[darthvader58]

PR Checklist

• Addresses an existing open issue: fixes 🐛 Bug: '.only' in parallel, typscript test gives confusing error #5579
• That issue was marked as status: accepting prs
• Steps in CONTRIBUTING.md were taken

Overview

Problem

When using Mocha in parallel mode with TypeScript via ts-node (CJS mode), if a test file contains .only and triggers a FORBIDDEN_EXCLUSIVITY error, the actual error message was buried. Instead of showing:

`.only` is not supported in parallel mode

Users would see a confusing module loading error:

Cannot use import statement outside a module

Root Cause

In lib/nodejs/esm-utils.js, the requireModule function had a fallback mechanism: when require() failed, it would attempt to use dynamic import(). This was intended to handle ESM/CJS interop issues.

However, when a Mocha-specific error (like FORBIDDEN_EXCLUSIVITY) was thrown during require(), it wasn't recognized as a "real" error that should be surfaced immediately. Instead, the code fell through to the import() fallback, which bypassed ts-node's CJS loader and produced a misleading syntax error.

Solution

Added a check to detect Mocha errors using the existing isMochaError() utility function. When a Mocha error is caught during require(), it's now thrown immediately instead of falling back to import().

This follows the same pattern established in PR #5572, which added similar handling for ts-node's TSError.

Changes Made

Files Modified:

1. lib/nodejs/esm-utils.js - Added Mocha error detection
2. test/node-unit/esm-utils.spec.js - Added test case
3. test/node-unit/fixtures/mock-mocha-forbidden-exclusivity-err.ts - Created test fixture

Code Changes:

• Imported isMochaError from ../errors
• Added check: if (isMochaError(requireErr)) { throw requireErr; }
• Positioned after the existing TSError check for consistency

Testing

• Created a test fixture that simulates the FORBIDDEN_EXCLUSIVITY error
• Added test case verifying Mocha errors are surfaced correctly
• All 580 existing tests continue to pass

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: darthvader58 / name: Shashwat Raj (1f0c3cd, 556f25f)

[JoshuaKGoldberg]

Makes sense to me, thanks! Since @mark-wiemer self-requested a review I'll defer to Mark.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.40%. Comparing base (e06cce7) to head (556f25f).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5647   +/-   ##
=======================================
  Coverage   88.40%   88.40%           
=======================================
  Files          66       66           
  Lines        4787     4788    +1     
  Branches      977      977           
=======================================
+ Hits         4232     4233    +1     
  Misses        555      555           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[mark-wiemer]

Thank you!