A function to decode a ROT13 string

rot13.js

function rot13 (str) { // LBH QVQ VG!
  const allCapsRegex = /[A-Z]/

  let decoded = ''
  let char = 0

  while (char < str.length) {
    if (true === allCapsRegex.test(str[char])) {
      decoded += String.fromCharCode(
        str[char].charCodeAt(0) <= 77 ? str[char].charCodeAt(0) + 13 : str[char].charCodeAt(0) - 13
      )
    } else {
      decoded += str[char]
    }

    char++
  }
  
  return decoded
}