Worker using synchronous XMLHttpRequest to get data from GUI

I'd like a Web Worker which is deep in a call stack to be able to make a synchronous request to get information from the GUI.

The GUI itself is not blocked--it's able to process messages. But the JavaScript on the worker's stack is not written in async / await style. It is just a lot of synchronous code. So if the GUI tried to send a response back to the worker with a postMessage, that would just be stuck in the onmessage() queue.

I've found at least one hack that works in today's browsers. The worker can postMessage to the GUI for the information it wants--along with some sort of ID (e.g. a UUID). Then it can make a synchronous XMLHttpRequest--which is not deprecated on workers--to some server out on the web with that ID.

While the worker is waiting on that http request, the GUI processes the information request. When it's done, it does an XMLHttpRequest to POST to that same server with the ID and the data. The server then uses that information to fulfill the blocking request it is holding open for the worker. This fulfills the synchronous request.

It may seem hare-brained to outsource synchronization between the GUI and the worker to a server. But I'll do it if I have to, because it does not fit the use case to force the worker code to be written in asynchronous style. Also, I'm assuming that someday the browser will be able to do this kind of synchronization natively. But it looks like the one mechanism which could have been used--SharedArrayBuffer, has been disabled for the time being.

(More bizarre options like compiling a JavaScript interpreter into the worker so the JS stack could be suspended and restarted at will are not on the table--not just due to size and performance, but the inability to debug the worker using the browser's developer tools.)

So...

• Is there any way for a synchronous XMLHttpRequest to be fooled into making a request of something coming from within the browser itself (maybe via a custom link scheme?) If the GUI thread could directly answer an XMLHttpRequest that would cut out the middleman.

• Could the same functionality be provided via some kind of plugin? I'm thinking maybe synchronization could be done as an abstraction. If someone doesn't have the plugin, it falls back to using the network as a synchronization surrogate. (And presumably if they ever re-enable SharedArrayBuffer, it could just use that.)

I'm wondering also if there is some kind stock JS-ready service which already implements the protocol for the echo server...if anyone knows of one. Seems quite easy to write.

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