fix: detect esm only pkg w/ main and module fields

[jonchurch]

Problem

In the Compare feature, the heuristic for legacy fallback for dual vs esm only detection has a bug currently where it assumes main contains CJS when the package also has a module field.

That reports ESM only packages w/ both esm main and module fields like lodash-es as dual packages.

  "type": "module",
  "jsnext:main": "lodash.js",
  "main": "lodash.js",
  "module": "lodash.js",

Above is an ESM only package description, but currently that will be reported as "ESM + CJS"

Solution

Infer if the main field should be CJS or ESM. If main is .cjs extension then we for sure know it's CJS. If we have a .js file and no "type": "module" then we have a CJS file.

📸

Before:

After:

[vercel]

The latest updates on your projects. Learn more about Vercel for GitHub.

Project	Deployment	Actions	Updated (UTC)
npmx.dev	Ready	Preview, Comment	Feb 14, 2026 7:11am

2 Skipped Deployments

Project	Deployment	Actions	Updated (UTC)
docs.npmx.dev	Ignored	Preview	Feb 14, 2026 7:11am
npmx-lunaria	Ignored		Feb 14, 2026 7:11am

[codecov]

Codecov Report

❌ Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
shared/utils/package-analysis.ts	0.00%	0 Missing and 2 partials ⚠️

📢 Thoughts on this report? Let us know!

[coderabbitai]

📝 Walkthrough

Walkthrough

The changes modify the module format detection logic in detectModuleFormat within the package analysis utility. Previously, when both module and main fields were present, the function always returned 'dual'. The new implementation conditionally returns 'dual' or 'esm' based on whether the main field ends with .cjs or .js (when package type is not module). Test cases have been added to verify both dual and ESModule detection scenarios with type, module, and main fields present.

Suggested reviewers

• danielroe
• ghostdevv

🚥 Pre-merge checks | ✅ 2

✅ Passed checks (2 passed)

Check name	Status	Explanation
Merge Conflict Detection	✅ Passed	✅ No merge conflicts detected when merging into main
Description check	✅ Passed	The pull request description clearly explains the bug in module format detection and provides a specific solution aligned with the code changes.

_{✏️ Tip: You can configure your own custom pre-merge checks in the settings.}

✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment

Tip

Issue Planner is now in beta. Read the docs and try it out! Share your feedback on Discord.

---

_{Comment @coderabbitai help to get the list of available commands and usage tips.}