I ask because I wrote some Lisp macros (I work in a Lisp that compiles to JS) to implement a few async patterns I need, and making sure that exceptions are trapped and threaded into the callback chain correctly was the most complicated part.
From a practical perspective, it doesn't make sense to try to catch exceptions in asynchronous code, anyway. Once you do something asynchronous, you lose the stack and thus the try block. The way to catch thrown exceptions in asynchronous code is with domains, which something as low level as async would not be expected to handle.
Sure, but what if the error is thrown at you as an exception in the first place—which happens a fair amount, because that's how the JS runtime tells you when something is wrong? How do you get from there to the callback way?
What the Lisp macro I mentioned does is generate a separate try-catch around each block of code that runs at a different time and thus might throw an exception that would not otherwise get caught. In that way it catches every exception that's thrown, converts it to an error object, and passes the error back through the callback chain. The async library could do the same, albeit with a lot more code. I'm curious why it doesn't.
From a practical perspective, it doesn't make sense to try to catch exceptions in asynchronous code, anyway.
I don't think that's right. Asynchronous code is just synchronous code that runs at different times. Each block of synchronous code can generate exceptions. I agree that if you don't catch them then, they become useless; but you can catch them then. The reason this is not a "practical perspective" in JS is not that it doesn't make sense, it's that the language doesn't support it. Even the minimum code necessary to catch every exception involves so many try-catch blocks as to obscure the rest of the program. So no one writes such code by hand in JS.
Yet it is, I think, code that one wants, because without it you don't have a consistent error model. You end up having one model for first-class errors—the ones you detect and pass to callbacks before an exception has a chance to arise—and a second one for the dregs—the ones that come from any code that didn't know about or follow the callback convention (which, critically, includes the language runtime). The latter kind of error either crashes the server or gets caught by a top-level handler so it "only" crashes the request it was processing. That's a half-baked system.
Its on you to catch that, not your libraries. This shouldn't be terribly common, though. The only thing I can remember having to wrap in a try/catch in the codebase I work on is JSON.parse.
> The async library could do the same, albeit with a lot more code. I'm curious why it doesn't.
It couldn't, without domains. try-catch wouldn't do it. Domains are something that is not very well understood, in my experience, and expected to happen at a higher level than libraries like async.
As for domains, I don't know what you mean by them, but if they're catching errors at a higher level than the async library, my guess is that they must be some more sophisticated sort of top-level handler; perhaps something that keeps track of which async calls are in progress and attempts to bind exceptions back to their context? Whatever it is, it sounds complicated.
But what I understand least of all is how you guys all seem to write Javascript code that generates almost no exceptions. To me that sounds almost like bug-free code. No null references, for example? I get stuff like that all the time.
We write in CoffeeScript, where a null reference check is so astonishingly easy to write that you use them everywhere you might get a null. I'm not sure what other exceptions you're seeing. We do basically no math, so /0 errors aren't a problem.
> For example, I don't know why you say that the async library couldn't try-catch every place that an exception might occur
Let's build a typical function you might pass to async:
function(next) { request.get(url, function(err, data) { JSON.parse(data); } }
Let's assume the server doesn't serve JSON like we expect - so JSON.parse throws an exception. The only thing async could have wrapped in a try/catch is the main function, but we've fired off a request and then the call stack wrapped up, including the try/catch. Next, an event occurs that calls our callbacks, not going through async at all. That's where the exception occurs. The stack trace generated by that exception doesn't contain any code in the async lib, so it can't possibly have a try/catch active.
Domains are a way of fixing this. You create a Domain and bind callbacks to it - if that callback throws an exception, the Domain instead emits an error event.
Re null reference checks, to get behavior analogous to a null exception you have not only to check for null, but also pass back an explicit error if you find it. That's a lot more work than adding in an extra question mark. Null checks that do nothing but not crash are a mixed blessing; 90+% of the time they do what you want, but when they don't, you get a silent failure and a debugging goose chase. I'd be surprised if you told me that that never happens.
I took a look at Node.js domains and they do seem really complicated. If I were working in Javascript instead of having control over the language, I doubt I would use them; I would probably just crash-and-restart as one of the other commenters described. That's not a good solution, but probably the best tradeoff given the alternatives.
That said, we get very thorough testing from our large user base, and we quickly fix crashers. Our server proc crash rate is almost 0, brought up by occasional spikes on releases.