minor updates based on comments.

Author: ehsannas

packages/firestore/src/local/indexeddb_document_overlay_cache.ts

@@ -147,7 +147,6 @@ export class IndexedDbDocumentOverlayCache implements DocumentOverlayCache {
   ): PersistencePromise<Map<DocumentKey, Overlay>> {
     const result = new Map<DocumentKey, Overlay>();
     let currentBatchId: number | undefined = undefined;
-    let currentCount = 0;
     // We want batch IDs larger than `sinceBatchId`, and so the lower bound
     // is not inclusive.
     const range = IDBKeyRange.bound(
@@ -164,16 +163,15 @@ export class IndexedDbDocumentOverlayCache implements DocumentOverlayCache {
         (_, dbOverlay, control) => {
           // We do not want to return partial batch overlays, even if the size
           // of the result set exceeds the given `count` argument. Therefore, we
-          // continue to aggregate the results even after `currentCount` exceeds
+          // continue to aggregate results even after the result size exceeds
           // `count` if there are more overlays from the `currentBatchId`.
           const overlay = fromDbDocumentOverlay(this.serializer, dbOverlay);
           if (
-            currentCount < count ||
+            result.size < count ||
             overlay.largestBatchId === currentBatchId
           ) {
             result.set(overlay.getKey(), overlay);
             currentBatchId = overlay.largestBatchId;
-            ++currentCount;
           } else {
             control.done();
           }

packages/firestore/src/local/memory_document_overlay_cache.ts

@@ -32,7 +32,6 @@ import { PersistenceTransaction } from './persistence_transaction';
 export class MemoryDocumentOverlayCache implements DocumentOverlayCache {
   // A map sorted by DocumentKey, whose value is a pair of the largest batch id
   // for the overlay and the overlay itself.
-
   private overlays = new SortedMap<DocumentKey, Overlay>(
     DocumentKey.comparator
   );

packages/firestore/src/model/document_key.ts

@@ -53,6 +53,10 @@ export class DocumentKey {
 
   /** Returns the collection group (i.e. the name of the parent collection) for this key. */
   getCollectionGroup(): string {
+    debugAssert(
+      !this.path.isEmpty(),
+      'Cannot get collection group for empty key'
+    );
     return this.path.get(this.path.length - 2);
   }