feat(jest-snapshot): support ESM snapshotResolver and snapshotSerializers by soltonigiri · Pull Request #16402 · jestjs/jest

soltonigiri · 2026-08-28T09:37:20Z

Summary

Jest loads both snapshot configuration options inside the test sandbox, where native ECMAScript module configuration files need the VM modules flag.

This change loads both options outside the sandbox with Jest's existing transform-aware loader. CommonJS and transformed TypeScript modules still work. The configured module order stays unchanged, and the project configuration ID scopes the snapshot resolver cache.

The tests cover the default and legacy test runners, including a .js resolver with ESM syntax that runs through Babel without a fixture install or the VM modules flag. The configuration docs include ECMAScript module examples and explain the transform and default-export behavior.

Related to #11167.
Closes #12014.

Test plan

yarn build
Focused snapshot unit tests: 2 suites, 18 tests, and 4 snapshots passed.
Snapshot configuration integration tests under both the default and legacy runners: 2 suites, 6 tests, and 2 snapshots passed per runner.
ESM resolver transform integration test with Babel under both runners: 1 test passed per runner.
yarn test-types: 24 files, 83 tests, and 1,585 assertions passed.
Type checking, lint, repository constraints, changelog validation, dependency checks, copyright header validation, package manager verification, and whitespace checks passed.

…zers

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commit: b9194a4

SimenB · 2026-09-01T07:39:10Z

Thanks for picking this up! Docs tabs and extra tests are great 👍

A few things I'd like carried over / cleaned up before merge:

Nothing covers a transformed ESM config module. feat(snapshot): support snapshotResolver and snapshotSerializers written in ESM #12014 had e2e/transform/transform-esm-snapshotResolver/ plus a case in e2e/__tests__/transform.test.ts. The test plan mentions "Transformed TypeScript resolver integration test under both runners". I can't find it in the diff, did it get lost?
createCustomSnapshotResolver still does interopRequireDefault(await localRequire(path, true)).default. Passing applyInteropRequireDefault: true already unwraps it, so the outer call is a no-op that happens to work out. Drop the interopRequireDefault import.
LocalRequire is typed (...) => unknown | Promise<unknown>, which collapses to plain unknown. It's always async now - make it generic and Promise<T>-returning.

soltonigiri · 2026-09-01T10:00:12Z

Thanks! I've addressed all three points 👍

I've kept the transformed TypeScript coverage in transform-snapshotResolver and added the transformed ESM fixture and its transform.test.ts coverage alongside it. Both cases pass under Circus and Jasmine2.

I also removed the redundant interopRequireDefault call and made LocalRequire generic with a Promise<T> return type.

SimenB

Thanks, all three points from last round look good.

The transform fixture is the one real problem - see the comment on it (and apologies for sending you on a wrong path). loadSnapshotSetup I'd also like moved before merge. Everything else is small.

SimenB · 2026-09-01T10:57:37Z

@@ -0,0 +1,6 @@
+{


.mjs can't be transformed at all - requireOrImportModule picks the extension branch and hands off to native import() before the hook exists. There's also no transform key, no babel config and no deps here, and setting "transform": {} doesn't change the result.

My bad for essentially asking you to copy the test in #12014 verbatim without reading it closer.

Give it an extension the pipeline can claim. .ts, or .js with ESM syntax and no type: module, both go through require() and hit babel.

Note that the current tests doesn't need the yarn install thing. that might change dependening on how you do the transform ofc 🙂

SimenB · 2026-09-01T10:59:38Z

  }
+
+  /** Loads snapshot config modules outside the test sandbox. */
+  async loadSnapshotSetup(): Promise<SnapshotSetup> {


Only reads this._config, and skips the sandbox by design. Runtime is the documented subclass seam and everything on it loads through the sandbox - this looks like an override point and isn't.

Should be a free function in jest-snapshot instead. loadSnapshotSetup(config)?.

SimenB · 2026-09-01T11:02:25Z

+  localRequire?: Promise<LocalRequire> | LocalRequire,
 ): Promise<SnapshotResolver> => {
-  const key = config.rootDir;
+  const key = `${config.rootDir}\0${config.snapshotResolver ?? ''}`;


Use config.id instead. It's sha1(rootDir + configPath + projectIndex) from normalize.ts:365, and it's already what jest-runner keys its resolvers on. What you have here still collides for two projects that share a rootDir and a resolver but differ in transform, and switching lets the comment above disappear as well.

