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SimenB · 2026-05-09T14:08:38Z

Summary

Test plan

Tests added

…SM syntax but lacks a type:module marker

netlify · 2026-05-09T14:08:43Z

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commit: b39c8e9

Copilot

Pull request overview

This PR updates jest-runtime’s ESM/CJS interop so that when a .js file contains ESM syntax (e.g. export) but lacks an ESM marker (.mjs / "type":"module"), Jest can detect that situation and fall back to native ESM loading (addressing #16144).

Changes:

Introduces CjsParseError and uses es-module-lexer to detect “module syntax” on CJS parse/compile failures, enabling a native-ESM retry.
Updates the CJS-as-ESM synthetic-module path to return null on CjsParseError, allowing callers to fall back to loadEsmModule / requireEsm.
Adds regression coverage via unit tests and new E2E coverage, plus updates dependencies and changelog.

Reviewed changes

Copilot reviewed 16 out of 19 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
yarn.lock	Bumps/aligns `es-module-lexer` resolution to support new runtime dependency.
scripts/cleanE2e.mjs	Excludes the new e2e fixture’s `node_modules/` from cleanup.
packages/jest-runtime/src/internals/syntheticBuilders.ts	Makes CJS-as-ESM synthetic builder return `null` on `CjsParseError` to enable native-ESM fallback.
packages/jest-runtime/src/internals/ModuleExecutor.ts	Adds `CjsParseError` and detects ESM syntax via `es-module-lexer` when `compileFunction` fails.
packages/jest-runtime/src/internals/EsmLoader.ts	Uses the `null` synthetic-module signal to fall back to native ESM loading.
packages/jest-runtime/src/internals/CjsLoader.ts	Updates `require()` fallback to use `instanceof CjsParseError` and surface original causes correctly.
packages/jest-runtime/src/internals/CjsExportsCache.ts	Throws `CjsParseError` when CJS export parsing fails due to detected ESM syntax.
packages/jest-runtime/src/internals/tests/ModuleExecutor.test.ts	Updates tests to cover `CjsParseError` behavior.
packages/jest-runtime/src/internals/tests/CjsExportsCache.test.ts	Adds coverage asserting `CjsParseError` is thrown for ESM-syntax sources.
packages/jest-runtime/src/tests/test_esm_interop_root/node_modules/esm-no-marker/package.json	Adds a fixture package used to reproduce “ESM syntax in `.js` without marker”.
packages/jest-runtime/src/tests/test_esm_interop_root/node_modules/esm-no-marker/index.js	Adds the ESM-syntax `.js` fixture module.
packages/jest-runtime/src/tests/test_esm_interop_root/import-esm-no-marker.mjs	Adds an ESM entrypoint importing from the fixture package.
packages/jest-runtime/src/tests/runtime_esm_cjs_interop.test.ts	Adds integration tests covering both `import` and `require()` fallback behavior.
packages/jest-runtime/package.json	Adds `es-module-lexer` runtime dependency.
e2e/native-esm-js-esm-syntax-fallback/package.json	Adds new e2e fixture config to exercise the fallback end-to-end.
e2e/native-esm-js-esm-syntax-fallback/tests/esm-fallback.test.js	Adds fixture test verifying the fallback works.
e2e/tests/nativeEsmJsEsmSyntaxFallback.test.ts	Adds new e2e harness test + snapshot assertion.
e2e/tests/snapshots/nativeEsmJsEsmSyntaxFallback.test.ts.snap	Snapshot for the new e2e harness test.
CHANGELOG.md	Adds a main-branch fix entry for `jest-runtime`.

Copilot

Pull request overview

Copilot reviewed 19 out of 25 changed files in this pull request and generated 6 comments.

…compileFunction vm.compileFunction throws a SyntaxError from the VM realm; instanceof Error fails cross-realm. Replace the narrowing check with jest-util's isError() (backed by util.isNativeError / Error.isError) which works across realms. Co-Authored-By: Claude Sonnet 4.6 <[email protected]>

Copilot

Pull request overview

Copilot reviewed 20 out of 26 changed files in this pull request and generated 1 comment.

Copilot

Pull request overview

Copilot reviewed 23 out of 31 changed files in this pull request and generated 2 comments.

Copilot

Pull request overview

Copilot reviewed 22 out of 30 changed files in this pull request and generated 4 comments.

-          : esmError;
-      });
+      if (!(error instanceof CjsParseError)) throw error;
+      return this.loadEsmModule(modulePath);


fix(jest-runtime): fall back to native ESM when a .js file contains E…

80406c0

…SM syntax but lacks a type:module marker

SimenB added a commit to SimenB/jest that referenced this pull request May 9, 2026

chore: add changelog entry for jestjs#16152

966b692

changelog

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SimenB force-pushed the esm-fallback branch from 9a0a963 to d9f5dcc Compare May 9, 2026 14:09

SimenB marked this pull request as ready for review May 9, 2026 14:09

Copilot AI review requested due to automatic review settings May 9, 2026 14:09

SimenB mentioned this pull request May 9, 2026

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

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Copilot started reviewing on behalf of SimenB May 9, 2026 14:10 View session

Copilot AI reviewed May 9, 2026

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SimenB added 2 commits May 10, 2026 07:31

ci

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assert caching

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SimenB requested a review from Copilot May 10, 2026 05:49

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Copilot started reviewing on behalf of SimenB May 10, 2026 05:56 View session

SimenB and others added 3 commits May 10, 2026 08:06

more comments

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types

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SimenB requested a review from Copilot May 10, 2026 20:12

Copilot started reviewing on behalf of SimenB May 10, 2026 20:13 View session

bump cjs as well

c2a5628

SimenB force-pushed the esm-fallback branch from 2585ae7 to c2a5628 Compare May 10, 2026 20:17

Copilot AI reviewed May 10, 2026

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Comment thread packages/jest-runtime/src/internals/EsmLoader.ts Outdated

SimenB added 3 commits May 10, 2026 22:34

clean some stuff up

4ba7330

review comment

617b7ea

test and simplify

3a1d69e

SimenB requested a review from Copilot May 11, 2026 06:26

Copilot started reviewing on behalf of SimenB May 11, 2026 06:28 View session

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Comment thread packages/jest-runtime/src/internals/EsmLoader.ts

Comment thread packages/jest-runtime/src/internals/CjsLoader.ts Outdated

SimenB mentioned this pull request May 11, 2026

chore(runtime): avoid magical null value #16160

Merged

SimenB added 6 commits May 11, 2026 09:46

Merge branch 'main' into esm-fallback

d0f5fc3

correct comment

51f70bf

sorted

c09f47e

oops

9b4b227

missing throw

751b475

less comment

76a476a

SimenB requested a review from Copilot May 11, 2026 08:02

Copilot AI reviewed May 11, 2026

View reviewed changes

SimenB added 4 commits May 11, 2026 10:05

not needed

f864a0d

exercise bug fix

1c3e5bb

copilot comment

f02ff98

run tests on more nodes

b39c8e9

SimenB changed the title ~~fix(jest-runtime): fall back to native ESM when a .js file contains ESM syntax but lacks a type:module marker~~ fix(jest-runtime): fall back to native ESM when a .js file contains ESM syntax May 11, 2026

SimenB merged commit c24dcd4 into jestjs:main May 11, 2026
90 checks passed

SimenB deleted the esm-fallback branch May 11, 2026 13:14

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