🐛 Bug: Mocha v11.7.3 and v11.7.4 fails with error ERR_REQUIRE_ESM

[Geam]

Bug Report Checklist

• I have read and agree to Mocha's Code of Conduct and Contributing Guidelines
• I have searched for related issues and issues with the faq label, but none matched my issue.
• I have 'smoke tested' the code to be tested by running it outside the real test suite to get a better sense of whether the problem is in the code under test, my usage of Mocha, or Mocha itself.
• I want to provide a PR to resolve this

Expected

Unit tests runs as expected

Actual

Unit tests fail with error ERR_REQUIRE_ESM

After some investigation, this problem is the same as #5387 which was fixed in 11.7.1 and 11.7.2 but is broken again in 11.7.3 and 11.7.4 .
The problem has been re-introduced with PR #5408 and the issue might be in lib/nodejs/esm-utils.js line 108 in the PR, the condition reads

      if (
        /\.(cts|mts|ts)$/.test(file) || // <- or
        importErr.code === 'ERR_UNKNOWN_FILE_EXTENSION'
      ) {
        throw requireErr;
      }

but in the PR description it is

If the file being imported is a .cts, .mts, or .ts and the caught error from import() is for an unknown file extension, we know it's from a Node.js version that doesn't support those file extensions.

or in the code, and in description.

Note : While making the minimal reproducible example, it appears that if the imported module has no async bit the tests are not run and fail silently.

Minimal, Reproducible Example

// src/main.test.ts
import "./other.js";

describe("the test", function () {
  it("should work", function () {
    console.log("ok");
  });
});

// src/other.ts
export const ok = () => {
  console.log("ok");
};

// there must be an async involved somewhere or the error does not trigger
await Promise.resolve("something");

// simulate an error in a module
throw new Error("stop");

npx mocha '--import=data:text/javascript,import{register}from"node:module";import{pathToFileURL}from"node:url";register("ts-node/esm",pathToFileURL("./"));' --enable-source-maps --inspect 'src/*.test.ts'

Versions

v11.7.3, v11.7.4
Nodejs v24.3.0

Additional Info

No response

[mini-bomba]

I've just spent 15 minutes debugging the exact same issue.
This issue can be triggered by even a simple syntax error in a test file.
Here's my reproduction steps:

• download the typescript mocha example
• apply this patch to the repository
• npm run test - tests should run without errors
• introduce any syntax error to the index.spec.ts file (such as a single ( with no corresponding ))
• npm run test - mocha throws the require() error instead of the actual syntax error due to the mistake described by the initial post

[mark-wiemer]

Thanks guys, let me confirm with @JoshuaKGoldberg but changing "or" to "and" looks like a good candidate for a fix

[mark-wiemer]

I'll note this bug only repros if Node is already in "module" mode. For Mocha, since it doesn't have "type": "module" in its package.json, that's not the case. The repro commit above does work, and changing the mentioned "or" to an "and" does fix it. This looks like the intended fix, I'll just need to find a way to write an automated test for this within Mocha!

[mark-wiemer]

Ref:

• 🛠 Repo: CI run failures (test failures) #5361 (comment)
• https://nodejs.org/docs/v22.20.0/api/module.html#customization-hooks
• https://github.com/mochajs/mocha/compare/fix-5361-esm-loader

[JoshuaKGoldberg]

👍 Great investigation @mark-wiemer, I concur!