-
Notifications
You must be signed in to change notification settings - Fork 29.2k
New issue
Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.
By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.
Already on GitHub? Sign in to your account
esm: 'module.exports' export on ESM CJS wrapper #53848
base: main
Are you sure you want to change the base?
Conversation
Review requested:
|
There was a problem hiding this comment.
Choose a reason for hiding this comment
The reason will be displayed to describe this comment to others. Learn more.
lgtm
I may not follow with the motivation. If this feature is going to be used in "transpile a CJS module (which was transpiled from ESM) back into ESM and run it on the web", can this be added to transpilers instead? |
It is also helpful to support CJS transpiled into ESM (ie modern bundled code) running in Node.js against an external that is unknown as to whether it is CJS or ESM, where being able to know if the external is a CJS wrapper can allow lowering the required require(esm) interop pattern. |
The reason why we introduced the __esModule was:
I think to implement a counterpart, we at least need 1 - there should at least be some integration in CJS -> ESM transpilers adopting this pattern to run things in browsers, which is what this primarily is useful for - it's very rare to see CJS code transpiled into ESM to be run in Node.js, as far as I know. Suppose transpilers disagree and some adopt a different flavor, or if this trick doesn't turn out to be actually working because no integration test has been written yet (this is all on paper for now), we'd be left with something that doesn't work and nobody uses. #52166 included an integration test using output from TypeScript, I think we need something similar in this PR too. |
@joyeecheung I think the requirement that this new pattern already be in use before we have created the pattern is an unrealistic requirement to make. I have given two clear use cases for this as a feature, with the direct use case in #53848 (comment) immediately being solved by landing this PR.
RollupJS is both a Node.js and browser bundler, so this is not quite true. When using RollupJS in Node.js applications to, say, speed up startup times, this is very much a real use case.
There's no trick or flavour here, simply the question - is the ES module namespace that I have imported an ESM wrapper around a CJS module? And I think that is a useful question to answer by supporting this flag to answer it. What more would you like to see tested with regards to this? |
Specifically the problem is needing to know when in Node.js if an arbitrary ESM namespace is an ESM CJS wrapper or not. With that answer, yes, new CJS to ESM interop patterns can emerge as I have discussed, since they very much need this information. But the check is useful in its own right, and does not depend on their future emergence either. |
Isn't this a pattern primarily meant for code run in the browser? Then it should be possible to create a bundler integration that demonstrate how this can be used in the browser first?
RollupJS supports CommonJS output too. As far as I know typically people bundle code into CommonJS format to speed up startup times (ESM loading is slower than CommonJS loading in the first place), or to use the bundle with SEA or snapshots since they do not support ESM. But choosing the ESM output for a Node.js environment is rare. If it's common enough it seems strange that this need for
I was referring to this use case:
In #52166 we added a test fixture with the output of the actual TypeScript compiler. Before the PR, loading default exports of real ESM in transpiled ESM -> CJS code leads to an error. After it works intuitively. I think for this PR we'd need CJS -> ESM code emitted by an existing transpiler that doesn't load an external CJS without |
I agree that it's useful in its own right, though currently for code only targeting Node.js this can also be worked around by checking whether the path exists in |
For the specific case in OP, it seems the problem all stems from the proposed interop layer, which AFAIK no transpiler has implemented yet. The layer seems redundant for ESM -> CJS code, however. If you don't do that layer, and simply transpile this: const depMod = require('dep');
const depNs = depMod.__esModule ? depMod : { default: depMod };
console.log(depNs.default); to just this (IIUC, this is already what Rollup currently produce with import depMod from 'dep';
const depNs = depMod.__esModule ? depMod : { default: depMod };
console.log(depNs.default); Things already just work automatically. If |
The entire ecosystem does not need to move to support Your scheme in #53848 (comment) does not work for a real ES module (not faux ESM) without a default export, since it will bail that |
If what you need is the ability fully comparable with |
The I think we're focusing a little too much on some hypothetical major adoption of this feature (I'm not really not sure what you mean by browser adoption), and a little too less on the problems it solves, and what reasons we might not want to land it. @joyeecheung if you are against this flag, perhaps you can more clearly state your underlying concerns? |
I’ve been building SvelteKit apps for a few years now, and SvelteKit uses Vite with Rollup to generate ESM production builds. Startup time isn’t a concern for long-lived servers, and these builds aren’t bundling the external dependencies; I still need my So no, I wouldn’t say that building for ESM output in a Node.js environment is rare. |
I've already stated this in #53848 (comment)
In general, adding more underscore underscore property should be frowned upon. |
So it isn't transpiling CJS to ESM either, which is what this PR is about? |
This PR is not claiming to set a convention that will work for all bundlers, rather it is providing a solution to the problem that when building CJS requires as externals into ESM imports, we have an interop problem in distinguishing CJS ESM wrappers from real ESM and that currently has no viable solution. And I would quite like to be able to solve this problem for our Node.js users.
