refactor(router-core): server-only fast path optimization for interpolatePath

[Sheraff]

interpolatePath is in the hot path of almost everything, so anything we can squeeze out of it might be worth it. This PR proposes a code-path that doesn't require parseSegment, but only works for 2 kinds of segments: /$param and /$

The lines-of-code to performance ratio isn't good enough for client-side code, so this hot path is gated behind an isServer check, and should be fully removed by dead-cod elimination for client-side builds.

before

after

Summary by CodeRabbit

• Performance
  • Faster server-side path interpolation for dynamic routes and common wildcard cases, preserving existing behavior (including trailing-slash and parameter handling).
• Tests
  • Expanded and reorganized tests to verify consistent interpolation behavior across server and client contexts; end-user behavior unchanged.

_{✏️ Tip: You can customize this high-level summary in your review settings.}

[coderabbitai]

📝 Walkthrough

Walkthrough

Adds a server-aware fast-path to interpolatePath for simple dollar-based templates, threads a server flag from RouterCore into interpolatePath calls, and parameterizes tests to run with server: true and server: false.

Changes

Cohort / File(s)	Summary
Path interpolation fast-path packages/router-core/src/path.ts	Imported isServer, added server?: boolean to InterpolatePathOptions, updated interpolatePath signature, and implemented a server-mode fast path for simple $id/$ splat segments that encodes params, tracks used/missing params, and returns early; falls back to existing parser for brace-based templates.
RouterCore server propagation packages/router-core/src/router.ts	Updated interpolatePath call sites (in buildRoute and buildLocation) to pass server: this.isServer.
Tests: server-aware parameterization packages/router-core/tests/path.test.ts	Reworked tests to run under describe.each([{ server: true }, { server: false }]) and pass the server flag to interpolatePath, preserving expected interpolation outputs.

Sequence Diagram(s)

(omitted — changes are internal and do not introduce a multi-component sequential flow warranting a diagram)

Estimated code review effort

🎯 4 (Complex) | ⏱️ ~45 minutes

Possibly related PRs

• refactor(router-core): interpolatePath skips optional segments w/ nullish param value #5934 — refactors optional-segment handling in interpolatePath, overlapping the interpolation logic.
• refactor: no additional package for isServer #6485 — changes around isServer exports/usage that relate to threading server-awareness into router-core.
• refactor(router-core): improve interpolatePath performance by pre-compiling a single regex decoder #6471 — modifications to interpolatePath implementation and decoder handling that intersect with this change.

Suggested labels

performance

Suggested reviewers

• schiller-manuel
• nlynzaad

Poem

🐰 Hoppity-hop, the server leads the race,
Dollar-wildcards find their place,
Fast-path hops through segments neat,
Params encoded, missing tracked — so sweet! 🥕

🚥 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: a server-only fast path optimization added to interpolatePath function in router-core package.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.

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

✨ Finishing touches

• 📝 Generate docstrings

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[nx-cloud]

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---

☁️ Nx Cloud last updated this comment at 2026-01-25 13:34:31 UTC

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commit: 129d310

[coderabbitai]

Actionable comments posted: 1

🤖 Fix all issues with AI agents

In `@packages/router-core/src/path.ts`:
- Line 279: The condition currently uses "isServer ?? server" which ignores the
test parameter "server" because "isServer" is always defined; change the
evaluation to prefer the function parameter first (e.g., use "server ??
isServer" or an explicit ternary like "server !== undefined ? server :
isServer") so tests can override the imported "isServer" constant; update the
condition where "isServer ?? server" appears in path.ts accordingly.