ceasarCipher.js

const ceasarCipher = (str, num)  => {
  const alphabetArr = 'abcdefghijklmnopqrstuvwxyz'.split('')
  
  let myNum = num % alphabetArr.length

  const strArr = str.toLowerCase().split('')

  let resArr = []

  for (let c in strArr) {
    let letter = strArr[c]
    
    const index = alphabetArr.indexOf(letter)

    if (index < 0) {
      resArr.push(letter)
      continue
    }

    const newIndex = index + myNum

    const pos = newIndex >= 0 ? 
      newIndex < alphabetArr.length ? 
        newIndex : 
        newIndex - alphabetArr.length    
      : alphabetArr.length + newIndex 

    if (str[c] === str[c].toUpperCase()) {
      resArr.push(alphabetArr[pos].toUpperCase())
      continue
    }
    resArr.push(alphabetArr[pos])
  }

  return resArr.join('')
}

let given = 'azyB ,Azyc'
let expected = 'yxwZ ,Yxwa'

let result = ceasarCipher(given, -262)

console.log(`with given '${given}' we expect '${expected}' and  we got '${result}', they equlity is '${expected ===  result}' `)

given = 'azyB ,Azyc'
expected = 'cbaD ,Cbae'

result = ceasarCipher(given, 496)

console.log(`with given '${given}' we expect '${expected}' and  we got '${result}', they equlity is '${expected ===  result}' `)