annevk Domenic: Atomics & SAB let you 'sleep' workers waiting something, but that requires an isolated origin, and it's not trivial to pack data into an SAB. Could message ports have a postMessage type thing that synchronously waits for a response from the other side? Like atomics, the sleeping side must be a worker.

API sketch:

In a worker:

console.log(
  self.postSyncCall({ action: 'sum', numbers: [2, 2] })
);

…logs 4.

In a page:

worker.addEventListener('synccall', (event) => {
  if (event.data.action === 'sum') {
    event.respondWith(async () => {
      const result = await someAsyncMathsSupercomputer.sum(event.data.numbers);
      return result;
    });
  }
});

Is this is stupid idea?