NodeJS EventEmitter test with Mocha

Question

I have a problem testing ldapjs client search operation. It returns an EventEmitter that you have to make listen for some specific event. I wrapped this operations to promisify it and to define my logic and I would like to unit-test it.

findUser(username) {
    return new Promise((resolve, reject) => {
        logger.debug('Searching user: ', username);
        this.ldapClient.bind(user.name, .user.password, err => {
            if (err) return reject(err);
            else
                this.ldapClient.search(root, {filter: `(cn=${username})`}, (errSearch, resSearch) => {
                    if (errSearch) return reject(errSearch);
                    const entries = [];
                    resSearch.on('searchEntry', entry => entries.push(entry.object));
                    resSearch.on('searchReference', referral => reject(new Error(`Received search referall: ${referral}`)));
                    resSearch.on('error', err => reject((err));
                    resSearch.on('end', result => {
                        if (result.status === 0 && entries.length === 1) {
                            return resolve({
                                cn: entries[0].cn,
                                objectclass: entries[0].objectclass,
                                password: entries[0].password
                            });
                        } else {
                            return reject(new Error(`Wrong search result: ${result}`));
                        }
                    });
                });
        });
    });
}

I am using mockery and Sinon to replace ldapjs dependency inside my module:

beforeEach(function () {
    searchEM = new EventEmitter();
    sandbox = sinon.createSandbox();
    ldapClientStub = Stubs.getLdapClientStub(sandbox);
    ldapClientStub.bind.yields(null);
    ldapClientStub.search.withArgs('o=ldap', {filter: `(cn=${findParam})`}).yields(null, searchEM);

    mockery.registerMock('ldapjs', Stubs.getLdapStub(ldapClientStub));
    mockery.registerAllowable('../src/client');

    UserClientCls = require('../src/client').default;
    userClient = new UserClientCls(config.get());
});

it('should return user with given username', function (done) {
    setTimeout(() => {
        searchEM.emit('searchEntry', users[1]);
        searchEM.emit('end', {status: 0});
        console.log('emitted');
    }, 500);
    searchEM.on('end', res => console.log(res));

    userClient.findUser(findParam)
        .then(user => {
            user.cn.should.equal(users[1].attributes.cn);
            user.objectclass.should.equal(users[1].attributes.objectclass);
            user.password.should.equal(users[1].attributes.password);
            return done();
        })
        .catch(err => done(err));
});

The problem is that listeners defined inside findUser are never called (but the function itself is called). The listener I defined in the test (just to debug the behaviour) is correctly called.

I do not understand if I miss something about how EventEmitters works or if I am doing the test in a wrong way. Or maybe I wrote a bad piece of code that cannot be tested.

Answer 1

I found a solution to my problem. I extended the base EventEmitter: I added the logic to store which event I want to emit and overrode its on method with a logic to emit my fake event.

class TestEventEmitter extends EventEmitter {
    constructor() {
        super();
    }

    setFakeEmit(fakeEmit) {
        this.fakeEmit = fakeEmit;
    }

    on(eventName, cb) {
        super.on(eventName, cb);
        if (super.eventNames().length === 4)
            this.fakeEmit.forEach(f => this.emit(f.name, f.obj));
    }
}

So, in beforeEach I can stub ldapClientStub.search to make it return my TestEventEmitter:

beforeEach(function() {
    searchEM = new TestEventEmitter();
    searchEM.setFakeEmit([{
        name: 'searchEntry',
        obj: { object: users[1].attributes }
    }, {
        name: 'end',
        obj: { status: 0 }
    }]);

    ...

    ldapClientStub.search.withArgs('o=ldap', { filter: `(&(cn=${findParam})(objectclass=astsUser))` }).yields(null, searchEM);
})

This solution may be not very elegant, but it works. If someone can post a better solution I'll be glad to have a look.