Cannot call Firestore from Cloud Functions unit tests - firebase

Question

Developing Google cloud functions locally.
Trying to test functions that invoke Firestore.
Here is a minimal example.
Emulators are running.
The function addMessage() works completely fine when invoked from the browser.
The function fails with error TypeError [ERR_INVALID_ARG_TYPE]: The "path" argument must be of type string. when invoked from tests.
Question: Why is this error occurring and how can I invoke the Firestore function successfully from the tests?
functions/index.js:
require ('dotenv').config();
const functions = require ('firebase-functions');
const admin = require ('firebase-admin');
admin.initializeApp();
exports.addMessage = functions.https.onRequest (async (req, res) => {
const original = req.query.text;
const writeResult = await admin.firestore().collection ('messages').add ({text: original});
const docSnap = await writeResult.get();
const writtenText = docSnap.get ('text');
res.send (`Message with text: ${writtenText} added.`);
});
functions/test/index.test.js:
const admin = require ('firebase-admin');
const firebase_functions_test = require ('firebase-functions-test')({
projectId: 'my-project-id'
}, '/path/to/google-application-credentials.json');
const testFunctions = require ('../index.js');
describe ('addMessage()', () => {
it ('returns Message with text: Howdy added.', (done) => {
const req = {query: {text: 'Howdy'} };
const res = {
send: (body) => {
expect (body).toBe (`Message with text: Howdy added.`);
done();
}
};
testFunctions.addMessage (req, res);
});
});
Starting jest from functions folder:
(Among other test-related output):
FAIL test/index.test.js
● addMessage() › returns Message with text: Howdy added.
TypeError [ERR_INVALID_ARG_TYPE]: The "path" argument must be of type string. Received type object
7 | exports.addMessage = functions.https.onRequest (async (req, res) => {
8 | const original = req.query.text;
> 9 | const writeResult = await admin.firestore().collection ('messages').add ({text: original});
| ^
10 | const docSnap = await writeResult.get();
11 | const writtenText = docSnap.get ('text');
12 | res.send (`Message with text: ${writtenText} added.`);
at GrpcClient.loadProto (node_modules/google-gax/src/grpc.ts:166:23)
at new FirestoreClient (node_modules/#google-cloud/firestore/build/src/v1/firestore_client.js:118:38)
at ClientPool.Firestore._clientPool.pool_1.ClientPool [as clientFactory] (node_modules/#google-cloud/firestore/build/src/index.js:326:26)
at ClientPool.acquire (node_modules/#google-cloud/firestore/build/src/pool.js:87:35)
at ClientPool.run (node_modules/#google-cloud/firestore/build/src/pool.js:164:29)
at Firestore.request (node_modules/#google-cloud/firestore/build/src/index.js:983:33)
at WriteBatch.commit_ (node_modules/#google-cloud/firestore/build/src/write-batch.js:496:48)
Caused by: Error:
at WriteBatch.commit (node_modules/#google-cloud/firestore/build/src/write-batch.js:415:23)
at DocumentReference.create (node_modules/#google-cloud/firestore/build/src/reference.js:283:14)
at CollectionReference.add (node_modules/#google-cloud/firestore/build/src/reference.js:2011:28)
at Object.<anonymous>.exports.addMessage.functions.https.onRequest (index.js:9:71)
at Object.addMessage (node_modules/firebase-functions/lib/providers/https.js:50:16)
at Object.done (test/index.test.js:17:19)
Environment:
"node": "10"
"firebase-admin": "^8.12.1",
"firebase-functions": "^3.7.0"
"firebase-functions-test": "^0.2.1",
"jest": "^25.5.4"

Answer 1

Solved by adding jest.config.js to functions/:
module.exports = {
testEnvironment: 'node'
};
Solution based on this (but NB I needed to place jest.config.js in functions/, not the project root).
Also tried this but appeared to do nothing.
Tests now run perfectly except that they end with a Jest error:
Jest did not exit one second after the test run has completed.
This usually means that there are asynchronous operations that weren't stopped in your tests. Consider running Jest with `--detectOpenHandles` to troubleshoot this issue.
This seems to be harmless.
Thanks #gso_gabriel for the valuable links!