Why is the worker's onmessage executing after a macro task?

[Codes MagicCodes Magic is verified member.]

In JavaScript, the `onmessage` event handler in a Web Worker is executed as a task on the worker's event loop. However, the event loop is not the same as the main thread's event loop.

When a message is posted to the worker using `postMessage()`, the worker's event loop is notified, and the `onmessage` event handler is added to the worker's task queue.

Here's what happens next:

1. *Microtask queue*: The worker's task queue has a microtask queue that runs before the macrotask queue. However, the `onmessage` event handler is not a microtask.
2. *Macrotask queue*: The `onmessage` event handler is executed as a macrotask, which means it runs after any pending macrotasks in the worker's task queue.

Now, if there's a pending macrotask in the worker's task queue, it will run before the `onmessage` event handler. This is why the `onmessage` event handler might seem to execute after a macrotask.

To illustrate this, consider the following example:

// In the worker
self.onmessage = (event) => {
  console.log('onmessage');
};

// Post a message to the worker
self.postMessage('Hello');

// Add a macrotask to the worker's task queue
setTimeout(() => {
  console.log('Macrotask');
}, 0);

In this example, the output will be:
```
Macrotask
onmessage
```

The `onmessage` event handler is executed after the macrotask because the macrotask was added to the worker's task queue before the `onmessage` event handler.

Keep in mind that the order of task execution can vary depending on the specific use case and the worker's task queue.