chore: introduce TSTyche for type testing by mrazauskas · Pull Request #6532 · fastify/fastify

mrazauskas · 2026-02-25T14:07:32Z

Context #6033

This PR is introducing TSTyche for testing TypeScript types.

I have a branch that is replacing tsd with tstyche here #6525. To be honest, I wanted to look through it once again, but that is too massive to review.

This is clean up PR that allows to run the same type tests files using tsd and tstyche.

Changes

expectError() is replaced with expectNotAssignable() or // @ts-expect-error
- mostly when tsds expectError() is provided a type argument (like expectError<string>("abc")), it catches assignability error and therefore it is replaced with expectNotAssignable() for better readability
- otherwise it is replaced with // @ts-expect-error followed by an error message, these are checked (and required!) by TSTyche
expectNotDeprecated() is removed. It does nothing. All exported types and expressions would pass this test.

Other Changes

test/types/import.ts, it was testing that FastifyListenOptions and FastifyLogFn are exported. I aded tests for FastifyListenOptions to test/types/instance.test-d.ts and FastifyLogFn is already tested in test/types/logger.test-d.ts and test/types/request.test-d.ts
the test:typescript script is renamed to test:types
ESLint config is updated to unused constructs in type test files

Checklist

run npm run test && npm run benchmark --if-present
commit message and code follows the Developer's Certification of Origin
and the Code of conduct

mrazauskas · 2026-02-25T14:09:37Z

+expectAssignable<FastifyInstance>(fastify().register(import('./dummy-plugin.mjs')))
+expectAssignable<FastifyInstance>(fastify().register(import('./dummy-plugin.mjs'), { foo: 1 }))


After tsconfig.json was added, TypeScript requires ESM file to be imported.

mrazauskas · 2026-02-25T14:10:33Z

-expectAssignable<ServerWithTypeProviderAndLogger>(
-  // @ts-expect-error
-  serverWithTypeProviderAndLogger.register(testPluginWithTypeProviderAndLogger)
-)
+// @ts-expect-error  No overload matches this call.
+serverWithTypeProviderAndLogger.register(testPluginWithTypeProviderAndLogger)


Same as other // @ts-expect-error tests above. expectAssignable() seems to be redundant.

mrazauskas · 2026-02-25T14:11:23Z

    expectType<((...args: [payload: string]) => typeof res)>(res.code(200).send)
    expectType<((...args: [payload: number]) => typeof res)>(res.code(400).send)
    expectType<((...args: [payload: { error: string }]) => typeof res)>(res.code(500).send)
-    expectError<(payload?: unknown) => typeof res>(res.code(200).send)


The exact types was tested few lines above.

mrazauskas · 2026-02-25T14:11:36Z

    expectType<((...args: [payload: string]) => typeof res)>(res.code(200).send)
    expectType<((...args: [payload: number]) => typeof res)>(res.code(400).send)
    expectType<((...args: [payload: { [x: string]: unknown; error?: string }]) => typeof res)>(res.code(500).send)
-    expectError<(payload?: unknown) => typeof res>(res.code(200).send)


The exact types was tested few lines above.

mrazauskas · 2026-02-26T08:44:39Z

+      '@typescript-eslint/no-unused-expressions': 'off',
+      '@typescript-eslint/no-unused-vars': 'off'


These are added only temporary. I plan to remove them after migrating the assertions. For instance, introducing .toHaveProperty() in three places allows enabling no-unused-expressions.

mrazauskas · 2026-02-26T08:47:08Z

-  // preClose hook types should be exported correctly https://github.com/fastify/fastify/pull/5335
-  /* eslint-disable @typescript-eslint/no-unused-vars */
  preCloseAsyncHookHandler,
  preCloseHookHandler


I included tests for both preClose* types, so they are not reported as unused vars by ESLint anymore.

Eomm

Why do we need it?

mrazauskas · 2026-03-07T10:34:42Z

Why do we need it?

@Eomm Not sure I fully understand your questions. See the discussion in #6033. Is the approach somehow unclear? Or which part? Happy to provide more details, but at the moment it is not clear what is missing.

