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SimenB · 2026-05-08T07:25:22Z

Summary

Test plan

Unit tests added

…ule` CJS without `.default`

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commit: 59bde21

Copilot

Pull request overview

Fixes a Jest runtime regression in CJS→ESM interop where CJS modules that set __esModule: true but do not provide a .default export (e.g. tslib) ended up with an undefined ESM default export, breaking default-import consumers under VM ESM.

Changes:

Adjust buildCjsAsEsmSyntheticModule to only unwrap the ESM default to exports.default when a .default property is present.
Add VM ESM unit coverage for __esModule CJS modules without .default, asserting both default and named exports behavior.
Add a CHANGELOG.md fix entry for jest-runtime.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
packages/jest-runtime/src/internals/syntheticBuilders.ts	Updates CJS-as-ESM default export selection logic for `__esModule` modules missing `.default`.
packages/jest-runtime/src/tests/test_esm_interop_root/import-esmodule-no-default.mjs	Adds an ESM entry point that re-exports from a `__esModule` CJS fixture with no `.default`.
packages/jest-runtime/src/tests/test_esm_interop_root/esmodule-no-default.cjs	Adds a CJS fixture that sets `__esModule` and exports only named helpers (tslib-like).
packages/jest-runtime/src/tests/runtime_esm_cjs_interop.test.ts	Adds unit tests validating correct default and named exports for tslib-style modules.
CHANGELOG.md	Documents the regression fix under `## main` → `### Fixes`.

SimenB · 2026-05-08T07:32:27Z

+  // Honors the Babel/Webpack __esModule convention: when the CJS module sets
+  // __esModule and has an explicit .default property, use that as the ESM
+  // default. We only unwrap when 'default' is actually present — packages like
+  // tslib set __esModule without a .default, so without the 'in' guard their
+  // named exports (e.g. __extends) would become unreachable via the default.
+  // Note: Node core never applies this convention; it always exposes the whole
+  // module.exports as the ESM default. Jest diverges intentionally for
+  // backward compatibility with Babel-compiled code.


I'm thinking we align with Node in Jest 31. But it's a breaking change

or actually - the code was only added in #16050. Probably better to not diverge from Node at all in this case

@jmuransky hello! Just FYI that I've removed the interop handling added in #16050 as it diverges from Node behaviour 🙂 First commit in this PR had a fix for it, but I think it's better to align with Node behaviour. I think we'll need to consider diverging from Node in a broader context rather than part of require(esm).

If we need the interop to better align with bundlers then that's probably a runtime option somehow rather than something we apply by default. Could quite easily be a project config option

I understand. Sure it makes perfect sense to align with Node behavior.
I will just try out current version and see if it works against my project, because I remember I added that in because it was breaking my real use case back then with that version of the code

It might not align with bundlers tho. but yeah, needs some thinking about 🙂

It seems like a fully unrelated.

I probably just need to use new version of JSDOM (I have frozen version just for testing this feature and I am not updating anything but jest). So this error at most could result in system warning user to actually use newer JSDOM (nicer error message) and that's it. I'll try things out again once I update my dependencies

Alright. With new JSDOM I no longer have crashes. On the other hand I do get the original error (and the reason why I wrote the unwrapping). Specifically running jest on the mixed cjs-esm repository ends up with specifically:

import thunk from 'redux-thunk'

returning to me wrapper instead of actual default function (on said frozen project, the version of redux-thunk is 2.4.2 ).

As a side note - running the project within browser works. So there is a difference between jest runtime and webpack/browser resolution.

@SimenB should I look into this and make a fix that work both both cases (one that made you write this PR and my case)? Or you want to take care of this?

Oh sorry, missed your comments here! Please file an issue, yeah. Ideally with a reproduction 😀 but obviously feel free to attempt a fix as well 🙂 but note that if it's purely unwrapping the interop I think we'd need an option added to ProjectConfig. Bundlers apply the interop usually, but that's not really following a spec - more for backwards compat.

That said, it seems redux-thunk has a native ESM file: https://www.runpkg.com/[email protected]/package.json#24. Odd we don't pick that up 🤔

Right. I created an issue for this: #16144

Might be version 3.1 works. What I am using for testing is not latest version of the project, but old one I deliberately froze so that I can test interoperability. It means quite a few thing will have old versions of dependencies. Still I would expect the system to work with them.

