fix: support async generators as response resolvers #2108

kettanaito · 2024-03-25T11:58:31Z

Fixes "Cannot read properties of undefined (reading 'get')" when using generators/async generators as resolvers #2109
Adds failing integration tests for using a generator (and async generator) function as response resolvers.
Supports async generators as response resolvers (generally, supports any iterators/async iterators!).
Adds missing type tests for generators and async generators as response resolvers (needs fix: accept a narrower response body type by default #2107 to properly pass).

kettanaito · 2024-03-25T13:06:52Z

src/core/handlers/RequestHandler.ts

@@ -256,6 +266,9 @@ export abstract class RequestHandler<
      return null
    }

+    // Preemptively mark the handler as used.
+    // Generators will undo this because only when the resolver reaches the
+    // "done" state of the generator that it considers the handler used.
    this.isUsed = true


It's important to mark the handler as used immediately, even if using generators then opts-out from this behavior. This is what we promise right now, so let's keep that promise.

kettanaito · 2024-03-25T13:26:23Z

src/core/handlers/RequestHandler.ts

@@ -57,6 +56,11 @@ export type AsyncResponseResolverReturnType<
      MaybeAsyncResponseResolverReturnType<ResponseBodyType>,
      MaybeAsyncResponseResolverReturnType<ResponseBodyType>
    >
+  | AsyncGenerator<


Older versions of TypeScript (4.7 - 5.2) have trouble digesting this type:

FAIL resolver-generator.test-d.ts > supports async generator function as response resolver TypeCheckError: Argument of type '() => AsyncGenerator<StrictResponse<JsonBodyType>, StrictResponse<JsonBodyType>, any>' is not assignable to parameter of type 'HttpResponseResolver<never, never, { value string; }>'. Type 'AsyncGenerator<StrictResponse<JsonBodyType>, StrictResponse<JsonBodyType>, any>' is not assignable to type 'AsyncResponseResolverReturnType<{ value string; }>'. Type 'AsyncGenerator<StrictResponse<JsonBodyType>, StrictResponse<JsonBodyType>, any>' is not assignable to type 'AsyncGenerator<ResponseResolverReturnType<{ value string; }>, ResponseResolverReturnType<{ value string; }>, ResponseResolverReturnType<{ value string; }>>'. The types returned by 'next(...)' are incompatible between these types. Type 'Promise<IteratorResult<StrictResponse<JsonBodyType>, StrictResponse<JsonBodyType>>>' is not assignable to type 'Promise<IteratorResult<ResponseResolverReturnType<{ value string; }>, ResponseResolverReturnType<{ value string; }>>>'. Type 'IteratorResult<StrictResponse<JsonBodyType>, StrictResponse<JsonBodyType>>' is not assignable to type 'IteratorResult<ResponseResolverReturnType<{ value string; }>, ResponseResolverReturnType<{ value string; }>>'. Type 'IteratorYieldResult<StrictResponse<JsonBodyType>>' is not assignable to type 'IteratorResult<ResponseResolverReturnType<{ value string; }>, ResponseResolverReturnType<{ value string; }>>'. Type 'IteratorYieldResult<StrictResponse<JsonBodyType>>' is not assignable to type 'IteratorYieldResult<ResponseResolverReturnType<{ value string; }>>'. Type 'StrictResponse<JsonBodyType>' is not assignable to type 'ResponseResolverReturnType<{ value string; }>'.

Reproduction steps

TypeScript 4.7

pnpm test:ts

Hi, @Andarist! Sorry for troubling you once again. If you have a minute, could you please lend me a hand with this type issue?

I would need to see a slimmed-down repro. The one that I tried put together in a rush errors in the current TS version too: TS playground

Thanks for giving it a try, @Andarist. Here's a minimal repro:

pnpm install pnpm build pnpm test:ts test/typings/resolver-generator.test-d.ts

That is not exactly minimal ;p it still depends on everything in the MSW's codebase. I could really use a self-contained TS playground here.

Yes, I can do that.

FWIW I did compile and test locally, and it all seemed to work, though I did not acutally test the full range of TS versions (but I can't imagine it wouldn't work, maybe I'm jinxing myself)

Happy to give this a try once it's here 👀 Installing different typescript dependency versions triggers the test:ts into using that version for testing. The CI will also cover all the ranges for us. So excited we may actually ship this!

Sent #2213.

FWIW if you're looking for multi-version testing and aren't too tied to vitest, https://github.com/tstyche/tstyche is one option I've heard of which automatically runs multiple versions of TS without having to install them explicitly.

