Missing required parameter [code_verifier]

@sasivarnan The solution was fairly simple. In the initial call of generateAuthURL() I use code_challenge_method: ‘plain’ and save the code_challenge that I use. Then when the user is redirected back to my platform I call the generateAuthURL() method again with the same saved code_challenge, and then the requestAccessToken() method with the code I have received.

The 1.2.0 version published 6 days ago (thanks @refarer!) allows the token to be passed on the constructor. So, now you could do something like this:

1. Create the endpoint to start the authentication process: generate state and challenge and call generateAuthURL; persist these values to recreate the OAuth2User later on;
2. Create another endpoint for the callback: recreate the auth with state and challenge and call requestAccessToken passing the code received; store the token returned by that function;
3. Pass the token on the OAuth2UserOptions during user creation.

Using firebase functions, my simplified code is:

// authenticate.ts
export const authenticate = functions
  .region('southamerica-east1')
  .https.onRequest(async (req, res) => {
    res.redirect(await generateAuthURL());
  });

// authenticationHandler.ts
export const authenticationHandler = functions
  .region('southamerica-east1')
  .https.onRequest(async (req, res) => {
    const { code } = req.query;
    await handleAuthCode(code as string);
    res.send('OK');
  });

// auth.ts
let user: auth.OAuth2User | null = null;
const getUser = async () => {
  if (!user) {
    const { token } = (await getPlatformTokens()) ?? {};
    user = new auth.OAuth2User({
      client_id: <CLIENT_ID>,
      client_secret: <SECRET>,
      callback: <CALLBACK_URL>,
      scopes: ['tweet.read', 'tweet.write', 'users.read', 'offline.access'],
      token: token ? JSON.parse(token) : undefined,
    });
  }
  return user;
};

let client: Client | null = null;
const getClient = async () => {
  if (!client) client = new Client(await getUser());
  return client;
};

export const generateAuthURL = async () => {
  const state = randomBytes(12).toString('hex');
  const challenge = randomBytes(12).toString('hex');
  await updatePlatformTokens({
    state,
    challenge,
  });
  const user = await getUser();
  return user.generateAuthURL({
    state,
    code_challenge_method: 'plain',
    code_challenge: challenge,
  });
};

export const handleAuthCode = async (code: string) => {
  const user = await getUser();
  const { state, challenge } = (await getPlatformTokens()) ?? {};
  if (state && challenge) {
    user.generateAuthURL({
      state,
      code_challenge_method: 'plain',
      code_challenge: challenge,
    });
    const { token } = await user.requestAccessToken(code);
    await updatePlatformTokens({
      token: JSON.stringify(token),
    });
  }
};