Intercepting a FormData request containing a File causes a timeout for node >20

[pkawula]

Prerequisites

• I confirm my issue is not in the opened issues
• I confirm the Frequently Asked Questions didn't contain the answer to my issue

Environment check

• I'm using the latest msw version
• I'm using Node.js version 18 or higher

Node.js version

v22.11.0

Reproduction repository

https://github.com/pkawula/msw-node-22-fail

Reproduction steps

Clone the provided repository and use node 20 or 22, then run npm i && npm t.

As an example I'm attaching screenshots from:

1. Node 18.17.1

2. Node 20.14.0

3. Node 22.11.0

Current behavior

Tests are failing with timeout

Expected behavior

Tests should pass.

[kettanaito]

Hi, @pkawula. Thanks for reporting this.

My hunch tells me this is either a bug in Undici or in JSDOM (we don't officially support JSOM and I highly encourage developers to migrate from it).

For triage, can you please try installing undici explicitly, grabbing their FormData directly, and using it in tests? Does that make any difference? There were some issues around FormData in the past, but I trust they have been resolved and should be available in v22.

As the next step, you can try updating to v23 and see if the issue is still there (perhaps there was a bugfix that wasn't backported, although those are rare).

[thenick775]

+1

I'm seeing this on node 23.7.0 and 22.13.1, where 20.12.1 works just fine.

  http.post(
    `${gbaServerLocationPlaceholder}/api/save/upload`,
    async ({ request }) => {
      console.log('before formdata');
      const formData = await request.formData(); // never makes it past here
      console.log('after formdata');
      const save = formData.get('save') as File;
      const saveName = save.name;

      await delay();

      return new HttpResponse(null, { status: saveName == '400' ? 400 : 200 });
    }
  ),

"jsdom": "^26.0.0",
"msw": "^2.7.0",

Edit: the only thing that made this work on higher node versions was adding this to my setup.ts:

import 'blob-polyfill';

after installing:

npm i -D blob-polyfill