This argument is circular - you are asking for adoption, but then also claiming you are concerned about adoption. The concern and request cannot be for the same thing? Would you be less concerned if we renamed the flag to There is also another side to this PR and that is that using this flag, it is possible to define the representation of an ESM module in require(esm). I would have thought you'd be interested in that given it can be used as another tool to solve require(esm) interop issues. |
Not saying that it isn't a problem, but it seems very well like an issue we can follow up later. This path needs code originally in CJS being transpiled to ESM and loading an external ESM to be hit. That doesn't seem to be an urgent issue - for one not that many people actually configure to transpile CJS to ESM for it to be run in Node.js, if they need to ship ESM they likely would just write in ESM in the first place and never hit this path (probably what @GeoffreyBooth does too), let alone attempting to loading real external ESM from said CJS code (considering how hacky CJS -> ESM can be in terms of handling dynamic exports and imports and all the wonky require extensions, this practice already has bigger fishes to fry anyway). This is unlike the problem that
I don't think I claimed to be concerned about adoption, rather I am concerned about the lack of adoption of a premature solution that's currently purely on paper, and we have no integration tests to verify that this solution actually works as intended.
I'd be more interested to see working prototypes, the current description of the solution is too vague and abstract, and we don't know for sure whether it actually works as intended. |
Actually I realized that this is a breaking change, because unlike import * as empty from './empty.cjs';
assert.deepStrictEqual({...empty}, { default: {} }); // it will throw after this change Unlike import { isCJSModule } from 'node:module'; // Or use process.getBuiltinModule();
import * as empty from './empty.cjs';
isCJSModule(empty); // true
|
The other aspect of this PR, is the ability to provide a As mentioned in the OP, this is a breaking change to go out alongside the unflagging of require(esm), as it deals with interop cases introduced by require(esm). If we do not ship alongside unflagging of require(esm), we create a compat gap where ESM bundles consisting of both CJS and ESM code cannot properly reflect and be reflected in require(esm) workflows, which would be good to avoid. |
I will flesh out some further tests this weekend to explicitly demonstrate the specific scenarios being designed for. Let's resume discussion then. |
We already don't have |
We have many requests to backport require(esm) to v18 and v20, I feel pretty strongly that we should not couple the release of this with require(esm), as we don't see any requests for this from end users so far. |
doc/api/modules.md
Outdated
@@ -233,6 +232,23 @@ This property is experimental and can change in the future. It should only be us | |||
by tools converting ES modules into CommonJS modules, following existing ecosystem | |||
conventions. Code authored directly in CommonJS should avoid depending on it. | |||
|
|||
To create an ESM module that should provide an arbitrary value to CommonJS, the | |||
`__cjsModule: true` marker can be used instead: |
There was a problem hiding this comment.
Choose a reason for hiding this comment
The reason will be displayed to describe this comment to others. Learn more.
I think we should consider discussing this in a separate issue. It's not the first time that we run into the need to save library users a loaded.default
pickup when they use a native loader. The same has been happening for import d from 'esm_transpiled_to_cjs'
as well and seems to have already been causing great pain for years - babel/babel#12363 contains an great description of the problem. I think we should invent a marker that helps both transpiled ESM and real ESM indicate the intent to unwrap a single default export when the module is being loaded by import cjs
or require(esm)
(and ideally, we should error when we found named exports along with this marker, because unwrapping breaks them). Something like exports.unwrapSingleDefault = true;
in transpiled ESM and export const unwrapSingleDefault = true
in real ESM may suffice - but we also need to get the buy-in from transpilers to implement it first.
