feat!: change root package.json type to module

[mark-wiemer]

PR Checklist

• Addresses an existing open issue: fixes 🛠️ Repo: Switch from CJS to ESM internally #5400
• That issue was marked as status: accepting prs
• Steps in CONTRIBUTING.md were taken

Overview

My version of #6034, rebased to reduce merge conflicts and respect Josh's fork

• Changes package.json to "type": "module" so we Mocha is now an ESM-first package
• Renames js files to cjs and mjs files to js
• Revert options.mjs to CJS (options.cjs) due to rewiremock usage and low ROI for implementing (risky!) replacement

---

• change ESM files back to using mjs for clarity? Need alignment with Josh as other maintainer
• several files in lib/cli were reverted to CJS, was that intentional? Only options.mjs is called out in description.
• test/integration/compiler-cjs.spec.cjs ?
• need lint rule to ensure js extensions aren't used in spaces where only cjs is expected, e.g. npm run test-node:unit
• comments in feat!: change root package.json type to module #6034

[codecov]

Codecov Report

❌ Patch coverage is 97.27273% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.15%. Comparing base (7c9b004) to head (bbdeaf4).

Files with missing lines	Patch %	Lines
.wallaby.cjs	0.00%	1 Missing ⚠️
bin/mocha.js	0.00%	1 Missing ⚠️
lib/cli/index.cjs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6078      +/-   ##
==========================================
+ Coverage   78.83%   82.15%   +3.31%     
==========================================
  Files          63       62       -1     
  Lines        4546     4566      +20     
  Branches      989      991       +2     
==========================================
+ Hits         3584     3751     +167     
+ Misses        962      815     -147     

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[mark-wiemer]

Playwright ESM test is failing on Windows + Node 20.19.0, all other tests passing, haven't tested any other OS or Node version yet:

$ npx playwright test
2026-06-16 20:17:34

Running 4 tests using 4 workers

  ✓  1 [chromium] › test\browser\run.spec.mjs:6:3 › browser suite: tdd (351ms)
  ✘  2 [chromium] › test\browser\run.spec.mjs:6:3 › browser suite: esm (297ms)
  ✓  3 [chromium] › test\browser\run.spec.mjs:6:3 › browser suite: qunit (301ms)
  ✓  4 [chromium] › test\browser\run.spec.mjs:6:3 › browser suite: bdd (320ms)


  1) [chromium] › test\browser\run.spec.mjs:6:3 › browser suite: esm ───────────────────────────────

    Error: expect(received).toBeGreaterThan(expected)

    Expected: > 0
    Received:   0

      18 |     );
      19 |
    > 20 |     expect(results.total).toBeGreaterThan(0);
         |                           ^
      21 |   });
      22 | }
      23 |
        at C:\Users\markw\my-stuff\hello-hello\packages\mocha\packages\mocha\test\browser\run.spec.mjs:20:27

    Error Context: test-results\run-browser-suite-esm-chromium\error-context.md