Treat empty maps correctly in Document.get()

[schmidt-sebastian]

Fixes #2204

This fixes the types for MapValue.fields so that tslint catches undefined violations and removes the violation in getProtoField that caused #2204

Note that is seems pretty involved to write a test for this since this code is only hit in Query processing and not in DocumentSnapshot.get() (see

firebase-js-sdk/packages/firestore/src/api/database.ts

Line 1346 in 4016987

const value = this._document

). If requested, I can add this test next week.

[mikelehen]

Can we not add a test for this?

Also, I don't exactly understand the issue. proto.fields can be missing if the document is empty? Is there a similar issue with empty maps inside a document? Or is only the root of the document an issue for some reason? If you can find a way to add comments to make it clearer, that might be nice. :-)

[schmidt-sebastian]

I fixed the browser tests and was now able to write a test that fails on master and succeeds with this PR. I did not add any inline comments as the types should speak for themselves.

As for your questions - I don't think this.proto.fields should ever be missing (but the types allow for it now, since this.proto is a fields?: ApiClientObjectMap<Value>. The issue the customer ran into is a nested empty map that did not contain the fields property when read back from IndexedDb (despite

firebase-js-sdk/packages/firestore/src/remote/serializer.ts

Line 563 in c7e5012

fields: this.toFields(map)

).

[schmidt-sebastian]

I fixed the browser tests and was now able to write a test that fails on master and succeeds with this PR. I did not add any inline comments as the types should speak for themselves.

As for your questions - I don't think this.proto.fields should ever be missing (but the types allow for it now, since this.proto is a fields?: ApiClientObjectMap<Value>. The issue the customer ran into is a nested empty map that did not contain the fields property when read back from IndexedDb (despite

firebase-js-sdk/packages/firestore/src/remote/serializer.ts

Line 563 in c7e5012

fields: this.toFields(map)

).

[mikelehen]

LGTM