Conditional catch clause

Brendan Eich brendan at
Thu Dec 20 12:38:03 PST 2012

John J Barton wrote:
>         Depending on the design, this could be anywhere from "only
>         errors raise exceptions" to "developers must supply a
>         algorithm to decide". Languages with a centrally controlled
>         type system (I have in mind Java) provide a relatively simple
>         way to separate these exceptions.
>     And how is this relevant? I'm not snarking back. JS != Java.
> Java is one example of a language that supports non-error uses of the 
> try/catch exception mechanism. Part of that support includes a way for 
> debuggers to distinguish errors uses from non-error uses. Encouraging 
> additional non-error use

Again, I object. StopIteration is not "encouraging additional non-error 
[uses of try/catch]" outside of specialized, written-by-experts 
libraries such as

The exceptions-are-not-all-errors cat is out of the bag. You don't seem 
to agree but you haven't rebutted directly. I cry foul.

> of the JavaScript try/catch mechanism without a similar means to 
> separate them from error uses

Again, without a type system, how?

Testing catch (e if e instanceof Error) was an explicit intended use of 
catch guards from the ES3 days. With standard catch you would write an 
if-else chain and remember to re-throw in final else. This is quite 
doable now, so what's the problem? Note that DOMException instanceof 
Error per

Yes, instanceof does not work cross-frame. This has been debated on 
es-discuss a lot. For same-realm uses, it suffices, as people here have 
pointed out recently. Exception catching may favor same-realm, I'm not 
sure. But this is a separate issue.

> will make features like onExceptionUnwind tedious for developers to use.

This is just the lowest tier of the API and we're not done yet.

To be frank, I think your frustration is perfectly understandable, but 
not grounds for general exhortations or judgments against particulars 
(StopIteration) that are not relevant (for-of handles outside of 
specialized taskjs settings), or at the very least not decisive in light 
of precedent (instanceof Error testing). Anyway, I hope to have shed 
some light here. How'd I do?


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