There was a problem hiding this comment.
Choose a reason for hiding this comment
The reason will be displayed to describe this comment to others. Learn more.
Another alternative is to unwrap based on the presence of __esModule
, but my hunch is the ship has already sailed and it is now too late to use this for unwrapping, as users of these transpiled packages may have already been relying on doing the unwrapping themselves.
There was a problem hiding this comment.
Choose a reason for hiding this comment
The reason will be displayed to describe this comment to others. Learn more.
I've motivated this in a separate issue in #54085 now.
There was a problem hiding this comment.
Choose a reason for hiding this comment
The reason will be displayed to describe this comment to others. Learn more.
I still think we need a more informative name (I don't see any need for underscored names, this would be declared explicitly by library authors, if export default
is a thing, export const unwrapDefault
doesn't seem too off either), and this should also be applied to the setExports('default')
call for import cjs
as explained in #54085 (comment) - IIUC that addresses your "what if depMod is CJS -> ESM transpiled" use case, if depMod
defines this marker, the transpiled consumer no longer needs to unwrap the default again:
import * as depMod from 'deps';
// depMod is { default: transpiledNS.default, ...namedExportsFromCjsModuleLexer, __esModule: true, unwrapDefault: true }
const depNs = depMod.__esModule ? depMod : { default: depMod }; // Takes first branch
console.log(depNs.default); // It's the original CJS module.exports, which was exported as `export default` in the transpiled ESM i.e. transpiledNs.default.
Compared to adding the marker to the wrapper, this has the advantage of not having to break existing import cjs
users which may have been relying on a pristine namespace in the past 5 years and can be backported to v18-22.
Also, in the document we should warn users that this would make the named exports invisible to require(esm)
, so if they have any named exports, they should make sure to make it a property of the object that is being exported as default.
export default class Point {};
export function distance() { }
export const unwrapDefault = true;
// There is no way to get function distance from the module via `require()`
// unless the module defines `Point.distance = distance`.
// This is different from the behavior that ESM-transpiled-to-CJS libraries currently
// provide to their CJS consumers should they wish to ship real ESM.
const { distance } = require('point');
distance(); // throws
@joyeecheung as discussed offline I've pushed up an explicit test in 33165b5 that fully demonstrates the interop modes in question here. I've also separately motivated the require(esm) representation component in #54085, although this PR should still land both due to the transitive interop requirements. |
8566dea
to
4688513
Compare
doc/api/modules.md
Outdated
@@ -233,6 +232,23 @@ This property is experimental and can change in the future. It should only be us | |||
by tools converting ES modules into CommonJS modules, following existing ecosystem | |||
conventions. Code authored directly in CommonJS should avoid depending on it. | |||
|
|||
To create an ESM module that should provide an arbitrary value to CommonJS, the | |||
`__cjsModule: true` marker can be used instead: |
There was a problem hiding this comment.
Choose a reason for hiding this comment
The reason will be displayed to describe this comment to others. Learn more.
I still think we need a more informative name (I don't see any need for underscored names, this would be declared explicitly by library authors, if export default
is a thing, export const unwrapDefault
doesn't seem too off either), and this should also be applied to the setExports('default')
call for import cjs
as explained in #54085 (comment) - IIUC that addresses your "what if depMod is CJS -> ESM transpiled" use case, if depMod
defines this marker, the transpiled consumer no longer needs to unwrap the default again:
import * as depMod from 'deps';
// depMod is { default: transpiledNS.default, ...namedExportsFromCjsModuleLexer, __esModule: true, unwrapDefault: true }
const depNs = depMod.__esModule ? depMod : { default: depMod }; // Takes first branch
console.log(depNs.default); // It's the original CJS module.exports, which was exported as `export default` in the transpiled ESM i.e. transpiledNs.default.
Compared to adding the marker to the wrapper, this has the advantage of not having to break existing import cjs
users which may have been relying on a pristine namespace in the past 5 years and can be backported to v18-22.
