fix(router-core): don't set isServer if NODE_ENV test

[roduyemi]

Summary by CodeRabbit

• Bug Fixes
  • Enhanced server environment detection to properly handle test environments separately from production configurations.

[coderabbitai]

📝 Walkthrough

Walkthrough

The isServer constant in packages/router-core/src/isServer/server.ts is converted from a boolean literal to a conditional expression that returns undefined in test environments and true otherwise, changing both its value and typing.

Changes

Cohort / File(s)	Summary
Environment-Based Server Export packages/router-core/src/isServer/server.ts	Modified isServer export from constant true to conditional that yields undefined when NODE_ENV is 'test' and true otherwise. Type signature now includes undefined in test environments.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~7 minutes

Poem

🐰 A rabbit hops through code so bright,
Where servers know if day or night,
But tests now make the truth unclear—
undefined whispers: "I'm not here!"
In test-land, facts dissolve to mist,
While true servers can't resist! 🌙

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately describes the main change: modifying the isServer export to handle the test environment case by returning undefined when NODE_ENV is 'test'.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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

✨ Finishing touches

• 📝 Generate docstrings

🧪 Generate unit tests (beta)

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

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[nx-cloud]

View your CI Pipeline Execution ↗ for commit c34ebb7

Command	Status	Duration	Result
nx run tanstack-router-e2e-bundle-size:build --...	✅ Succeeded	1m 26s	View ↗

---

☁️ Nx Cloud last updated this comment at 2026-02-21 21:44:51 UTC

[roduyemi]

@Sheraff may you please help review and merge this? Thanks

[pkg-pr-new]

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commit: c34ebb7

[roduyemi]

@Sheraff there is a bug in your codebase whereby the @tanstack/router-core/isServer variable is set in test mode. May you please either review and merge this PR or raise another to fix it please? It’s blocking us (and I’m sure many others) from being able to upgrade to the latest version. Thanks 🙏

[roduyemi]

@Sheraff? 🥹

[schiller-manuel]

closed in favor of #6727 to get bundle size measurements