NextAuth signIn pass parameter to callback - next.js

Question

I am using NextAuth to enable users to sign up/in with their Google account and also to link their Google account to their current account on my site.
In order to differentiate between signing up and linking an account when already signed in, I want to pass an extra parameter to signIn that I can access in the signIn callback that will allow me to take the correct action. I have tried:
signIn("google", null, { linkAccount: "true" });
However, this is only passed into the signIn request as a query parameter and is not passed through to the callback. Is there any way I can make a custom argument accessible in the callback?
Edit: Including more code below.
Call to next-auth's signIn client API:
signIn("google", null { linkAccount: "true" });
[...nextauth.js]
import NextAuth from 'next-auth';
import GoogleProvider from 'next-auth/providers/google';
import axios from 'axios';
const authOptions = (req) => ({
providers: [
GoogleProvider({
clientId: process.env.GOOGLE_CLIENT_ID,
clientSecret: process.env.GOOGLE_CLIENT_SECRET,
}),
],
secret: "secret",
callbacks: {
async signIn({
user, account, profile, email, credentials
}) {
// GOAL: How can I specify to this endpoint that I am just linking an account?
let res = await axios.post('http://localhost:8000/users/third_party_sign_in', {
third_party_id: user.id,
email: user.email,
type: account.provider
justLink: true|false
}, { withCredentials: true })
let path;
if (res.data.action === "login") {
path = `/?action=${res.data.action}&id=${res.data.user_id}&email=${user.email}&third_party=${account.provider}`
} else if (res.data.action === "create") {
path = `/?action=${res.data.action}&name=${user.name}&email=${user.email}&third_party=${account.provider}&third_party_id=${user.id}`
}
return path;
},
async redirect({ url }) {
return Promise.resolve(url)
}
},
});
function testNextApiRequest(req) {
if (req.query.nextauth
&& req.query.nextauth.length === 2
&& req.query.linkAccount) {
/// logs for signIn API call but not for callback
console.log("QUERY PARAMS: ", req.query);
}
}
export default (req, res) => {
testNextApiRequest(req);
return NextAuth(req, res, authOptions(req));
}

Answer 1

I also spent a lot of time on this trying to figure out how to get a param in a callback when using the signIn function.
Here's the solution
call signIn like you were doing
signIn("google", null, { linkAccount: "true" });
Now in [...nextauth].ts you want to parse req.query BEFORE passing it to next-auth like so
authOptions is just a function that returns next-auth callbacks and config.
export default async function auth(req: NextApiRequest, res: NextApiResponse) {
console.log(req.query); // This will have your linkAccount param
return await NextAuth(req, res, authOptions(req, res));
}
Now that you have access to the params you can do whatever logic you want. This will only work for some callbacks like redirect. Trying to get the params in the session callback is still proving to be impossible for me.
It isn't great, honestly it's pretty bad if you do db queries you'll be slowing down all the requests but I think this is currently the best way to do it. Hope it helps!
More discussion here https://github.com/nextauthjs/next-auth/discussions/901