twitch identity provider flow

twitch_oauth.js

///////////////////////////
// Client
///////////////////////////

// when the user clicks the 'login with twitch' button...
document.location =
    `https://id.twitch.tv/oauth2/authorize` +
    `?client_id=${TWITCH_CLIENT_ID}` +
    `&redirect_uri=${redirect}` +
    `&response_type=token`;

// when you get redirected back to your app:
const hash = window.location.hash.replace('#', '');
const params = new URLSearchParams(hash);
const twitch_access_token = params.get('access_token');

// try to login on the server with the token
await window.fetch(
  backendLoginEndpoint,
  {
    body: JSON.stringify({ twitch_access_token }),
    headers: {
      'Content-Type': 'application/json',
    },
    method: 'POST',
  }
);

///////////////////////////
// Server (login endpoint)
///////////////////////////
const resp = await fetch("https://id.twitch.tv/oauth2/validate", {
  headers: { Authorization: "Bearer " + twitch_access_token },
});
const { client_id, login, user_id } = await resp.json();

if (client_id !== TWITCH_CLIENT_ID) {
  throw new Error("client id mismatch");
}
// Ask twitch for user details
const userDataResp = await fetch(
  "https://api.twitch.tv/helix/users?id=" + user_id,
  {
    headers: {
      Authorization: "Bearer " + twitch_access_token,
      "Client-Id": client_id,
    },
  }
);