Use of CommonJS promises: rejection vs. exceptions

Question

I have a function, downloadAsync(), that returns a CommonJS promise (using Q). It can fail in two ways:

1. The file can already be downloaded, in which case we know immediately.
2. The download process could fail, in which case we know some time later.

In case (1), since I know before anything asynchronous happens, I could throw an exception. In case (2), I'd have to reject the promise.

My question is, should my API be uniform and always signal error by rejecting the promise? Or should I throw exceptions for immediately-determinable invalid state conditions? As another example, if the user passed me an invalid argument, it would seem much more sensible to throw an error than to reject the promise.

Some clarity on how "exceptional" promise rejection really is would be great, as well; does usage there map one-to-one with exception usage practices, or do we use it for non-exceptional failure as well?

Answer 1

If you want to know what you should do when your function (which implements Q) should do when presented invalid input that could be detected synchronously, I looked at the source code for Q, to see what Q did.

Here's an example.

if (fallback === undefined) {
    fallback = function (op) {
        return reject("Promise does not support operation: " + op);
    };
}

When Q is presented with invalid input, Q invokes the resolver with a rejection object. Therefore you could be justified in having your API behave similarly, rather than trying to modify the behavior of the underlying library.

Plus, look at it from a standpoint of anyone who consumes your API. Do they want to develop and maintain two exception handling code paths, or one?

Answer 2

I don't think you need to worry about the synchronous case. The way promises work, callbacks should execute synchronously when thay are added to an already resolved(or rejected) promise. (Well, at least they do in Dojo where I use them...)

The only reason to make an reject/exception distinction would be if having a promise thrown was actually important to your application, since I don't think you can throw exception from inside a promise callback. (Again, this is Dojo promises, not 100% sure how yours work...)