[nanami-0713]

What happens

Under environment: 'jsdom' with jsdom ≥ 30.1.0, every path that goes through makeCompatBlob throws:

TypeError: Cannot read properties of undefined (reading '_bytes')

This breaks URL.createObjectURL(new Blob(...)) and Request/FormData bodies holding a jsdom Blob (#11336). jsdom 30.0.1 still works; #11295 (shipped in 5.0.2) doesn't cover it because the failure happens one step earlier than the _buffer → _bytes rename it fixed.

Root cause

jsdom 30.1.0 moved the implementation object from the hidden Symbol(impl) own property into a private class field (#impl in wrapper). createCompatUtils finds the impl by scanning the own symbols of a Blob instance, so implSymbol is now undefined and blob[implSymbol] returns undefined:

// own symbols on `new window.Blob(['x'])`
// jsdom 30.0.1 -> [ Symbol(impl) ]
// jsdom 30.1.0 -> []

Fix

Use jsdom's own implForWrapper, which can unwrap both layouts. It is not exported from the package entry, but jsdom has no exports map, so the module subpath can be required directly. The new subpath (lib/generated/idl/utils.js, present in 29 and 30.1) is tried first, the old one (lib/jsdom/living/generated/utils.js, older releases) second, and the existing symbol scan stays as a last-resort fallback.

Testing

• Repo unit tests (test/unit/test/environments/jsdom.spec.ts) still pass on the repo's jsdom 29: 25/25, including the strengthened URL.createObjectUrl works properly assertions added here. Note the repo suite runs jsdom 29, where the old symbol path still works, so the 30.1-specific break cannot go red there — that's why the scratch-project run below is part of the verification.
• Scratch project pinning jsdom@30.1.0 and vitest from pnpm pack of this branch, running the two tests from the issue:
  • build of main (unpatched): 2 failed with exactly Cannot read properties of undefined (reading '_bytes')
  • build of this branch: 2 passed
• Verified implForWrapper resolution on both sides: jsdom 29.1.1 and 30.1.0 both export it from lib/generated/idl/utils.js (the old subpath is gone by 29 already), and on 30.1.0 implForWrapper(blob)._bytes yields the blob's bytes while the blob has zero own symbols.
• eslint passes on both changed files.

AI disclosure

The fix and this description were drafted with AI assistance (ZCode/GLM); the reproduction and verification chain above was executed by me on this machine against real jsdom 29/30.1 installs and a real build of this branch.

Fixes #11336

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[nanami-0713]

I am the author of this PR—a real human developer—and I would like to request a manual review after seeing my account flagged as a suspected bot by agentscan. Although this account is relatively new (registered in August 2026), it is my primary development account and hosts 34 public repositories.
As required by the AI ​​policy in the CONTRIBUTING guidelines, I have already disclosed at the end of the PR description that the work involved "AI assistance and local verification by the author."
I have locally executed the full reproduction steps for the fix; maintainers are welcome to re-run any specific step they choose.
Thanks.