caesar-vigenere.js

const a = 'abcdefghijklmnopqrstuvwxyz'

const ceasar = (str, rot) => {
  return str.split('').map((letter) => {
    if (a.indexOf(letter.toLowerCase()) < 0) {
      return letter
    }
    let rotLetter = a[(a.indexOf(letter.toLowerCase()) + rot) % a.length]
    return letter.toLowerCase() === letter ? rotLetter : rotLetter.toUpperCase()
  }).join('')
}

const vigenere = (str, key, direction) => {
  key = key.replace(/[ ,\?!]/g, '')
  let i = 0
  return str.split('').map((letter) => {
    if (a.indexOf(letter.toLowerCase()) >= 0) {
      const keyLetterInd = a.indexOf(key[i % key.length].toLowerCase())
      const index = (a.indexOf(letter.toLowerCase()) + (direction ? keyLetterInd : a.length - keyLetterInd)) % a.length
      i++
      return letter.toLowerCase() === letter ? a[index] : a[index].toUpperCase()
    } else {
      return letter
    }
  }).join('')
}