I'll have a deeper look at this during this weekend and can write you all my findings/way forward with this. Hopefully I'll find some solution that handles all the cases :)

Copilot

Pull request overview

Copilot reviewed 8 out of 8 changed files in this pull request and generated 1 comment.

| datasource | package | from | to | | ---------- | ------- | ------ | ------ | | npm | jest | 30.3.0 | 30.4.2 | ## [v30.4.2](https://github.com/jestjs/jest/blob/HEAD/CHANGELOG.md#3042) ##### Fixes - `[jest-runtime]` Fix named imports from CJS modules whose `module.exports` is a function with own-property exports ([#16150](jestjs/jest#16150)) ## [v30.4.1](https://github.com/jestjs/jest/blob/HEAD/CHANGELOG.md#3041) ##### Features - `[jest-config, jest-core, jest-runner, jest-schemas, jest-types]` Allow custom runner configuration options via tuple format `['runner-path', {options}]` ([#16141](jestjs/jest#16141)) ##### Fixes - `[jest-runtime]` Align CJS-from-ESM default export with Node: `module.exports` is always the ESM default, `__esModule` unwrapping is no longer applied ([#16143](jestjs/jest#16143)) ## [v30.4.0](https://github.com/jestjs/jest/blob/HEAD/CHANGELOG.md#3040) ##### Features - `[babel-jest]` Support collecting coverage from `.mts`, `.cts` (and other) files ([#15994](jestjs/jest#15994)) - `[jest-circus, jest-cli, jest-config, jest-core, jest-jasmine2, jest-types]` Add `--collect-tests` flag to discover and list tests without executing them ([#16006](jestjs/jest#16006)) - `[jest-config, jest-runner, jest-worker]` Add `workerGracefulExitTimeout` config option to control how long workers are given to exit before being force-killed ([#15984](jestjs/jest#15984)) - `[jest-config]` Add support for `jest.config.mts` as a valid configuration file ([#16005](jestjs/jest#16005)) - `[jest-config, jest-core, jest-reporters, jest-runner]` `verbose` and `silent` can now be set per-project; the project-level value overrides the global value for that project's tests ([#16133](jestjs/jest#16133)) - `[@jest/fake-timers]` Accept `Temporal.Duration` in `jest.advanceTimersByTime()` and `jest.advanceTimersByTimeAsync()` ([#16128](jestjs/jest#16128)) - `[@jest/fake-timers]` Accept `Temporal.Instant` and `Temporal.ZonedDateTime` in `jest.setSystemTime()` and `useFakeTimers({now})` ([#16128](jestjs/jest#16128)) - `[@jest/fake-timers]` Support faking `Temporal.Now.*` ([#16131](jestjs/jest#16131)) - `[jest-mock]` Add `clearMocksOnScope(scope)` on `ModuleMocker` for clearing every mock function exposed on a scope object ([#16088](jestjs/jest#16088)) - `[jest-resolve]` Add `canResolveSync()` on `Resolver` so callers can detect when a user-configured resolver only exports an `async` hook ([#16064](jestjs/jest#16064)) - `[jest-runtime]` Use synchronous `evaluate()` for ES modules without top-level `await` on Node versions that support it (v24.9+), and prefer the synchronous transform path when a sync transformer is configured ([#16062](jestjs/jest#16062)) - `[jest-runtime]` Support `require()` of ES modules on Node v24.9+ ([#16074](jestjs/jest#16074)) - `[jest-runtime]` Validate TC39 import attributes (`with { type: 'json' }`) on ESM imports ([#16127](jestjs/jest#16127)) - `[@jest/transform]` Add `canTransformSync(filename)` on `ScriptTransformer` so callers can pick the sync vs async transform path ([#16062](jestjs/jest#16062)) - `[jest-util]` Add `isError` helper ([#16076](jestjs/jest#16076)) - `[pretty-format]` Support React 19 ([#16123](jestjs/jest#16123)) ##### Fixes - `[expect-utils]` Fix `toStrictEqual` failing on `structuredClone` results due to cross-realm constructor mismatch ([#15959](jestjs/jest#15959)) - `[@jest/expect-utils]` Prevent `toMatchObject`/subset matching from throwing when encountering exotic iterables ([#15952](jestjs/jest#15952)) - `[fake-timers]` Convert `Date` to milliseconds before passing to `@sinonjs/fake-timers` ([#16029](jestjs/jest#16029)) - `[jest]` Export `GlobalConfig` and `ProjectConfig` TypeScript types ([#16132](jestjs/jest#16132)) - `[jest-circus]` Prevent crash when `asyncError` is undefined for non-Error throws ([#16003](jestjs/jest#16003)) - `[jest-circus, jest-jasmine2]` Include `Error.cause` in JSON `failureMessages` output ([#15967](jestjs/jest#15967)) - `[jest-config]` Fix preset path resolution on Windows when the preset uses subpath `exports` ([#15961](jestjs/jest#15961)) - `[jest-config]` Allow `collectCoverage` and `coverageProvider` in project config without a validation warning ([#16132](jestjs/jest#16132)) - `[jest-config]` Project config validator now emits "is not supported in an individual project configuration" instead of "probably a typing mistake" for known global-only options ([#16132](jestjs/jest#16132)) - `[jest-environment-node]` Fix `--localstorage-file` warning on Node 25+ ([#16086](jestjs/jest#16086)) - `[jest-reporters]` Apply global coverage threshold to unmatched pattern files in addition to glob/path thresholds ([#16137](jestjs/jest#16137)) - `[jest-reporters, jest-runner, jest-runtime, jest-transform]` Fix coverage report not showing correct code coverage when using `projects` config option ([#16140](jestjs/jest#16140)) - `[jest-runtime]` Resolve `expect` and `@jest/expect` from the internal module registry so test-file imports share the same `JestAssertionError` as the global `expect` ([#16130](jestjs/jest#16130)) - `[jest-runtime]` Improve CJS-from-ESM interop: `__esModule`/Babel default unwrap, broader named-export coverage, and shared CJS singleton across importers ([#16050](jestjs/jest#16050)) - `[jest-runtime]` Load `.js` files with ESM syntax but no `"type":"module"` marker as native ESM ([#16050](jestjs/jest#16050)) - `[jest-runtime]` Extend the `.js`-with-ESM-syntax fallback to `require()` on Node v24.9+ - falls back to `require(esm)` when the CJS parser rejects ESM syntax ([#16078](jestjs/jest#16078)) - `[jest-runtime]` Fix deadlocks and double-evaluation in concurrent ESM and wasm imports ([#16050](jestjs/jest#16050)) - `[jest-runtime]` Fix error when `require()` is called after the Jest environment has been torn down ([#15951](jestjs/jest#15951)) - `[jest-runtime]` Fix missing error when `import()` is called after the Jest environment has been torn down ([#16080](jestjs/jest#16080)) - `[jest-runtime]` Fix virtual `unstable_mockModule` registrations not respected in ESM ([#16081](jestjs/jest#16081)) - `[jest-runtime]` Apply `moduleNameMapper` when resolving modules with `require.resolve()` and the `paths` option ([#16135](jestjs/jest#16135)) ##### Chore & Maintenance - `[@jest/fake-timers]` Upgrade `@sinonjs/fake-timers` ([#16139](jestjs/jest#16139)) - `[jest-runtime]` Use synchronous `linkRequests` / `instantiate` for ESM linking on Node v24.9+ ([#16063](jestjs/jest#16063))

