Remembering that time I had to XOR in JavaScript...

xor.js

const xor = function (a, b) {
  if (!Buffer.isBuffer(a)) a = new Buffer.from(a);
  if (!Buffer.isBuffer(b)) b = new Buffer.from(b);
  let res = []
  const len = (a.length > b.length) 
    ? b.length 
    : a.length;
  for (let i = 0; i < len; i++) {
    res.push(a[i] ^ b[i]);
  }
  return new Buffer.from(res);
}