How to call an asynchronous JavaScript function and block the original caller

Question

I have an interesting situation that my usually clever mind hasn't been able to come up with a solution for :) Here's the situation...

I have a class that has a get() method... this method is called to get stored user preferences... what it does is calls on some underlying provider to actually get the data... as written now, it's calling on a provider that talks cookies... so, get() calls providerGet() let's say, providerGet() returns a value and get() passes it along to the caller. The caller expects a response before it continues its work obviously.

Here's the tricky part... I now am trying to implement a provider that is asychronous in nature (using local storage in this case)... so, providerGet() would return right away, having dispatched a call to local storage that will, some time later, call a callback function that was passed to it... but, since providerGet() already returned, and so did get() now by extension to the original called, it obviously hasn't returned the actual retrieved data.

So, the question is simply is there a way to essentially "block" the return from providerGet() until the asychronous call returns? Note that for my purposes I'm not concerned with the performance implications this might have, I'm just trying to figure out how to make it work.

I don't think there's a way, certainly I know I haven't been able to come up with it... so I wanted to toss it out and see what other people can come up with :)

edit: I'm just learning now that the core of the problem, the fact that the web sql API is asychronous, may have a solution... turns out there's a synchronous version of the API as well, something I didn't realize... I'm reading through docs now to see how to use it, but that would solve the problem nicely since the only reason providerGet() was written asychronously at all was to allow for that provider... the code that get() is a part of is my own abstraction layer above various storage providers (cookies, web sql, localStorage, etc) so the lowest common denominator has to win, which means if one is asychronous they ALL have to be asychronous... the only one that was is web sql... so if there's a way to do that synchronously my point become moot (still an interesting question generically I think though)

edit2: Ah well, no help there it seems... seems like the synchronous version of the API isn't implemented in any browser and even if it was it's specified that it can only be used from worker threads, so this doesn't seem like it'd help anyway. Although, reading some other things it sounds like there's a way to pull of this trick using recursion... I'm throwing together some test code now, I'll post it if/when I get it working, seems like a very interesting way to get around any such situation generically.

edit3: As per my comments below, there's really no way to do exactly what I wanted. The solution I'm going with to solve my immediate problem is to simply not allow usage of web SQL for data storage. It's not the ideal solution, but as that spec is in flux and not widely implemented anyway it's not the end of the world... hopefully when its properly supported the synchronous version will be available and I can plug in a new provider for it and be good to go. Generically though, there doesn't appear to be any way to pull of this miracle... confirms what I expected was the case, but wish I was wrong this one time :)

Answer 1

spawn a webworker thread to do the async operation for you. pass it info it needs to do the task plus a unique id. the trick is to have it send the result to a webserver when it finishes.

meanwhile...the function which spawned the webworker sends an ajax request to the same webserver use the synchronous flag of the xmlhttprequest object(yes, it has a synchronous option). since it will block until the http request is complete, you can just have your webserver script poll the database for updates or whatever until the result has been sent to it.

ugly, i know. but it would block without hogging cpu :D

basically

function get(...) {
    spawnWebworker(...);
    var xhr = sendSynchronousXHR(...);
    return xhr.responseTEXT;
}