caesars-cipher.js

// Caesars Cipher

/*
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) {
  return str.split('').map((char) => {
    const charCode = char.charCodeAt();

    if (charCode >= 65 && charCode <= 90) {

      if (charCode > 77) {
        return String.fromCharCode(charCode - 13);
      }
      else {
        return String.fromCharCode(charCode + 13);
      }
    }
    else {
      return char;
    }
  }).join('');
}
rot13('SERR PBQR PNZC');