How to call this type of async function in Typescript

Question

How to type an async callback function in onAuthStateChanged?

There is an error "Promise returned in function argument where a void return was expected." from this line.

async (user: User | null) => {
  const unpdatedUser = user ? await adminUser(user) : null;
  callback(unpdatedUser);
};

here is the full code.

type DispatchType = Dispatch<SetStateAction<User | null>>;

export function onUserStateChange(callback: DispatchType) {
  onAuthStateChanged(auth, async (user: User | null) => {
    const unpdatedUser = user ? await adminUser(user) : null;
    callback(unpdatedUser);
  });
}

async function adminUser(user: User | null) {
  return get(ref(database, 'admins')).then((snapshot) => {
    if (snapshot.exists()) {
      const admins = snapshot.val();
      const isAdmin = admins.include(user?.uid);
      return { ...user, isAdmin };
    }
    return user;
  });
}

Answer 1

You've got some type confusion around the Firebase Auth User type and your own User type that includes an isAdmin Boolean.

I'd go with something like this...

import { User as FirebaseUser } from "firebase/auth";

// this is your user type
interface User extends FirebaseUser {
  isAdmin: boolean;
}

You should also check for a null user parameter before attempting to spread it into {...user, isAdmin } .

Your adminUser function should return type Promise<User | null> Promise<User | null>

async function adminUser(user: FirebaseUser | null): Promise<User | null> {
  // null user
  if (!user) {
    return user;
  }

  const isAdmin =
    (await get(ref(database, "admins"))).val()?.include(user.uid) ?? false;
  return { ...user, isAdmin };
}