fix(jest-runtime): restore module.exports as ESM default for `__esMod…

8a60a8f

…ule` CJS without `.default`

Copilot AI review requested due to automatic review settings May 8, 2026 07:25

Copilot started reviewing on behalf of SimenB May 8, 2026 07:26 View session

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SimenB commented May 8, 2026

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align with node

9e53549

SimenB changed the title ~~fix(jest-runtime): restore module.exports as ESM default for __esModule CJS without .default~~ fix(jest-runtime): align CJS-from-ESM default with Node, drop __esModule unwrapping May 8, 2026

SimenB requested a review from Copilot May 8, 2026 07:47

Copilot started reviewing on behalf of SimenB May 8, 2026 07:49 View session

update e2e test as well

9f49471

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Comment thread packages/jest-runtime/src/__tests__/runtime_esm_cjs_interop.test.ts

remove file header

59bde21

SimenB merged commit 211cdbd into jestjs:main May 8, 2026
90 checks passed

SimenB deleted the cjs-default branch May 8, 2026 08:10

jmuransky mentioned this pull request May 8, 2026

[Bug]: ESM-CJS interoperability issue of getting incorrectly imported default module #16144

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claude Bot mentioned this pull request May 16, 2026

chore(deps)(deps-dev): bump the testing group with 6 updates Han5991/fe-lab#96

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