Please see suggested Pull Request #1983 for the fix

Environment details

• OS: MacOS
• Node.js version:
• npm version: 21.5.0
• @google-cloud/firestore version: 7.2.0

Steps to reproduce

1. Install the latest version of this library from https://www.npmjs.com/package/@google-cloud/firestore
2. Initialise the firestore object using a JSON private key (Service Account)
3. Get any object like the original firestore object or Make a WriteBatch, CollectionReference, DocumentSnapshot
4. Run JSON.stringify() method on any object obtained in the step above (3), the private key is exposed because the spelling of toJSON method in https://github.com/googleapis/nodejs-firestore/blob/main/dev/src/index.ts#L748 is not correct
   it should be toJSON instead of toJson, so the method is not getting activated which was meant to prevent serialisation of internal this._settings object when the firestore object is being serialised via JSON.stringify