Eomm

Pinged the TS team

LGTM - let's remove tsd after this PR

mcollina

lgtm

mcollina

lgtm

Not a ts user

Signed-off-by: mrazauskas <[email protected]>

Tony133 · 2026-05-13T13:48:18Z

I wanted to let you know that all Fastify libraries and plugins have been migrated from tsd to tstyche for type checking 🙌 🎉 🚀

I think we can merge this PR so that we can proceed with the complete removal of tsd from this repository as well, since there is also this PR: #6525 🔥

The only PR left to merge is this one regarding the Fastify CLI: fastify/fastify-cli#886 🔥

Tony133

LGTM

- Replace tsd with tstyche for type testing - Update test:types script to use tstyche - Convert initialization.test-d.ts to initialization.tst.ts - Update index.d.ts to use ES module export syntax PR of Reference: - fastify/fastify#6532 - fastify/fastify#6525

* refactor(types): migrate from tsd to tstyche - Replace tsd with tstyche for type testing - Update test:types script to use tstyche - Convert initialization.test-d.ts to initialization.tst.ts - Update index.d.ts to use ES module export syntax PR of Reference: - fastify/fastify#6532 - fastify/fastify#6525 * refactor(types): add toBeCallableWith assertion for app.register * refactor(types): add route-level constraints assertion * refactor(types): add child.get constraints assertion * Apply suggestions from code review Co-authored-by: Tom Mrazauskas <[email protected]> * Apply suggestions from code review Co-authored-by: Tom Mrazauskas <[email protected]> * Update test/types/initialization.tst.ts * fix(scripts): update test command to run all test files * refactor(types): update type definitions and test initialization --------- Co-authored-by: Tom Mrazauskas <[email protected]>

chore: introduce TSTyche for type testing

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github-actions Bot added typescript TypeScript related github actions Github actions related labels Feb 25, 2026

mrazauskas commented Feb 25, 2026

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Should we move out from tsd? #6033

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mrazauskas added 2 commits February 26, 2026 08:02

Merge branch 'main' into introduce-tstyche

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clean up

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mrazauskas commented Feb 26, 2026

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Merge branch 'main' into introduce-tstyche

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Merge branch 'main' into introduce-tstyche

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Merge branch 'main' into introduce-tstyche

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Should we move out from tsd? #6033

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mrazauskas added 3 commits March 9, 2026 13:56

rebase

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Merge branch 'main' into introduce-tstyche

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Merge branch 'main' into introduce-tstyche

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rebase

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mrazauskas added 2 commits April 2, 2026 18:49

Merge branch 'main' into introduce-tstyche

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update tstyche

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Comment thread test/types/content-type-parser.test-d.ts

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Puppo mentioned this pull request May 7, 2026

refactor(types): migrate from tsd to tstyche fastify/secure-json-parse#175

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refactor(types): migrate from tsd to tstyche fastify/deepmerge#93

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refactor(types): migrate from tsd to tstyche fastify/fastify-kafka#171

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This was referenced May 7, 2026

refactor(types): migrate from tsd to tstyche fastify/otel#153

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refactor(types): migrate from tsd to tstyche fastify/fastify-error#164

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refactor(types): migrate from tsd to tstyche fastify/fast-json-stringify-compiler#86

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Tony133 mentioned this pull request May 11, 2026

refactor(types): migrate from tsd to tstyche fastify/fastify-cli#886

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refactor(types): migrate from tsd to tstyche nearform/fast-jwt#614

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refactor(types): migrate from tsd to tstyche nearform/sql#233

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rebase

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Signed-off-by: mrazauskas <[email protected]>

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refactor(types): migrate from tsd to tstyche nearform/fastify-constraints#473

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Migrate tsd tests to tstyche nearform/node-test-parser#237

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Eomm merged commit 436e1f0 into fastify:main May 17, 2026
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mrazauskas deleted the introduce-tstyche branch May 17, 2026 12:43

This was referenced May 18, 2026

chore: migrate type tests to TSTyche assertions (part one) #6726

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chore: migrate type tests to TSTyche assertions (part two) #6727

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Puppo mentioned this pull request May 22, 2026

refactor(types): migrate from tsd to tstyche, and move from CommonJS to ESM nearform/fastify-casbin#160

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Puppo mentioned this pull request Jun 2, 2026

refactor(types): migrate from tsd to tstyche nearform/fastify-casbin-rest#166

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refactor(types): migrate from tsd to tstyche mcollina/fastify-html#95

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		expectAssignable<FastifyInstance>(fastify().register(import('./dummy-plugin.mjs')))
		expectAssignable<FastifyInstance>(fastify().register(import('./dummy-plugin.mjs'), { foo: 1 }))

		'@typescript-eslint/no-unused-expressions': 'off',
		'@typescript-eslint/no-unused-vars': 'off'

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mrazauskas commented Feb 25, 2026 •

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