CaesarsCipher.js

/**
 * Caesar's Cipher - also known as the ROT13 Cipher is when
 * a letter is replaced by the one that is 13 spaces away
 * from it in the alphabet. If the letter is in the first half
 * of the alphabet we add 13, if it's in the latter half we
 * subtract 13 from the character code value.
 */

/**
 * Decrypt a ROT13 cipher
 * @param {String} str - string to be decrypted
 * @return {String} decrypted string
 */
function rot13 (str) {
  const response = []
  const strLength = str.length

  for (let i = 0; i < strLength; i++) {
    const char = str.charCodeAt(i)

    if (char < 65 || (char > 90 && char < 97) || char > 122) {
      response.push(str.charAt(i))
    } else if ((char > 77 && char <= 90) || (char > 109 && char <= 122)) {
      response.push(String.fromCharCode(str.charCodeAt(i) - 13))
    } else {
      response.push(String.fromCharCode(str.charCodeAt(i) + 13))
    }
  }
  return response.join('')
}

// Caesars Cipher Example
const encryptedString = 'Uryyb Jbeyq'
const decryptedString = rot13(encryptedString)

console.log(decryptedString) // Hello World