onSnapshot() results in `unknown` snapshot data type

[samtstern]

[REQUIRED] Describe your environment

• Operating System version: macOS
• Browser version: Chrome latest
• Firebase SDK version: 9.0.0-beta.1
• Firebase Product: firestore

[REQUIRED] Describe the problem

Steps to reproduce:

Relevant Code:

index.ts

import { collection, getFirestore, onSnapshot } from "firebase/firestore";

function main() {
  const firestore = getFirestore();
  const colRef = "col";

  onSnapshot(
    collection(firestore, colRef),
    (snap) => {
      if (snap.empty) {
        return;
      }
      const documents = snap.docs.map((doc) => ({
        id: doc.id,
        ...doc.data(), // error: Spread types may only be created from object types
      }));
    },
    (error) => {
      console.warn(error);
    }
  );
}

In this code snap is QuerySnapshot<unknown> which makes it hard to use .data(). This can be fixed by either:

• Casting doc.data() as any
• Using onSnapshot<any>

Is this the intended behavior in the new SDK?

[samtstern]

cc @puchesjr

[schmidt-sebastian]

TypeScript does not retain the type of the generic class unless the type is part of the API surface. Unfortunately, CollectionReference<T> does not refer to T in any of its members. This means that we have to change the API to accommodate this. I will prepare a PR, but we need to obtain approval for this API change internally.