feat!: change root package.json type to module by JoshuaKGoldberg · Pull Request #6034 · mochajs/mocha

JoshuaKGoldberg · 2026-06-06T18:28:43Z

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 · 2026-06-07T01:37:44Z

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

mark-wiemer · 2026-06-07T03:10:48Z

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 · 2026-06-07T03:12:33Z

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 · 2026-06-07T03:25:28Z

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! ;)

mark-wiemer · 2026-06-07T03:15:13Z

 ```

-`mocha.run()` returns a `Runner` instance which emits many [events](https://github.com/mochajs/mocha/blob/9a7053349589344236b20301de965030eaabfea9/lib/runner.js#L52) of interest.
+`mocha.run()` returns a `Runner` instance which emits many [events](https://github.com/mochajs/mocha/blob/9a7053349589344236b20301de965030eaabfea9/lib/runner.cjs#L52) of interest.


Suggested change

`mocha.run()` returns a `Runner` instance which emits many [events](https://github.com/mochajs/mocha/blob/9a7053349589344236b20301de965030eaabfea9/lib/runner.cjs#L52) of interest.

`mocha.run()` returns a `Runner` instance which emits many [events](https://github.com/mochajs/mocha/blob/9a7053349589344236b20301de965030eaabfea9/lib/runner.js#L52) of interest.

Applied in my PR (along with the changes in main to change blob/9a7.../lib to blob/main/lib

mark-wiemer · 2026-06-07T03:17:13Z


 ```js
-// test/setup.js
+// test/setup.cjs


I'm on the fence about changing docs to recommend .cjs extensions for projects that others are creating. It might seem that Mocha only supports CJS files but of course we also support ESM. I think we should keep all examples of user code using the generic .js, both for simplifying this PR and for clarity.

Suggested change

// test/setup.cjs

// test/setup.js

Applied the change to setup.js in my PR

mark-wiemer · 2026-06-07T03:17:19Z


 ```bash
-mocha --file "./test/setup.js" "./test/**/*.spec.js"
+mocha --file "./test/setup.cjs" "./test/**/*.spec.js"


Suggested change

mocha --file "./test/setup.cjs" "./test/**/*.spec.js"

mocha --file "./test/setup.js" "./test/**/*.spec.js"

Applied this change in my PR

mark-wiemer · 2026-06-07T03:19:22Z

Hmm, I thought we agreed to keep mjs and cjs extensions and avoid js if at all possible. Maybe this is a premature review, nbd if you're already planning on reverting this change.

#5400 (comment)

If we are going to keep some files as CJS and some as ESM in v12 (which I'm OK with), we should always use .cjs and .mjs file extensions, never .js and ideally even add a lint rule for that. Just for absolute clarity :)

Me, with a like from Josh

Applies to multiple files

JoshuaKGoldberg · 2026-06-07T15:24:33Z

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

mark-wiemer · 2026-06-17T03:35:10Z

See also my PR: #6078

mark-wiemer · 2026-06-17T04:10:38Z

+      return resolved;
+    }
+
+    if (path.extname(fixture) === ".js") {


Should we even keep this safety-check? Or should we just remove it and manually fix failing tests?

(I will remove it and see if any tests fail... if none fail, I will be scared of false negatives though!)

mark-wiemer · 2026-06-17T04:12:12Z

-    ? fixture
-    : path.resolve(__dirname, "fixtures", fixture);
+
+  for (const extension of [".cjs", ".js", ".mjs", ".ts"]) {


This "find a fixture with a matching name and non-matching extension" seems like a significantly new behavior :/

mark-wiemer · 2026-06-17T04:19:27Z

              },
              "to throw",
-              "Unable to read /something/wherever: bad file message",
+              "Unable to read /something/wherever: bad file message: Sinon-provided bad file message",


is this a change we want to keep?

mark-wiemer · 2026-06-17T04:25:19Z

      sourceType: "module",
      ecmaVersion: 2025,
    },
+    rules: {


why this change?

mark-wiemer · 2026-06-17T04:27:15Z

        '" "' +
        path.join("bin", "mocha") +
-        '" -R json --require test/compiler-fixtures/js.fixture "test/compiler-cjs/*.js"',
+        '" -R json --require test/setup.cjs --require test/compiler-fixtures/js.fixture.cjs "test/compiler-cjs/*.js"',


why the new require in these few files?

JoshuaKGoldberg added 4 commits June 6, 2026 14:26

feat!: change root package.json type to module

194cc4b

npm run format:fix

0926dd7

integration test fixes

fa1a132

Merge branch 'main'

051c180

mark-wiemer reviewed Jun 7, 2026

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mark-wiemer mentioned this pull request Jun 17, 2026

feat!: change root package.json type to module #6078

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mark-wiemer reviewed Jun 17, 2026

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Comment thread test/integration/options/ui.spec.cjs

mark-wiemer reviewed Jun 17, 2026

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Comment thread eslint.config.cjs

sourceType: "module",

ecmaVersion: 2025,

},

rules: {

mark-wiemer Jun 17, 2026

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why this change?

mark-wiemer reviewed Jun 17, 2026

View reviewed changes

	`mocha.run()` returns a `Runner` instance which emits many [events](https://github.com/mochajs/mocha/blob/9a7053349589344236b20301de965030eaabfea9/lib/runner.cjs#L52) of interest.
	`mocha.run()` returns a `Runner` instance which emits many [events](https://github.com/mochajs/mocha/blob/9a7053349589344236b20301de965030eaabfea9/lib/runner.js#L52) of interest.

	mocha --file "./test/setup.cjs" "./test/*/.spec.js"
	mocha --file "./test/setup.js" "./test/*/.spec.js"

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JoshuaKGoldberg commented Jun 6, 2026

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JoshuaKGoldberg commented Jun 7, 2026

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JoshuaKGoldberg commented Jun 7, 2026

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