A super basic character shift cipher

basicCipher.js

const alphabet = [
  "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"
];

// Encrypt

function encrypt(plainText) {
  const ptLength = plainText.length;
  const encryptedText = [];
  for (let i = 0; i < ptLength; i += 1) {
    const letter = plainText.charAt(i);
    const indexInAlphabet = alphabet.indexOf(letter);
    let index = indexInAlphabet - 1;
    if (indexInAlphabet - 1 < 0) index = 25;
    encryptedText.push(alphabet[index]);
  }
  return encryptedText.join("");
}

// Decrypt

function decrypt(plainText) {
  const ptLength = plainText.length;
  const encryptedText = [];
  for (let i = 0; i < ptLength; i += 1) {
    const letter = plainText.charAt(i);
    const indexInAlphabet = alphabet.indexOf(letter);
    let index = indexInAlphabet + 1;
    if (indexInAlphabet + 1 > 25) index = 0;
    encryptedText.push(alphabet[index]);
  }
  return encryptedText.join("");
}