How to test a Server Sent Events (SSE) route in NodeJS?

Question

I have a Server Sent Events route on my NodeJS app that clients can subscribe to for getting real-time updates from the server. It looks like follows:

router.get('/updates', (req, res) => {
    res.writeHead(200, {
        'Content-Type': 'text/event-stream',
        'Cache-Control': 'no-cache',
        'Connection': 'keep-alive'
    })

    const triggered = (info) => {
        res.write(`\ndata: ${JSON.stringify(info)}\n\n`)
    }

    eventEmitter.addListener(constants.events.TRIGGERED, triggered)

    req.on('close', () => {
        eventEmitter.removeListener(constants.events.TRIGGERED, triggered)
    })
})

Testing a traditional route using supertest is simple enough in node:

test('Should get and render view', async() => {
    const res = await request(app)
        .get('/')
        .expect(200)
    
    expect(res.text).not.toBeUndefined()
})

However, this does not work when testing a SSE route.

Does anyone have any ideas on how to test a SSE route with Node? It doesn't necessarily have to be tested with supertest. Just looking for ideas on how to test it, supertest or otherwise.

I have an idea about how to integration test this. Basically, one would have to spin up a server before the test, subscribe to it during the test and close it after the test. However, it doesn't work as expected in Jest when I use beforeEach() and afterEach() to spin up a server.

Answer 1

I would mock/fake everything used by the endpoint, and check if the endpoint executes in the right order with the correct variables. First, I would declare trigger function and close event callback outside of the endpoint so that I could test them directly. Second, I would eliminate all global references in all functions in favor of function parameters:

let triggered = (res) => (info) => {
    res.write(`\ndata: ${JSON.stringify(info)}\n\n`);
}

let onCloseHandler = (eventEmitter, constants, triggered, res) => () => {
    eventEmitter.removeListener(constants.events.TRIGGERED, triggered(res));
}

let updatesHandler = (eventEmitter, constants, triggered) => (req, res) => {
    res.writeHead(200, {
        'Content-Type': 'text/event-stream',
        'Cache-Control': 'no-cache',
        'Connection': 'keep-alive'
    });

    eventEmitter.addListener(constants.events.TRIGGERED, triggered(res));

    req.on('close', onCloseHandler(eventEmitter, constants, triggered, res));
};

router.get('/updates', updatesHandler(eventEmitter, constants, triggered));

With this code, the test cases would be like:

test("triggered", () => {
    let res;

    beforeEach(() => {
        res = generateFakeRespone();
    });

    it("should execute res.write with the correct variable", () => {
        trigger(res)("whatever");

        expect(res.write).to.have.been.called.once;
        expect(res.write).to.have.been.called.with(`\ndata: ${JSON.stringify("whatever")}\n\n`);
    });
});


test("onCloseHandler", () => {
    let res;
    let eventEmitter;
    let constants;
    let triggered;

    beforeEach(() => {
        res = Math.random();
        eventEmitter = generateFakeEventEmitter();
        constants = generateFakeConstants();
        triggered = generateFakeTriggered();
    });

    it("should execute eventEmitter.removeListener", () => {
        onCloseHandler(eventEmitter, constants, triggered, res);

        expect(eventEmitter.removeListener).to.have.been.called.once;
        expect(eventEmitter.removeListener).to.have.been.called.with(/*...*/)
    });
});

test("updatesHandler", () => {
    beforeEach(() => {
        req = generateFakeRequest();
        res = generateFakeRespone();
        eventEmitter = generateFakeEventEmitter();
        constants = generateFakeConstants();
        triggered = generateFakeTriggered();
    });

    it("should execute res.writeHead", () => {
        updatesHandler(eventEmitter, constants, triggered)(req, res);

        expect(res.writeHead).to.have.been.called.once;
        expect(res.writeHead).to.have.been.called.with(/*...*/)
    });

    it("should execute req.on", () => {
        //...
    });

    // more tests ...
});

With this style of coding and testing, you have the ability to make very detailed unit test. The downside is that it take much more effort to test everything properly.