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

# Ben Wiley (10 days ago)

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.


// 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: benwiley4000/wildcard-export-override-example


# Jordan Harband (9 days ago)

Wouldn't the solution be, don't use import * as, but instead, explicitly import and re-export what you want?

# Guy Bedford (9 days ago)

Did you mean to have both examples use ‘export const a = 1’?

This ambiguous export case is supposed to be an explicit error from the spec. If the export is being stripped and not throwing an error sounds like a possible browser bug.

# Ben Wiley (9 days ago)

@Jordan: yes that also works, but for a less trivial example that is annoying to maintain. I prefer to narrow in sources of truth where possible. But yes that would satisfy the app user requirements, just make the library dev's job more annoying.

@Guy: no unintentional sorry. The intent was to show an actual override. :)

Le ven. 14 févr. 2020 04 h 17, Guy Bedford <guybedford at> a écrit :

# Augusto Moura (9 days ago)

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:

// 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
export { ApiError as ApiBError, * } from './api-b.js'; // exporting Bar and

// 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>


# Ben Wiley (9 days ago)


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 on.

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).