How do I link OAuth2 to an already create email and password account? (Node.js)

You'll need to build a custom integration to connect OAuth provider accounts with your regular email/password accounts, the OAuth providers themselves don't provide that functionality. OAuth providers only return specific credentials from the linked account on their service. One way to handle it, especially if you're using MongoDB for your database, is to create a collection just for OAuth credentials. You'd link those back to your regular accounts using something like a user ID. Make sure whatever you're using to link the OAuth creds to the user account won't change, for example don't use an email because that can change, whereas on most services a user ID or similar won't ever change. Same goes for the actual OAuth creds themselves, for example don't use a temporary auth token, or an email (unless specific cases where the provider doesn't allow users to change their email/whatever other info you use). Each OAuth provider does things a little differently, so you'll need to investigate what kind of data you can grab after a successful OAuth request (documentation moment). That's a basic rundown but yeah

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