Why did user object is undefined in NextAuth session callback?

Question

I use NextAuth for signIn with discord provider and I need to add userID into the session object. For that I use session callback but user object is undefined. When I try to add userID to the session object, I got an error like this :

[next-auth][error][JWT_SESSION_ERROR] https://next-auth.js.org/errors#jwt_session_error Cannot read properties of undefined (reading 'id')

Indeed, it seems the user object is undefined but I don't have any solution.

export default NextAuth({
  providers: [
    DiscordProvider({
      clientId: process.env.DISCORD_CLIENT_ID,
      clientSecret: process.env.DISCORD_CLIENT_SECRET,
      authorization: { params: { scope: 'identify' } }
    })
  ],
  session: {
    jwt: true
  },
  jwt: {
    secret: process.env.JWT_SECRET
  },
  callbacks: {
    async session({session, user}) {
      session.user.id = user.id
      return session
    },
    async jwt(token) {
      return token
    }
  }
})

Answer 1

You can do like that in your callback :-

callbacks: {
  jwt:({token,user})=>{
    if(user){
      token.id = user.userId;
    }
    return token;
  },
  session:({session,token}) =>{
    if(token){
      session.user.id = token.id;
    }
    return session;
  }
}

and don't forget to return the user in the function which is executed just before the callback, otherwise it tell undefined.