Mocha: How to assert after an event?

Question

I'm working with Electron and Johnny-Five to process some data I read with my Arduino Mega 2560 and I'm having some trouble testing my Arduino's connection.

This is the method I want to test (ignore the awful signature):

export let board: Board | null = null;

export function setArduinoBoard(f: (num: number, ...etc: any[]) => void, num: number, ...etc: any[]) {
    if (board == null)
        board = new Board({...});

    board = board.on("ready", () => f(num, etc));
}

And this is my test:

describe.only("setArduinoBoard()", function() {
    it("can communicate with the Arduino device", function(done) {
        const f = (num: number) => console.log(num);
        const spy = sinon.spy(f);
        setArduinoBoard(f, 0);
        assert(spy.called);
        done();
    });
});

What I want is for the assertion to wait until the function has been called. And I know it's called eventually because the console outputs 0 but it's only after the assertion fails.

Answer 1

The typical approach would be moving the done callback into the event handler. In this way, the test will wait until the callback is called. If the ready event is not fired, the callback won't be called and the test will timeout with an error after 2 seconds.

This means that you don't need to explicitly assert that the event f is called, and in fact, you don't event need to spy on it.

    it("can communicate with the Arduino device", function(done) {
        const f = (num: number) => {
            console.log(num);
            done();
        };
        setArduinoBoard(f, 0);
    });

If you need to assert something after the ready event has fired, you can add an assertion in the event handler and use a catch block to capture exceptions and pass them to the done callback.

    it("can communicate with the Arduino device", function(done) {
        const f = (num: number) => {
            try {
                assert.equal(num, 0);
                done();
                return;
            } catch (err) {
                done(err);
            }
        };
        setArduinoBoard(f, 0);
    });