Worker Messaging#

Our worker has a dispatcher, but it isn’t yet receiving or sending messages with the main module. Let’s set up the postMessage machinery used in workers.

Receive messages#

As explained in the MDN page for web workers, you handle incoming messages by defining an onmessage function. This can be at module scope or on the self variable, which is what we’ll do.

We’ll start of course with a test. We’ll start small:

test("handles incoming messages with onmessage", async () => {

This fails of course, so add the following to worker.js:

self.onmessage = async (e) => {
    // Unpack the message structure early to get early failure.
    const {messageType, messageValue} =;
    const responseMessage = await dispatcher({messageType, messageValue});
    if (responseMessage) {

This code acts as a mediator between worker messaging and the dispatcher:

  • Unpack the “protocol” of messageType and messageValue

  • Call the dispatcher

  • If the dispatched function returns something, post it back to the main module

This last point is a convenience. Our dispatched functions can always call self.postMessage themselves. But that’s binding the unique mechanics of web workers into our logic.

We now have something to confront. We need to test calling self.onmessage and we want to see if self.postMessage was called. Even better, called with what we expect.

Mocking to the rescue.

Mocking self.postMessage#

While self exists in happy-dom, self.postMessage doesn’t. We need to do two things in tests/setup.js:

  • Create a pretend aka “mock” implementation for postMessage

  • Register it on global

Add the following to tests/setup.js:

if (!globalThis["worker"]) {
    self.postMessage = vi.fn();

This allows our code to pretend to call self.postMessage. Of course, nothing happens, but we don’t care. Our worker.test.js code is in isolation, only interested in the worker, not the communication with main.

Testing self.onmessage#

We now have the pieces in place. We can test the self.onmessage function which receives messages from main:

test("handles incoming messages with onmessage", async () => {

    // Make a fake worker message
    const event = new MessageEvent("message"); = {messageType: "initialize", messageValue: null};

    // Spy on self.postMessage, then call our handler
    const spy = vi.spyOn(self, "postMessage");
    await self.onmessage(event);

    expect(spy).toHaveBeenCalledWith({messageType: "initialized"});
  • The first two expect just make sure our self has what we expect

  • We then simulate the event object from the main<->worker communication

  • Create a “spy” to watch the calling of self.postMessage

  • Call self.onmessage and see if the resulting message back, matched what we thought

Layout of our worker#

So what does this worker.js implementation look like? It’s a 3-layer cake:

  • The event listener which mediates with the main thread via messages

  • The dispatcher, which finds the right message handler, calls it, and returns the message

  • Handler functions which don’t really know the outside world beyond self