success.html

<script>

  const GITHUB_WORKER_URL = '';

	const code = new URL(location.href).searchParams.get('code');
	const error = new URL(location.href).searchParams.get('error');

	if (code) {
	// remove ?code=... from URL
		const path = location.pathname + location.search.replace(/\bcode=\w+/, '').replace(/\?$/, '');
		history.pushState({}, '', path);
		login(code);
	}

	if (error) {
		location.replace('/');
	}

  async function login(code) {
    try {
      const response = await fetch(GITHUB_WORKER_URL, {
        method: 'POST',
        mode: 'cors',
        headers: {
          'content-type': 'application/json',
        },
        body: JSON.stringify({ code }),
      });

      const result = await response.text();

      if (result.error) {
        alert(JSON.stringify(result, null, 2));
			}     
		} catch (error) {
      alert(error);
    }
  }

</script>