Also, in the document we should warn users that this would make the named exports invisible to require(esm)
, so if they have any named exports, they should make sure to make it a property of the object that is being exported as default.
export default class Point {};
export function distance() { }
export const unwrapDefault = true;
// There is no way to get function distance from the module via `require()`
// unless the module defines `Point.distance = distance`.
// This is different from the behavior that ESM-transpiled-to-CJS libraries currently
// provide to their CJS consumers should they wish to ship real ESM.
const { distance } = require('point');
distance(); // throws
4688513
to
9fcf217
Compare
@joyeecheung I've expanded the test cases to include various shapes include the string, object and also the faux esm cases of mixed, named and default exports, and separated the dependency into node_modules being consumed as CJS and ESM under this convention. For the name, I'm open to changing to a name that is not prefixed with I'd also like to revisit this being a major change. The benefit of supporting this at the same time as putting it on CJS ESM wrapper namespaces is that those namespaces are then properly supported under CJS require, so that closes the loop on the lifting and lowering sides of the pattern. Whereas if we only land one side individually we potentially break transitive usage. The argument here is that many objects support obtaining new fields as a non-breaking change, so surely this could be okay. If we don't treat this as a minor change, we would need to split it up into two PRs further - one for the CJS unwrapping in require(esm) that is non breaking, and another breaking change for adding this to the import wrapper. I'd really prefer not to have to do that if possible though. |
Codecov ReportAll modified and coverable lines are covered by tests ✅
Additional details and impacted files@@ Coverage Diff @@
## main #53848 +/- ##
==========================================
+ Coverage 88.39% 88.41% +0.01%
==========================================
Files 652 652
Lines 186565 186569 +4
Branches 36046 36032 -14
==========================================
+ Hits 164916 164952 +36
+ Misses 14908 14905 -3
+ Partials 6741 6712 -29
|
9fcf217
to
9c584e1
Compare
I've updated this PR to only add the //cc @nodejs/loaders |
There was a problem hiding this comment.
Choose a reason for hiding this comment
The reason will be displayed to describe this comment to others. Learn more.
nit: the commit message does not meet the guidelines (the first word after the subsystem should be an imperative verb). This can be addressed when landing.
9c584e1
to
6a2b577
Compare
This provides a
'module.exports'
export on the wrapper namespaces created for CommonJS modules when they are imported into ES modules. This mirrors the PR at #54563, which was split out from the original version of this PR which makesrequire(esm)
take the value of this export in its interop pattern when that is used.By adding this to the module namespaces for CJS in ESM, this effectively allows tools which transpile CommonJS into ESM to be able to consistently follow the same
require(esm)
interop for CJS externals, despite the importer being transpiled from CJS into ESM.This is marked as a major change, since it changes the shape of the CJS wrapper.
//cc @nodejs/loaders
For details on the exact transpilation problem this solves, it is the following:
Consider a CommonJS module importing a dependency in Node.js that is then transpiled into ESM, where that dependency might be any unknown module format:
Now if I run this module in Node.js, and
dep
is an ESM module,dep
will go through therequire(esm)
interop checks:module.exports
export per module: support 'module.exports' interop export name in require(esm) #54563, this will be used__esModule
marker already and has a default export, it will be wrapped with an__esModule
marker automatically added by Node.jsSo if I want to transpile this module into ESM, with that same dynamic dependency linkage, I end up with something like this:
So we now have a scheme to transpile arbitrary CJS to ESM in Node.js, while retaining the exact same interop semantics.
But there is now a new interop problem with such a scheme - in the case where depMod is itself a CJS module, instead of an ESM module, say:
When importing this module from ESM, Node.js will interpret this as a module namespace of the shape
depNs
havingModule { default: Dep }
, which will then through the inlined interop code give the value ofdepMod
as{ __esModule: true, default: Dep }
instead ofclass Dep
, the expectedmodule.exports
value!The fix, is to add this
module.exports
marker in the CJS ESM representation, then'module.exports' in depMod
is true and we we hit this first check in the inlined require CJS interop pattern of the transpiled code, and will get that back directly as we desired.Therefore fully solving the dynamic externalization against all of the cases where dep might respectively be CJS transpiled to ESM and real ESM with the conditional faux ESM interop layer for the consuming faux ESM as CJS transpiled into ESM per #52166.