MSW + Jest: Transferring of ReadableStream cannot be properly polyfilled in this engine about msw

[kettanaito]

Hi. Thanks for reporting this. @chawax, thanks for the extensive discussion. Let's get to the bottom of this.

Root cause

First, let's see why the request fails. If we inspect the actual error preserved by Undici, we see this:

    DataCloneError: Transferring of ReadableStream cannot be properly polyfilled in this engine
        at throwUnpolyfillable (/msw-request-with-body-ignored/node_modules/core-js/modules/web.structured-clone.js:120:9)
        at tryToTransfer (/msw-request-with-body-ignored/node_modules/core-js/modules/web.structured-clone.js:485:9)
        at structuredClone (/msw-request-with-body-ignored/node_modules/core-js/modules/web.structured-clone.js:520:17)
        at cloneBody (/msw-request-with-body-ignored/node_modules/undici/lib/fetch/body.js:277:21)

This hints that Jest is having serious trouble with ReadableStream.

This error is caused by core-js. One of our community members shared that removing core-js solved this error for them: mswjs/interceptors#475 (comment).

To be more specific, this is what happens:

1. undici tries to clone the response body using the standard structuredClone function. This is a global function in Node.js that can clone ReadableStream just fine.

// node_modules/undici/lib/fetch/body.js:277:21
const out2Clone = structuredClone(out2, { transfer: [out2] })

2. But, since we are using Jest, we need to rely on core-js to polyfill structuredClone. The structuredClone from core-js just throws if the data it tries to transfer is ReadableStream, claiming it as unpolyfillable:

// node_modules/core-js/modules/web.structured-clone.js:485:9
switch (type) {
  ...
  case 'MediaSourceHandle':
  case 'MessagePort':
  case 'OffscreenCanvas':
  case 'ReadableStream':
  case 'TransformStream':
  case 'WritableStream':
    throwUnpolyfillable(type, TRANSFERRING);
}

This is a bold statement, especially since we polyfill ReadableStream in jest.polyfills.js. But core-js never checks for that, just assumes a no-op and throws.

The solution seems to be, once again, force Jest to use the platform, to use Node.js where things function correctly.

[kettanaito]

Jest depends on core-js, there's nothing we can do about that. Node.js doesn't provide the means to directly import structuredClone, it's set directly on the internal messaging binding in C++. No way to suggest require('node:???').structuredClone to "polyfill" it in jest.polyfills.js.

There's literally nothing I can do here. Jest doesn't respect your environment. I highly recommend you migrate away from Jest to tools like Vitest and forget about these sorts of errors. I know it's not ideal, I know migrations take time, but it's either one-time migration or lifetime of dealing with this shenanigans that nobody but Jest can properly address.

[kettanaito]

Some folks are having success by downgrading undici to v5: #1934 (reply in thread). This may be a temporary option for those who cannot migrate to other testing frameworks as of now.

[chawax]

It looks like the problem I have too : #1915

[chawax]

Yes, migrating to Vitest is definitely the only long term solution

[kettanaito]

Also a note of warning: this seems to be specific to JSDOM. ReadableStream and other Node.js globals work fine in Jest's node environment. But since jest-environment-jsdom isn't based on jest-environment-node, you don't get that. You can try swapping it with HappyDOM to see if they respect the Node.js globals better.

[chawax]

Well, I was using HappyDOM in my project, and it looks like that was the reason why downgrading Undici didn't work for me. Using Jest Environement JSDom solved instead of HappyDOM solved my problem indeed.

[kettanaito]

Related.