SimenB · 2026-09-01T11:02:58Z

-  localRequire: Promise<LocalRequire> | LocalRequire = createTranspilingRequire(
-    config,
-  ),
+  localRequire?: Promise<LocalRequire> | LocalRequire,


Seems 100% unused now (outside of tests). add a TODO comment to mark it for removal in Jest 31?

SimenB · 2026-09-01T11:03:20Z

+};
+```
+
+Jest loads the module outside the test sandbox, so it does not use `jest.mock()` or `moduleNameMapper`. Your `transform` still applies. Jest loads `.mjs` and `.mts` files as native ESM, so they must be valid JavaScript as written.


"Your transform still applies" contradicts the sentence that follows it, and the second one is the accurate one: .mjs and .mts skip the transform entirely. Worth stating that outright rather than leaving a reader to reconcile the two.

This paragraph is also missing the default-export requirement that the snapshotSerializers section spells out, even though both paths go through applyInteropRequireDefault: true.

SimenB · 2026-09-01T11:03:30Z

+
 `printer` is a function that serializes a value using existing plugins.

+Jest loads serializers outside the test sandbox, so they do not use `jest.mock()` or `moduleNameMapper`. Your `transform` still applies. Jest loads `.mjs` and `.mts` files as native ESM, so they must be valid JavaScript as written. An ESM serializer must use a `default` export.


The transform sentence has the same problem here.

SimenB · 2026-09-01T11:04:21Z


 ### Features

+- `[jest-circus, jest-jasmine2, jest-runtime, jest-snapshot]` Support ESM `snapshotResolver` and `snapshotSerializers` configuration modules ([#16402](https://github.com/jestjs/jest/pull/16402))


Please call out the behaviour change in this entry. Loading these modules outside the sandbox means their transitive imports no longer go through moduleNameMapper or jest.mock, and as written it reads like pure feature work. Anyone whose serializer pulls in a mapped module will find out the hard way.

Also, while it hits circus and jasmine, I don't think it's relevant to people reading the changelog?

SimenB · 2026-09-01T11:07:39Z

+
+  it('should transform the snapshotResolver', () => {
+    const result = runJest(dir, ['-w=1', '--no-cache', '--ci=false'], {
+      nodeOptions: '--experimental-vm-modules --no-warnings',


I don't see why --experimental-vm-modules would be needed to run the tests?

soltonigiri · 2026-09-01T12:31:44Z

Thanks for the review, and no worries! I've addressed all eight comments 👍

The transform fixture now uses a .js resolver with ESM syntax, so it goes through Babel without the install step or VM modules flag. I moved loadSnapshotSetup to jest-snapshot, and both runners now call it before entering the test sandbox.

The resolver cache now keys on config.id, and the docs spell out the .mjs/.mts and default-export behavior. The changelog now notes that transitive imports no longer use jest.mock() or moduleNameMapper.

The focused snapshot tests and transform E2Es are green with both Circus and Jasmine2.

feat(jest-snapshot): support ESM snapshotResolver and snapshotSeriali…

8f45697

…zers

github-actions Bot added the require-changelog If a PR does requires a changelog entry label Aug 28, 2026

soltonigiri added 2 commits August 28, 2026 18:45

chore: add changelog entry for jestjs#16402

fc732bb

test: cover snapshot setup and label docs tabs

6e5f87e

soltonigiri closed this Aug 28, 2026

soltonigiri reopened this Aug 28, 2026

fix(jest-snapshot): address ESM resolver review feedback

7ecf8f0

Merge branch 'main' into feat/esm-snapshot-modules-11167

1175936

SimenB requested changes Sep 1, 2026

View reviewed changes

fix(jest-snapshot): address follow-up review feedback

291d8ff

soltonigiri added 2 commits September 1, 2026 22:22

test(e2e): configure ESM snapshot resolver transform

e7e51fe

Merge branch 'main' into feat/esm-snapshot-modules-11167

b9194a4


		`printer` is a function that serializes a value using existing plugins.

		Jest loads serializers outside the test sandbox, so they do not use `jest.mock()` or `moduleNameMapper`. Your `transform` still applies. Jest loads `.mjs` and `.mts` files as native ESM, so they must be valid JavaScript as written. An ESM serializer must use a `default` export.


		### Features

		- `[jest-circus, jest-jasmine2, jest-runtime, jest-snapshot]` Support ESM `snapshotResolver` and `snapshotSerializers` configuration modules ([#16402](https://github.com/jestjs/jest/pull/16402))

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