ASCII Shift Encryption&Decryption

/*

Ascii Shift Encryption/Decryption
The goal of this kata is to create a very simple ASCII encryption and decryption. The encryption algorithm should shift each character's charcode by the character's current index in the string (0-based).

The input strings will never require to go outside of the ASCII range.

Example:
  p | a | s | s | w | o | r | d # Plaintext
+ 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 # Shift (add)
  p | b | u | v | { | t | x | k # Ciphertext
The decryption should reverse this:

  p | b | u | v | { | t | x | k # Ciphertext
- 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 # Shift (subtract)
  p | a | s | s | w | o | r | d # Plaintext

*/

function asciiEncrypt(plaintext) {
    return [...plaintext].map((el,i)=>String.fromCharCode(el.charCodeAt()+i)).join``;
}
    
function asciiDecrypt(ciphertext) {
    return [...ciphertext].map((el,i)=>String.fromCharCode(el.charCodeAt()-i)).join``;
}