Internal state errors accessing prototype methods using DocumentSnapshot.get()

[MieleVL]

[REQUIRED] Describe your environment

• Operating System version: Windows 11
• Browser version: Google Chrome 104.0.5112.10
• Firebase SDK version: 9.9.3
• Firebase Product: firestore

[REQUIRED] Describe the problem

Steps to reproduce:

Passing a field to DocumentSnapshot.get() that does not exists on the document itself, but matches a property of Object.prototype throws Internal State errors (eg: toString). This happens because DocumentSnapshot.get() looks at mapvalue.fields as shown below, which will also match and return prototype methods.

return e = (e.mapValue.fields || {})[t.lastSegment()], e || null;

My suggestion would be to exclude prototype methods, by first checking if the key exists using Reflect.ownKeys(e.mapValue.fields || {}), e.mapValue.fields.hasOwnProperty(), or something similar.

Relevant Code:

import { doc, getDoc } from "firebase/firestore";

const docRef = doc(db, "cities", "SF");
const docSnap = await getDoc(docRef);

// Will throw an Internal State error if toString does not exist on the document
// Uncaught Error: FIRESTORE (9.9.3) INTERNAL ASSERTION FAILED: Unexpected state
docSnap.get('toString');

[MarkDuckworth]

@MieleVL, thanks for the suggestion. We will consider this change for the SDK.