NextAuth: Custom user data

I want to use custom user data but, I faced a trap. So I want to show you.

export default NextAuth({
  providers: [
    // ...some probiders
  ],
  callbacks: {
    async signIn({ user, account, profile, email, credentials }){
      // You cannot use spread syntax
      // user = {...[Your variable]}
      user.[Your variable] = {
        // ...some properties
      }
    },
    async jwt({ token, user }){
      // You cannot use spread syntax
      // token = {...user.[Your variable]}
      token.[Your variable] = user.[Your variable]
    },
    async session({ session, token }){
      // You cannot use spread syntax
      // session = {...token.[Your variable]}
      session.[Your variable] = token.[Your variable]
    }
  }
})