fix: fall back to fetch when prerender returns 4xx

[Kikobeats]

Bug and impact

prerender() starts got and Chrome in parallel, then treats any page.content() as success and cancels fetch. A CloudFront/WAF 403 document is still a successful page.content(), so a parallel 200 fetch is dropped. That is how https://www.un.org/en/about-us kept the challenge page while prerender=false returned the real HTML.

Root cause

Chrome success is defined as “got a document”, not “got a usable status”. Fetch is only used when Chrome throws.

Fix

If prerender statusCode >= 400, wait for fetch. Use fetch when it is a 2xx/3xx. Otherwise keep the prerender document (both blocked, or fetch rejected).

Validation

• prerender 4xx falls back to a successful fetch — mocked Chrome 403 + local 200 → mode: fetch, statusCode: 200
• prerender 4xx is kept when fetch is also unsuccessful — mocked Chrome 403 + local 403 → keep prerender 403
• pnpm exec ava test/mode.js passed

Made with Cursor

Summary by CodeRabbit

• Bug Fixes

  • Improved handling of prerendered pages returning client or server error statuses.
  • When available, the app now displays successfully fetched content instead of the prerendered error response.
  • Preserved prerendered error pages when the fallback fetch is unsuccessful.

• Tests

  • Added coverage for successful and unsuccessful fallback behavior, including response status, rendering mode, and displayed content.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: 502498fd-fe4a-4fe3-9391-05bf96c4f128

📥 Commits

Reviewing files that changed from the base of the PR and between 0063335 and b189095.

📒 Files selected for processing (2)

• src/index.js
• test/mode.js

Included review availability: Your plan includes up to 2 reviews per rolling hour; 1 remains after this review.

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📝 Walkthrough

Walkthrough

The prerender flow now checks error responses before canceling the fetch request. It returns a successful fetch response when available and preserves the prerender response when the fetch also fails. Tests cover both outcomes.

Changes

Prerender fallback handling

Layer / File(s)	Summary
Error response fallback logic src/index.js	For prerendered 4xx or 5xx responses, the flow returns a non-error fetch response when available. Otherwise, it continues with the prerender response.
Fallback mode validation test/mode.js	Tests verify that a successful fallback returns fetched content in fetch mode and that a failed fallback preserves prerender mode, status, and error HTML.

Estimated code review effort: 2 (Simple) | ~10 minutes

Merge Risk: ⚪ Minimal · up to b1890

This localized change makes prerendered 4xx responses fall back to a successful fetch while preserving the prerender result when both responses fail; no actionable merge-blocking risk remains after normal checks and review.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: using fetch as a fallback when prerender returns a 4xx response.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/prerender-4xx-fallback

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