Caesar cipher

Encrypts or decrypts a given string using the Caesar cipher.

• Use the modulo (%) operator and the ternary operator (?) to calculate the correct encryption/decryption key.
• Use the spread operator (...) and Array.prototype.map() to iterate over the letters of the given string.
• Use String.prototype.charCodeAt() and String.fromCharCode() to convert each letter appropriately, ignoring special characters, spaces etc.
• Use Array.prototype.join() to combine all the letters into a string.
• Pass true to the last parameter, decrypt, to decrypt an encrypted string.

const caesarCipher = (str, shift, decrypt = false) => {
  const s = decrypt ? (26 - shift) % 26 : shift;
  const n = s > 0 ? s : 26 + (s % 26);
  return [...str]
    .map((l, i) => {
      const c = str.charCodeAt(i);
      if (c >= 65 && c <= 90)
        return String.fromCharCode(((c - 65 + n) % 26) + 65);
      if (c >= 97 && c <= 122)
        return String.fromCharCode(((c - 97 + n) % 26) + 97);
      return l;
    })
    .join('');
};

caesarCipher('Hello World!', -3); // 'Ebiil Tloia!'
caesarCipher('Ebiil Tloia!', 23, true); // 'Hello World!'