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?

via Flame_Phoenix