What's the use case? Do you need to know exactly when the task is done? Does it vary in duration significantly? Can you split the call into two - one to start it, another to check the status given an ID?
It could be the user sending a computation to the server and wanting its interface to be updated as soon as the computation is done, it is feasible by regularly polling the backend after launching a worker process, but this adds complexity compared to simply opening a non-blocking socket a la nodejs & waiting for the worker to finish its job & sending the result back to the browser