Простой способ Ceasars Cipher Encryptor (сдвиг позиции буквы в алфавите )

CeasarsCipher.js

var alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZ".toLowerCase(),
    otherCharacters = '-=~\"\'#$%&*^:<>?/!{(|)}.1234567890\, ';

var shiftedAlphabet = alphabet.slice(3);
    shiftedAlphabet += alphabet.slice(0, 3);
    shiftedAlphabet += otherCharacters;
    alphabet += otherCharacters;
  
var str = 'I am amazing'.toLowerCase(),
    out = '';
for(var i = 0; i < str.length; i++){
  var numberd = alphabet.indexOf( str[i] )
  out += shiftedAlphabet[numberd];
}

console.log(out) // I am amazing => l dp dpdclqj