I am working out some kinks in my unit tests for a node app with a Firebase RTDB integration. I am trying to build tests for my Firebase listeners, but one is giving me fits.

For example, if I have this function:

export function my_function(callback) {
    database.ref(`test`).on('child_changed', async snapshot => {
        callback(snapshot);
    });
};

and this test:

describe('my_function', () => {
    beforeEach(() => {
        databaseStub = sinon.stub(database, 'ref').returns({
            on: sinon.stub()
        });
    });

    afterEach(() => {
        databaseStub.restore();
    });

    it("should call the callback function on 'child_changed'", () => {
        const callback = sinon.spy();
        const snapshot = {}; // for brevity; actual data is not needed

        databaseStub().on.callsArgWith(1, snapshot);

        my_function(callback);

        expect(callback).to.have.been.calledWith(snapshot);
    });
});

This test passes as expected. But, if I have this function:

export async function my_function(callback) {
    database.ref('test').on('child_added', async snapshot => {
        const snap = await database.ref('messages').once('value'); // <- here's the rub
        
        // prints "depots: {}" as expected
        console.log(`depots: ${ JSON.stringify(snap) }`);

        // calling with a new object, not the one passed in as in the previous example
        callback(snap);
    });
}

With this test:

describe('my_function', () => {
    beforeEach(() => {
        databaseStub = sinon.stub(database, 'ref').returns({
            on: sinon.stub(),
            once: sinon.stub().returns({}),
            update: sinon.stub()
        });
    });

    afterEach(() => {
        databaseStub.restore();
    });

    it("should call the callback function on 'child_added'", () => {
        const callback = sinon.spy();
        const snapshot = {}; // for brevity; actual data is not needed
        const messages = {}; // for brevity; actual data is not needed

        databaseStub().on.callsArgWith(1, snapshot);

        my_function(callback);

        expect(callback).to.have.been.calledWith(messages);
    });
});

my test fails with AssertionError: expected spy to have been called with arguments {}.

What am I missing?