async function handleRequest(request) {
  const token = request.headers.get('x-custom-token');

  if (token === 'token-123456') {
    return new Response('hello world');
  }

  // Incorrect key supplied. Reject the request.
  return new Response('Sorry, you have supplied an invalid token.', {
    status: 403,
  });
}

addEventListener('fetch', event => {
  event.respondWith(handleRequest(event.request));
});