Sinon spy.called no working

Background

I have a small server that receives data from a machine. Every time I receive a message I call a function in a dispatcher object that simply console.logs everything it receives.

Problem

The code works well as I can see the console.logs in the console, but Sinon spy.called doesn't work. It is always false no matter how many times I call dispatcher.onMessage.

Code

server.js

const eventDispatcher = {
    onMessage: console.log,
};

const server = (dispatcher = eventDispatcher) => {


    //this gets called everytime the server receives a message
    const onData = data => {

        //Process data
        //....
        dispatcher.onMessage(data);
    };


    const getDispatcher = () => dispatcher;

    return Object.freeze({
        getDispatcher
    });
};

test.js

describe("message sender", () => {

    const myServer = serverFactory();


    it("should send information to server", () => {
        dummyMachine.socket.write("Hello World!\r\n");

        const dataSpy = sinon.spy(myServer.getDispatcher(), "onMessage");
        expect(dataSpy.called).to.be.true; //always fails!
    });

});

Research

After reading similar posts I believe this happens due to some layer of indirection, as pointed in:

• Sinon Spy is not called if the spied method is called indirectly

And should be fixed via using this:

• Sinon.spy on a method is not invoked

However, looking at my code I really can't get what I am missing.

Question

• What am I doing wrong?

MCVE

Directory Structure

 Project_Folder
 |____package.json
 |____server.js
 |____test
      |____ dummyMachine_spec.js

package.json

{
  "name": "sinon-question",
  "version": "1.0.0",
  "description": "MCVE about a dummy machine connecting to a server for StackOverflow",
  "main": "server.js",
  "scripts": {
    "test": "NODE_ENV=test mocha --reporter spec --slow 5000 --timeout 5000 test/*_spec.js || true"
  },
  "author": "Pedro Miguel P. S. Martins",
  "license": "ISC",
  "devDependencies": {
    "chai": "^3.5.0",
    "mocha": "^3.3.0",
    "sinon": "^2.2.0"
  },
  "dependencies": {
    "net": "^1.0.2"
  }
}

server.js

"use strict";

const net = require("net");

const eventDispatcher = {
    onMessage: console.log,
};

const server = (dispatcher = eventDispatcher) => {

    let serverSocket;

    const onData = data => {
        //Process data
        dispatcher.onMessage(`I am server and I got ${data}`);
    };

    const start = (connectOpts) => {
        return new Promise(fulfil => {
            serverSocket = net.createConnection(connectOpts, () => {
                serverSocket.on("data", onData);   
                fulfil();
            });
        });
    };

    const stop = () => serverSocket.destroy();

    const getDispatcher = () => dispatcher;

    return Object.freeze({
        start,
        stop,
        getDispatcher
    });
};

module.exports = server;

test/dummyMachine.js

"use strict";


const chai = require("chai"),
    expect = chai.expect;

const sinon = require("sinon");
const net = require("net");
const serverFactory = require("../server.js");

describe("Dummy Machine", () => {

    const dummyMachine = {
        IP: "localhost",
        port: 4002,
        server: undefined,
        socket: undefined
    };

    const server = serverFactory();

    before("Sets up dummyReader and server", done => {

        dummyMachine.server = net.createServer(undefined, socket => {
            dummyMachine.socket = socket;
        });

        dummyMachine.server.listen(
            dummyMachine.port,
            dummyMachine.IP,
            undefined,
            () => {
                server.start({
                    host: "localhost",
                    port: 4002
                })
                .then(done);
            }
        );
    });

    after("Kills dummyReader and server", () => {
        server.stop();
        dummyMachine.server.close();
    });

    it("should connect to server", done => {
        dummyMachine.server.getConnections((err, count) => {
            expect(err).to.be.null;
            expect(count).to.eql(1);
            done();
        });

    });

    it("should send information to server", () => {
        dummyMachine.socket.write("Hello World\r\n");

        const dataSpy = sinon.spy(server.getDispatcher(), "onMessage");
        expect(dataSpy.called).to.be.true; //WORK DAAMN YOU!
    });
});

Instructions for MCVE

1. Download the files and create the directory structure indicated.
2. Enter project folder and type npm install on a terminal
3. Type npm test

The first test should pass, meaning a connection is in fact being made.

The second test will fail, even though you get the console log, proving that onMessage was called.