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) {
  const alphabetRegex = /[A-Za-z]/;
  const A = "A".charCodeAt(0);
  let numberOfLetters = 26;
  return str.split("").map(ch => {
    let currentCharCode = ch.charCodeAt(0);
    return (ch.match(alphabetRegex) ? 
      String.fromCharCode((((currentCharCode - A) + 13) % numberOfLetters) + A)
      : ch);
  }).join('');
}

rot13("SERR PBQR PNZC");