fix: compare prerelease versions against previous stable for size diff

[joshuaisaact]

🔗 Linked issue

Resolves #2313

🧭 Context

When viewing an old prerelease version (e.g. oxlint@1.17.0-alpha.0), it compared against the latest version (in oxlint's case this was 1.57.0), not the stable version before the prerelease.

Since the latest version is newer and smaller, this showed a "size increase" warning on the old alpha. The package size of oxlint has decreased in newer versions, but we shouldn't say that the old alpha has 'increased'.

We should be comparing the old alpha against the closest previous stable.

📚 Description

I changed the prerelease comparison logic in getComparisonVersion to find the previous stable version in semver order, the same approach already used for stable versions.

The old code used the latest dist-tag as a shortcut, which broke when viewing prereleases older than latest.

The old prerelease behavior was:

1. Look up latest dist-tag
2. If no latest or prerelease IS latest, return null
3. Otherwise compare against latest

The new behavior is:

Find the highest stable version before this prerelease, in semver order.

Which matches the non-alpha behaviour, removing the (bugged) shortcut.

Also updated the tests while I was there, because I am nice.

[vercel]

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

Project	Deployment	Actions	Updated (UTC)
npmx.dev	Ready	Preview, Comment	Mar 30, 2026 6:00am

2 Skipped Deployments

Project	Deployment	Actions	Updated (UTC)
docs.npmx.dev	Ignored	Preview	Mar 30, 2026 6:00am
npmx-lunaria	Ignored		Mar 30, 2026 6:00am

[github-actions]

Hello! Thank you for opening your first PR to npmx, @joshuaisaact! 🚀

Here’s what will happen next:

1. Our GitHub bots will run to check your changes.
   If they spot any issues you will see some error messages on this PR.
   Don’t hesitate to ask any questions if you’re not sure what these mean!

2. In a few minutes, you’ll be able to see a preview of your changes on Vercel

3. One or more of our maintainers will take a look and may ask you to make changes.
   We try to be responsive, but don’t worry if this takes a few days.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ All tests successful. No failed tests found.

📢 Thoughts on this report? Let us know!

[coderabbitai]

📝 Walkthrough

Walkthrough

The changes update the prerelease version comparison logic in getComparisonVersion. Previously, when dealing with a prerelease version, the function returned the packument's dist-tags.latest tag value. The updated implementation now constructs a filtered, semver-sorted list of stable versions and returns the highest stable version with a lower version number than the prerelease. Non-prerelease version handling remains unchanged. Corresponding test cases were updated to reflect this new prerelease comparison behaviour.

Possibly related PRs

• fix: don't diff install size with first version #2175: Modifies getComparisonVersion to change comparison baseline selection with semver-filtered stable lookup for prereleases and index guard adjustments.
• feat: show a warning when significant size increase #1673: Updates getComparisonVersion prerelease comparison logic to select the prior stable version instead of using dist-tags.latest.

Suggested reviewers

• ghostdevv
• danielroe

🚥 Pre-merge checks | ✅ 1

✅ Passed checks (1 passed)

Check name	Status	Explanation
Description check	✅ Passed	The pull request description clearly relates to and explains the changeset, detailing the bug fix for prerelease version comparison logic and the rationale for the 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

---

Thanks for using CodeRabbit! It's free for OSS, and your support helps us grow. If you like it, consider giving us a shout-out.

❤️ Share

• X
• Mastodon
• Reddit
• LinkedIn

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