feat(NODE-7441): add ChangeStream.bufferedCount

src/change_stream.ts

@@ -705,6 +705,11 @@ export class ChangeStream<
     return this.cursor?.resumeToken;
   }
 
+  /** Returns the currently buffered documents length of the underlying cursor. */
+  bufferedCount(): number {
+    return this.cursor?.bufferedCount() ?? 0;
+  }
+
   /** Check if there is any document still available in the Change Stream */
   async hasNext(): Promise<boolean> {
     this._setIsIterator();

test/integration/change-streams/change_stream.test.ts

@@ -1167,6 +1167,91 @@ describe('Change Streams', function () {
         );
       });
     });
+
+    // Note that `insertOne` in these tests is called with write concern `{ w: 'majority' }`.
+    // This is to avoid eventual consistency issues that make these tests brittle.
+    // A change stream won't see the new documents until they are persisted in the oplog and
+    // some MongoDB server versions (4.2-) used for testing default to write concern `{ w: 1 }`.
+    // Using write concern `{ w: 'majority' }` ensures that the new documents are persisted in 
+    // the oplog when the `insertOne` promise resolves, making these tests more reliable.
+    describe('#bufferedCount()', function () {
+      it(
+        'should return 0 before starting to consume the change stream (empty collection)',
+        { requires: { topology: 'replicaset' } },
+        async function () {
+          changeStream = collection.watch([]);
+          expect(changeStream.bufferedCount()).to.equal(0);
+          await changeStream.close();
+        }
+      );
+
+      it(
+        'should return 0 before starting to consume the change stream (non-empty collection)',
+        { requires: { topology: 'replicaset' } },
+        async function () {
+          // existing documents
+          await collection.insertOne({ a: 1 }, { writeConcern: { w: 'majority' } });
+          await collection.insertOne({ a: 2 }, { writeConcern: { w: 'majority' } });
+
+          changeStream = collection.watch([]);
+
+          // docs inserted after the change stream was created
+          await collection.insertOne({ a: 3 }, { writeConcern: { w: 'majority' } });
+          await collection.insertOne({ a: 4 }, { writeConcern: { w: 'majority' } });
+
+          expect(changeStream.bufferedCount()).to.equal(0);
+          await changeStream.close();
+        }
+      );
+
+      it('should return the underlying cursor buffered document count', {
+        metadata: { requires: { topology: 'replicaset' } },
+        async test() {
+          changeStream = collection.watch([]);
+          await initIteratorMode(changeStream);
+          await collection.insertOne({ a: 1 }, { writeConcern: { w: 'majority' } });
+          await collection.insertOne({ a: 2 }, { writeConcern: { w: 'majority' } });
+
+          expect(changeStream.bufferedCount()).to.equal(0);
+
+          // hasNext will trigger a batch fetch, buffering the documents
+          const hasNext = await changeStream.hasNext();
+          expect(hasNext).to.be.true;
+
+          expect(changeStream.bufferedCount()).to.equal(2);
+        }
+      });
+
+      it(`decreases as buffered documents are consumed`, {
+        metadata: { requires: { topology: 'replicaset' } },
+        async test() {
+          changeStream = collection.watch([]);
+          await initIteratorMode(changeStream);
+          await collection.insertOne({ a: 1 }, { writeConcern: { w: 'majority' } });
+          await collection.insertOne({ a: 2 }, { writeConcern: { w: 'majority' } });
+          await collection.insertOne({ a: 3 }, { writeConcern: { w: 'majority' } });
+
+          expect(changeStream.bufferedCount()).to.equal(0);
+
+          // `next` triggers a batch fetch, buffering the documents
+          // and then consumes one document from that buffer
+          for (let i = 2; i >= 0; i--) {
+            await changeStream.next();
+            expect(changeStream.bufferedCount()).to.equal(i);
+          }
+
+          await collection.insertOne({ a: 1 }, { writeConcern: { w: 'majority' } });
+          await collection.insertOne({ a: 2 }, { writeConcern: { w: 'majority' } });
+          await collection.insertOne({ a: 3 }, { writeConcern: { w: 'majority' } });
+
+          // `tryNext` also triggers a batch fetch
+          for (let i = 2; i >= 0; i--) {
+            await changeStream.tryNext();
+            expect(changeStream.bufferedCount()).to.equal(i);
+          }
+        }
+      });
+    });
   });
 
   describe('startAfter', function () {