Infering the boundary callback arguments

[Tenga]

Scope

Improves an existing behavior

Compatibility

• This is a breaking change

Feature description

Currently, when using the MSW boundaries, the callback provided to the boundary has no inferred types.

This is a slight issue when using boundaries and concurrency, as when using concurrency in Vitest, in certain cases you are required to use the local expect from the context instead of the imported expect.

So I went digging a bit as to what the root cause of this is, and eventually I ran into this issue on the TypeScript repository. I believe this issue also describes the basic interaction between Vitest's test typings and MSW boundary typings.

I applied Daniel's suggested alternative to boundary locally

declare class SetupServerApi extends SetupServerCommonApi implements SetupServer {
    constructor(handlers: Array<RequestHandler>);
-   boundary<Fn extends (...args: Array<any>) => unknown>(callback: Fn): (...args: Parameters<Fn>) => ReturnType<Fn>;
+   boundary<Fn>(callback: Fn & Function): Fn & Function;
    close(): void;
}

and indeed the boundary callback's arguments became correctly inferred

and I could use the expect from the context with inferred types for it.

I'm unsure if this change would bring something undesireable or if there's something I missed. Correct me if I'm wrong, but I believe that the proposed change's outcome is an improvement as what the current types are trying to do anyway.

[kettanaito]

Hi, @Tenga. Thanks for letting me know!

I'd love to use this suggestion but it doesn't seem to work in the boundary implementation:

T & Function is not guaranteed to be a function. I can use some help with this. @Andarist, if you have a moment, do you see something wrong with what I'm doing? The behavior is reproducible in #2100, pnpm build. Thank you.

[kettanaito]

Released: v2.2.9 🎉

This has been released in v2.2.9!

• 📄 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.

[Tenga]

Thanks for looking into this, both of you!

I just got around to updating MSW to 2.2.12. Unfortunately the changes had no effect on this issue on my end.

Parameters are still any unless manually provided to boundary.

I'm on typescript@3.4.3typescript@5.4.3. Are there any tsconfig settings needed to make this be correctly infered?

If not, can we reopen this or is the only other resolution to wait for the typescript to fix the issue linked in the first post?

[kettanaito]

@Tenga, are you certain you aren't using TS 4.3? MSW doesn't support any TypeScript versions older than 4.7.

[Tenga]

@Tenga, are you certain you aren't using TS 4.3? MSW doesn't support any TypeScript versions older than 4.7.

@kettanaito Sorry, yeah I mistyped that somehow. The project (and VS Code) is using typescript@5.4.3.

[Andarist]

could you share a repro case of the problem with the current version of MSW?

[Tenga]

@Andarist I've made a quick minimal repro repo that can be found here.

Below is what I get in the sample.test.ts in the project.

The project and branch I've originally encountered this is here. Pardon the wild mess the branch is in, in case you decide to go looking. 😁

[kettanaito]

@Tenga, in the code snippet above, I'm pretty sure expect is any because the entire argument is cast to any:

-({ expect }: any)
+({ expect })

Is there any difference if you omit any explicit types?

[Andarist]

That doesn't look like an explicit type annotation but rather like an inlay hint

[Tenga]

Yup, @Andarist is correct, there is no explicit typing in the screenshot. Bright gray text is the VS Code inlay hints that I've enabled to show the inferred types clearly everywhere.

The source from the screenshot is identical to the code in the repro case I've provided in my previous post.

https://github.com/Tenga/msw-boundary-infer-repro/blob/main/sample.test.ts#L6