vigenère-cipher.js

const message = 'alamaworek';
const key = 'blablabla';

function encode(message, key) {
  const shift = key
    .repeat(Math.ceil(message.length / key.length))
    .split('')
    .map(c => c.charCodeAt(0) - 'a'.charCodeAt(0));

  return message
    .split('')
    .map(c => c.charCodeAt(0) - 'a'.charCodeAt(0))
    .map((c, i) => (c + shift[i]) % 26 + 'a'.charCodeAt(0))
    .map(c => String.fromCharCode(c))
    .join('');
}

const encodedMsg = encode(message, key);
console.log(encodedMsg);