That's nice! Something to consider in the setup.

kettanaito · 2024-07-21T18:04:11Z

The fix from @jakebailey (#2213) seems to fix the compatibility with older TypeScript versions! 🎉 Super job here. Any objections to merge this?

jakebailey · 2024-07-21T18:06:39Z

This technically now enables passing in arrays, maps, etc; is that desirable? Should it be tested?

kettanaito · 2024-07-21T18:13:24Z

@jakebailey as the return value of the resolver? It would be nice to guard against that on runtime at least.

jakebailey · 2024-07-21T18:21:07Z

Actually, the old code may have already allowed that, at least for like .values()...

jakebailey · 2024-07-21T18:24:04Z

It's probably fine as is, I'm just paranoid given I know nothing about this codebase!

kettanaito · 2024-07-21T21:24:56Z

@jakebailey, well, now you know a little bit :) I'm infinitely grateful for your work on this. Looks like I know another TypeScript wizard now.

I will give that false matching types scenario a try before releasing this.

kettanaito · 2024-07-22T19:25:19Z

Yeah, @jakebailey, you're right. As long as the return type satisfies the Iterator type, there's no type error:

http.get('/user/:a/:b/:b', ({ params }) => {
  return []
  // ^ This compiles OK while it must error. 
})

jakebailey · 2024-07-22T19:29:18Z

I guess the question is whether or not that's bad or not; I can just as easily pass in a generator which never yields today, right?

kettanaito · 2024-07-23T10:45:20Z

That's certainly agains the established contract. Response resolvers either return/yield a Response instance or nothing. Nothing is an accepted value. But anything save for a Response instance is not. We don't have runtime guards against that now because we rely on type-level validation to prevent developers from returning non-Response stuff from their resolvers.

kettanaito · 2024-07-23T13:15:57Z

This only seems to be a problem when returning an empty array () => []. Arrays are iterable, and they will even respect the resolver return type (response type) so that () => [123] errors, but an empty array is okay. Exploring if I can tweak the types so that an empty iterator is not allowed but that's equivalent of having a generator that doesn't yield value, which is still okay in MSW world.

Huh.

kettanaito · 2024-07-23T13:22:37Z

Oddly enough, I'm beginning to like the array stuff. Since arrays are iterable, one can return an array of mocked responses at once (no need for a generator). If that happens, MSW will exhaust that array as it does a generator function, producing mocked responses in order, and stopping on the last response to then use it forever. I wouldn't document this behavior but I like it!

kettanaito · 2024-07-23T14:09:49Z

Released: v2.3.3 🎉

This has been released in v2.3.3!

📄 Release notes
📦 npm package

Make sure to always update to the latest version (npm i msw@latest) to get the newest features and bug fixes.

Predictable release automation by @ossjs/release.

test: add generator resolver tests

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kettanaito marked this pull request as draft March 25, 2024 11:58

fix: support async generator response resolvers

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kettanaito mentioned this pull request Mar 25, 2024

"Cannot read properties of undefined (reading 'get')" when using generators/async generators as resolvers #2109

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kettanaito added 2 commits March 25, 2024 13:52

fix: accept AsyncGenerator as response resolver type

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Merge branch 'main' into fix/generator-resolver

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kettanaito marked this pull request as ready for review March 25, 2024 12:54

kettanaito changed the title ~~test: add generator resolver tests~~ fix: support async generators as response resolvers Mar 25, 2024

fix(RequestHandler): mark as used early, opt-out in generators

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kettanaito commented Mar 25, 2024

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kettanaito added 2 commits March 25, 2024 14:19

fix(RequestHandler): unwrap AsyncGenerator generic from MaybePromise

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test: add return type tests for generators

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kettanaito commented Mar 25, 2024

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chore: revert MaybeAsync to AsyncGenerator generic

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kettanaito mentioned this pull request Mar 30, 2024

feat(HttpResponse): support explicitly empty response body via null generic type #2118

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Merge branch 'main' into fix/generator-resolver

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juliusmarminge mentioned this pull request Apr 4, 2024

test: implement polling and retries in test pingdotgg/uploadthing#731

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kettanaito added 3 commits July 18, 2024 18:11

Merge branch 'main' into fix/generator-resolver

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chore: print tsconfig used in type tests

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Merge branch 'main' into fix/generator-resolver

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jakebailey mentioned this pull request Jul 21, 2024

fix: use iterables instead of generators #2213

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fix: use iterables instead of generators (#2213)

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kettanaito requested a review from mattcosta7 July 21, 2024 18:04

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kettanaito added 2 commits July 23, 2024 15:17

test: add type test for resolver return type

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test(generator): simplify fetch assertions

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test(types): import from build, not source

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kettanaito merged commit d38fc3d into main Jul 23, 2024
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fix: support async generators as response resolvers #2108

fix: support async generators as response resolvers #2108

kettanaito commented Mar 25, 2024 •

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fix: support async generators as response resolvers #2108

fix: support async generators as response resolvers #2108

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kettanaito commented Jul 21, 2024

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kettanaito commented Jul 23, 2024

Released: v2.3.3 🎉

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