How to register multiple OAuth logins to the same user?

[oOBoomberOo]

I want the user to be able to register using one of the third-party providers and then have them connect other providers to the existing account instead of registering a new one. What would be the unique identity that tells who the user is in this case?

[saikarthikp9]

Did you figure this out? I guess if the email IDs match you can assume they are indeed the same user.

[oOBoomberOo]

Yes, I finally figured it out. You can set passReqToCallback to true in the strategy's option, which will make the first argument of the verifier function be the Request object. Then you can look up req.user? to see if the user is currently authenticated through another strategy or not and handle it accordingly.

new TwitterStrategy({
    callbackURL: "...",
    consumerKey: "...",
    consumerSecret: "...",
    passReqToCallback: true
}, (req, accessToken, refreshToken, profile, done) => {
    const hasAlreadyRegistered = hasRegistered(profile.id); // `id` is a unique ID from the platform that the user is authenticating. You can use it to look up if this account has been signed up before.
    if (req.user && !hasAlreadyRegistered) {
        // add this provider to the existing user
    }
    else if (hasAlreadyRegistered) {
        // find the existing user with that `profile.id` and return it
    }
    else {
        // create a new user and then add the provider
    }
})

[oOBoomberOo]

The passReqToCallback option can be used to pass the Request object to the verifier function, and then you can handle the connecting account logic by checking if the req.user object exists or not.