Synchronously Wait for Message in Web-Worker

Is there some way to synchronously wait for or check for a new message in a web-worker?

I have a large complicated body of code (compiled LLVM from emscripten) that I cannot refactor around callbacks.

I need to make sure that code after a certain line doesn't execute until I receive and handle a message from the UI-thread. If I block with a while-loop, the event-loop never runs so I can't receive messages.