[GrahamCampbell]

PR Checklist

• Addresses an existing open issue: fixes 🐛 Bug: parallel mode crashes at startup with Cannot find module './serializer' in 12.0.0-beta-9.5 #6054
• That issue was marked as status: accepting prs
• Steps in CONTRIBUTING.md were taken

Overview

#6011 renamed lib/nodejs/serializer.js to serializer.mjs and updated one of its three consumers (lib/nodejs/worker.js), but the other two still require the module without an extension. CommonJS resolution never tries .mjs, so both requires fail unconditionally, and as of 12.0.0-beta-9.5 any --parallel run crashes at startup with Cannot find module './serializer' (see #6054 for a minimal reproduction).

This PR updates the three remaining stale requires to reference ./serializer.mjs explicitly, mirroring what worker.js already does:

• lib/nodejs/buffered-worker-pool.js — loaded in the main process via Mocha.parallelMode(); this is the startup crash.
• lib/nodejs/reporters/parallel-buffered.js — loaded in the worker processes; masked by the first failure until that one is fixed.
• test/node-unit/serializer.spec.js — the serializer spec itself cannot currently load on main for the same reason (its sibling worker.spec.js was already updated in chore: convert lib/nodejs/ utility files to ESM #6011).

require() of a synchronous ESM graph is supported at Mocha's engines floor (^20.19.0 || >=22.12.0), so no loader changes are needed. Verified by applying the two lib/ changes to the published 12.0.0-beta-9.5 package: --parallel goes from the startup crash to passing, and serial runs are unaffected.

No new test is added: the fix makes the existing serializer spec loadable again, and any parallel-mode integration test exercises the repaired path by construction. #5969 (converting the worker-pool files themselves to ESM) will subsume these requires eventually; this is the minimal interim fix so the next beta doesn't ship with parallel mode unusable.

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: GrahamCampbell / name: Graham Campbell (78584fd)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.12%. Comparing base (8df3ba6) to head (78584fd).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6055      +/-   ##
==========================================
+ Coverage   78.08%   78.12%   +0.04%     
==========================================
  Files          63       63              
  Lines        4590     4590              
  Branches      977      977              
==========================================
+ Hits         3584     3586       +2     
+ Misses       1006     1004       -2     

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[GrahamCampbell]

The failing test is an existing flaky issue. I've prepared PRs to fix this:

• 11.x: test: synchronize watch integration tests on observed runs instead of sleeps #6059
• 12.x: chore: synchronize watch integration tests on observed runs instead of sleeps #6058

[JoshuaKGoldberg]

Thanks for the fix!