[JoshuaKGoldberg]

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

At long last:

• package.json's type is switched from "commonjs" to "module"
• Files are renamed from (.js, .mjs) to (.cjs, .js)

The only "backwards" change here is lib/cli/options.mjs -> lib/cli/options.cjs. It's used in requiremock tests and we haven't migrated to a mocker that supports ESM yet.

💖

[JoshuaKGoldberg]

Build failures seem to just be codecov/codecov-action#1876.

[mark-wiemer]

Tests are passing on my Windows machine, Node 20.19.0, with the patched #6040 (ref mark-wiemer@08f921b), no unusual output aside from expected EBADENGINE when installing docs dependencies :)

[mark-wiemer]

Build failures seem to just be codecov/codecov-action#1876.

That bug is old and about random failures, which matches #6036 . I've created a new bug for consistent failures today: #6042 .

[mark-wiemer]

Test failed on my Windows machine with Node v22.12.0:

  346 passing (3m)
  2 pending
  1 failing

  1) FIFO support
       should accept a test passed as a FIFO:
     Error: Couldn't parse JSON: Unexpected token '(', "(node:3652"... is not valid JSON

Original result output: (node:3652) ExperimentalWarning: CommonJS module C:\Users\markw\my-stuff\hello-hello\packages\mocha\packages\mocha\lib\cli\options.cjs is loading ES Module C:\Users\markw\my-stuff\hello-hello\packages\mocha\packages\mocha\lib\cli\one-and-dones.js using require().    
Support for loading ES Module in require() is an experimental feature and might change at any time
(Use `node --trace-warnings ...` to show where the warning was created)
{
  "stats": {
    "suites": 1,
    "tests": 1,
    "passes": 1,
    "pending": 0,
    "failures": 0,
    "start": "2026-06-07T03:12:02.281Z",
    "end": "2026-06-07T03:12:02.282Z",
    "duration": 1
  },
  "tests": [
    {
      "title": "should pass",
      "fullTitle": "suite from FIFO should pass",
      "file": "C:\\Users\\markw\\AppData\\Local\\Temp\\mocha-test-fifo-CH1ftF\\fifo-1",
      "duration": 0,
      "currentRetry": 0,
      "speed": "fast",
      "err": {}
    }
  ],
  "pending": [],
  "failures": [],
  "passes": [
    {
      "title": "should pass",
      "fullTitle": "suite from FIFO should pass",
      "file": "C:\\Users\\markw\\AppData\\Local\\Temp\\mocha-test-fifo-CH1ftF\\fifo-1",
      "duration": 0,
      "currentRetry": 0,
      "speed": "fast",
      "err": {}
    }
  ]
}
      at toJSONResult (test\integration\helpers.cjs:185:11)
      at invokeMocha.stdio (test\integration\fifo.spec.cjs:44:26)
      at ChildProcess.<anonymous> (test\integration\helpers.cjs:355:5)
      at ChildProcess.emit (node:events:524:28)
      at maybeClose (node:internal/child_process:1101:16)
      at ChildProcess._handle.onexit (node:internal/child_process:304:5)



ERROR: "test-node:integration" exited with 1.
ERROR: "test-node" exited with 1.

I'm going to pause my review now given this is in draft state and you're probably aware of some of these issues :) Happy to continue running Windows- or Linux-specific checks as needed, of course :) thanks for leading this one!

[mark-wiemer]

Partial review given draft state, matches what I want though! Mostly! ;)

[JoshuaKGoldberg]

Ha yeah, I figured I'd come back to this after the Codecov issues were done.

[mark-wiemer]

See also my PR: #6078

[JoshuaKGoldberg]

Closing out in favor of Mark's in-progress PR.