This is a proof-of-concept for in-browser communication using service workers, enabling synchronous
cancellation tokens and RPC calls in web workers. The service worker intercepts HTTP requests to paths like
/@cancellation@/{id}, effectively turning it into an HTTP server accessible to the current
site. Since workers can make HTTP requests via XMLHttpRequest, workers can communicate with the
main page synchronously, whereas the actual HTTP handler in the service worker is asynchronous and able to
call asynchronous APIs. See above for a diagram of the cancellation implementation, or the code
here.
Below is a console log of the page and a worker communicating over JSON-RPC, with the page making requests
to two addNumbers handlers in the worker (one that's fast, one that's slow but able to be
canceled). As an added bonus, the page makes a call to a hello handler, which the worker
implements as an RPC call back to the page itself (again, synchronously!).