如何使用 mocha/sinon/chai 正确测试异步 stream 错误事件处理？

Question

I am testing an async method that returns some data from a web request using the native https.request() method in NodeJS. I am using mocha, chai, and sinon with the relevant extensions for each.

The method I'm testing essentially wraps the boilerplate https.request() code provided in the NodeJS docs in a Promise and resolves when the response 'end' event is received or rejects if the request 'error' event is received. The bits relevant to discussion look like this:

async fetch(...) {
    // setup

    return new Promise((resolve, reject) => {
        const req = https.request(url, opts, (res) => {
            // handle response events
        });
        
        req.on('error', (e) => {
            logger.error(e);  // <-- this is what i want to verify
            reject(e);
        });

        req.end();
    });
}

As indicated in the comment, what I want to test is that if the request error event is emitted, the error gets logged correctly. This is what I'm currently doing to achieve this:

it('should log request error events', async () => {
    const sut = new MyService();
    const err = new Error('boom');
    const req = new EventEmitter();
    req.end = sinon.fake();
    const res = new EventEmitter();
    sinon.stub(logger, 'error');
    sinon.stub(https, 'request').callsFake((url, opt, cb) => {
        cb(res);
        return req;
    });

    try {
        const response = sut.fetch(...);
        req.emit('error', err);
        await response;
    } catch() {}

    logger.error.should.have.been.calledOnceWith(err);
});

This feels like a hack, but I can't figure out how to do this correctly using the normal patterns. The main problem is I need to emit the error event after the method is called but before the promise is fulfilled, and I can't see how to do that if I am returning the promise as you normally would with mocha.

Answer 1

I should have thought of this, but @Bergi's comment about using setTimeout() in the fake gave me an idea and I now have this working with the preferred syntax:

it('should log request error events', () => {
    const sut = new MyService();
    const err = new Error('boom');
    const req = new EventEmitter();
    req.end = sinon.fake();
    const res = new EventEmitter();
    sinon.stub(logger, 'error');
    sinon.stub(https, 'request').callsFake((url, opt, cb) => {
        cb(res);
        setTimeout(() => { req.emit('error', err); });
        return req;
    });

    return sut.fetch(...).should.eventually.be.rejectedWith(err)
        .then(() => {
            logger.error.should.have.been.calledOnceWith(err);
        });
});

I don't like adding any delays in unit tests unless I'm specifically testing delayed functionality, so I used setTimeout() with 0 delay just to push the emit call to the end of the message queue. By moving it to the fake I was able to just use promise chaining to test the call to the logger method.