Session callback params types are misleading

[baptisteArno]

Question 💬

session param in session callback is typed as Session but in reality, it can also be undefined.

How to reproduce ☕️

Add a session callback:

session: async ({ session, token, user }) => {
      token?.user ? (session.user = token.user as User) : (session.user = user)
      if (token.providerType) session.providerType = token.providerType
      return session
    }

Error: Cannot read property 'providerType' of undefined

Contributing 🙌🏽

Yes, I am willing to help answer this question in a PR

[balazsorban44]

session should always be defined.

When using strategy: "jwt", you will have token, when using strategy: "database", you will get user.

See the docs: https://next-auth.js.org/configuration/callbacks#session-callback

If you want to augment the default Session interface, see https://next-auth.js.org/getting-started/typescript#module-augmentation

[baptisteArno]

Sorry for the misleading issue content, I wanted to talk about token and user types. They can both be undefined but the types don't say they can so it results in potential runtime types errors

[balazsorban44]

I see. It is slightly complicated since the existence of those properties is dependent on the adapter and jwt.strategy values in this interface:

next-auth/src/core/types.ts

Line 17 in 49e4af1

export interface NextAuthOptions {

I haven't found a nice way of propagating that to the callbacks.session yet, but feel free to have a look at it. I assume it would involve some generics, but they should be totally optional.

[yyueniao]

I can work on this issue to correct the params type.

[yyueniao]

Fixing in #8404