xor-dexor-unxor.js

const key = (k, strLen) => Array
  .from({
    length: Math.ceil(strLen / k.length)
  })
  .fill(k)
  .join('')
  .split('')
  .map((v) => v.charCodeAt(0));

const xorStr = (s, {key: {ascii: asciiKey}}) => {
  const str = s.split('');
  const k = key(asciiKey, str.length);
  const x = str.map(
    (v, idx) => [
      '0',
      (v.charCodeAt(0) ^ k[idx])
      .toString('16')
    ]
    .join('')
    .slice(-2)
  );
  return x.join('');
};

const unXorStr = (s, {key: {ascii: asciiKey}}) => {
  const str = s
    .split('')
    .reduce((a, c, idx) => {
      if (idx % 2 === 0) {
        return a.concat(c);
      } else {
        return a
          .slice(0, -1)
          .concat(
            parseInt(a.slice(-1).concat(c).join(''), 16)
          )
      }
    }, []);
  const k = key(asciiKey, str.length);
  return str
    .map((v, idx) => String.fromCharCode(v ^ k[idx]))
    .join('');
};

const kk = 'abcd'
const s = 'The quick brown fox';

console.log({kk, s});

const a = xorStr(
  s,
  {key: {ascii: kk}}
);

console.log(a);

const b = unXorStr(a, {key: {ascii: kk}});
console.log(b)