the inputs below to test
  rot13("SERR PBQR PNZC");