Vigenère cipher - Clash of Code

cipher.js

// Vigenère cipher
// https://en.wikipedia.org/wiki/Vigen%C3%A8re_cipher
// Created for a Clash Of Code
// doesnt respect case. screw it.
function encrypt(text, code) {
  text = text.toLowerCase();
  code = code.toLowerCase();
  const alpha = [..."abcdefghijklmnopqrstuvwxyz".repeat(2)];
  let output = "";
  let key = code;

  while (key.length < text.length) {
    key += code;
  }

  if (key.length !== text.length) {
    key = key.slice(0, text.length);
  }

  for (let i = 0; i < text.length; i++) {
    const letter = text[i];
    if (!alpha.includes(letter)) {
      output += letter;
    } else {
      const letterAlphaIndex = alpha.indexOf(letter);
      const k = key[i];
      const kAlphaIndex = alpha.indexOf(k);
      output += alpha[letterAlphaIndex + kAlphaIndex];
    }
  }

  return output;
}

function decrypt(text, code) {
  text = text.toLowerCase();
  code = code.toLowerCase();
  const alpha = [..."abcdefghijklmnopqrstuvwxyz".repeat(2)];
  let output = "";
  let key = code;

  while (key.length < text.length) {
    key += code;
  }

  if (key.length !== text.length) {
    key = key.slice(0, text.length);
  }

  for (let i = 0; i < text.length; i++) {
    const letter = text[i];
    if (!alpha.includes(letter)) {
      output += letter;
    } else {
      const letterAlphaIndex = alpha.indexOf(letter);
      const k = key[i];
      const kAlphaIndex = alpha.indexOf(k);
      let indexToget = letterAlphaIndex - kAlphaIndex;
      if (indexToget < 0) {
        output += alpha[26 - Math.abs(indexToget)];
      } else {
        output += alpha[indexToget];
      }
    }
  }

  return output;
}

const encryptedMessage = encrypt("Je suis un message secret.", "lecode");
console.log(encryptedMessage);
console.log(decrypt(encryptedMessage, "lecode"));