/*
Description:
In this kata you will have to change every letter in a given string to the next letter in the alphabet. You will write a functionnextLetter to do this. The function will take a single parameter str (string).

EXAMPLES:

"Hello" --> "Ifmmp"

"What is your name?" --> "Xibu jt zpvs obnf?"

"zoo" --> "app"

"zzZAaa" --> "aaABbb"

Note: spaces and special characters should remain the same. Capital letters should transfer in the same way but remain capitilized.
*/
function nextLetter(str) {
let arr_en = ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z'];
let arr_EN = ['A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J', 'K', 'L', 'M', 'N', 'O', 'P', 'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X', 'Y', 'Z'];
  return str.replace(/[a-z]/gi,v=>{
  if (v===v.toLowerCase()){
  const index=arr_en.indexOf(v)
  return v=arr_en[(index+1)%26]}
  if (v===v.toUpperCase()){
  const index=arr_EN.indexOf(v)
  return v=arr_EN[(index+1)%26]}
  })
}
