This small Firestore app tests how onSnapshot() notifications fire when documents are updated in a transaction.

See https://groups.google.com/g/google-cloud-firestore-discuss/c/tORo0hykByc/m/RuYaMoCmBQAJ

1. Run npm install to install dependencies.
2. Edit firebase_config.ts to fill out your apiKey and projectId (not required if you only want to use the Firestore emulator).
3. See the test code in run_the_test.ts.

1. Run npm run build to generate the compiled JavaScript.
2. Open index.html in a web browser, and click the "Run Test" button.

1. Run npm run run to run the code in Node on the command line.

To connect to the Firestore emulator, specify -e. To enable Firestore debug logging, specify -v.

Example:

npm run run -- -e -v

The log lines like

[snapshot listener] GOT size=NNN ...

are printed every time that the snapshot listener is notified. I was wondering if it gets intermediate invocations as the transaction is partially committed; however, it only gets notified once before the transaction starts then once with the completed transaction.