Double wildcard "re-exports"... off-limits forever?

Ben Wiley therealbenwiley at
Fri Feb 14 19:09:33 UTC 2020


I think the rest import/export could be an interesting idea although it
doesn't quite solve my case since I would like to keep the original names
and treat one API as an override of the other (in your case it seems you're
trying to combine all exports from multiple libraries by changing names in
some cases). The rest export would also still require explicitly naming the
duplicate re-exports, which means redundant export declarations.

I did consider something very similar (like rest imports) while I was
trying to arrive at a solution though. It's a cool idea for sure!

Something that could avoid the dangerous situation that necessitated the
duplicate re-export rule in the first place, would be to have some kind of
"joint import/export" syntax designed for this use case, where multiple
import sources are accepted.


// api-base.js
export { Foo, ApiError };

// api-derived.js
export { Bar, ApiError };

// index.js
export * from './api-base.js', './api-derived.js';
// or...
import * as api from './api-base.js', './api-derived.js';

This way you're explicitly stating your intent for exports from the first
source to be override-able by those from the second source. Semantically,
at a high level you can think of this as having behavior similar to
Object.assign or an object rest spread, where first a set of exports is
formed from the first source, then the second set of exports is grafted on
top, possibly overriding some values, then a potential third source, and so

Various implementations could be considered, including reading the exports
from right-to-left to avoid registering the same export name twice
(although I'd guess the http requests wouldn't be fired until all export
paths are determined anyway, so might not make a meaningful difference).


On Fri, Feb 14, 2020 at 12:47 PM Augusto Moura <augusto.borgesm at>

> If I understand it correctly, I had a similar problem with generated apis
> from OpenApi, two apis have a error definition with the name ApiError, i
> want to reexport all classes (a lot of model definitions) from both apis.
> The problem is that using `export * from 'api-a'; export * from 'api-b';`
> raises a error that ApiError is a duplicated name. So I have 2 options, or
> I reexport all definitions from the apis explicitly (hundreds of `export {
> Foo } from 'api-b'`) just to rename the ApiError to ApiAError at then end
> or I don't rexport then together at all (splitting the reexports in 2 files
> and having the dev to import the necessary models from the different files).
> If we could have a rest-operator like construct for imports the problem
> would be solved, something like:
> ```js
> // api-a.js
> export { Foo, ApiError };
> // api-b.js
> export { Bar, ApiError };
> // apis.js
> export { ApiError as ApiAError, * } from './api-a.js'; // exporting Foo
> and ApiAError
> export { ApiError as ApiBError, * } from './api-b.js'; // exporting Bar
> and ApiBError
> // other ideas for syntax
> export { ApiError as  ApiAError }, * from './api-a.js'; // similiar to
> default and named imports
> export { ApiError as  ApiAError, ... } from './api-a.js'; // similar to
> spread syntax
> export { ApiError as  ApiAError, ...* } from './api-a.js'; // mix from
> spread syntax and wild card imports
> // this last is one is the one I like the most, because both wildcards and
> spread are already familiar in the language, and it reads like "import the
> rest and rexport as it is"
> ```
> Em sex., 14 de fev. de 2020 às 01:02, Ben Wiley <therealbenwiley at>
> escreveu:
>> Apologies if this has already been talked about at length at some point.
>> I was unable to find much in the way of relevant discussions.
>> I found a compelling use case for something which seems to be off-limits
>> in the JavaScript language, that is wildcard re-exporting where the same
>> export name appears in multiple of the export-forwarded imports.
>> e.g.
>> ```
>> // a.js
>> export const a = 1;
>> // b.js
>> export const b = 2;
>> // c.js
>> export * from './a.js';
>> export * from './b.js';
>> ```
>> The ideal use case would be shipping an "override library" that ships all
>> the default exports of an upstream library, except it replaces some of them
>> with its own overrides. The object-oriented folks might think of it like a
>> derived class. This can of course be accomplished alternatively by
>> exporting an object which merges all the named exports from each library,
>> but the major disadvantage I see is that we would no longer have access to
>> tree-shaking, since that object contains *all* of the exports. For a really
>> big upstream library, that could make a large difference in kilobytes
>> shipped to the browser. So preserving the named exports is desirable.
>> The protections against double-re-exporting vary. In Chrome and Firefox,
>> there are no runtime errors but the duplicated exports will be stripped and
>> unavailable. If you try Babel or Typescript, the compiler will throw an
>> error.
>> I understand *not* protecting against this could lead to very weird
>> debugging situations for unwitting users who didn't realize their wanted
>> import was being overwritten, however I'd love if there were a way to say
>> "I know what I'm doing, don't stop me." As far as I can immediately tell
>> nothing about ES imports would prevent the compiler from being able to know
>> the order of precedence for overridden exports, and the "ambiguity" would
>> be mainly from the perspective of an unwitting user. I recognize that
>> import trees may be processed in parallel, however since code execution is
>> delayed until the import tree is complete I would think we could resolve
>> any ambiguities by that time. However it's possible I missed something -
>> maybe there's a case related to circular imports which ruins this?
>> Anyway, I wrote up some more detailed thoughts on this problem, and some
>> demo code, here:
>> Ben
>> _______________________________________________
>> es-discuss mailing list
>> es-discuss at
> --
> Atenciosamente,
> Augusto Borges de Moura
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