perf(history): avoid temporary arrays in memory history

[LadyBluenotes]

🎯 Changes

Write the next memory-history URL and state directly at the destination index, then truncate any remaining forward entries. This avoids the two unused arrays returned by splice when a push creates a new branch. Backward/forward navigation and retained state stay unchanged.

Performance

Local Node.js 24.12.0 comparison of 6494e75362 and ea0bcb8690. Allocation rows are medians of three heap-sampling profiles with a 4,096-byte sampling interval; history setup is outside the measurement window.

Workload	Before	After	Change
100,000 one-entry branches, sampled allocation bytes	174,676,312	163,961,904	-6.1%
4,096 pushes discarding 64 forward entries each, sampled allocation bytes	9,232,904	4,443,224	-51.9%
Standalone memory-history bundle, gzip	1,225 bytes	1,220 bytes	-5 bytes

These allocation estimates include objects collected by GC; they are not retained-heap measurements. The standalone bundle uses Vite 8.0.14, minified ESM, es2022, and gzip level 9. Browser-history-only output is byte-identical.

Longer ordinary-push measurements were effectively unchanged: median elapsed time was 80.392 ms before and 80.315 ms after; CPU time was 100.221 ms before and 100.421 ms after, across nine counterbalanced samples of 150,000 pushes. This PR claims allocation savings, not a stable navigation-speed improvement.

✅ Checklist

• I have followed the steps in the Contributing guide.
• I have tested code changes locally with the relevant test commands, or tests do not apply to this pull request.
• I fully understand the code in this pull request, including any code generated with AI assistance.

🚀 Release Impact

• This change affects published code, and I have generated a changeset.
• This change is docs/CI/dev-only (no release).

Summary by CodeRabbit

• Bug Fixes
  • Improved memory-history behavior when navigating back and creating a new entry, correctly removing forward entries while preserving the active history.
  • Back and forward navigation now maintains the expected entries and state.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

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Review profile: CHILL

Plan: Advanced

Run ID: c56dc9d1-7262-40be-a1a7-f998b5bddcd5

📥 Commits

Reviewing files that changed from the base of the PR and between 6494e75 and ea0bcb8.

📒 Files selected for processing (3)

• .changeset/history-forward-entries.md
• packages/history/src/index.ts
• packages/history/tests/createMemoryHistory.test.ts

Included review availability: Your plan provides up to 10 included reviews per hour; 7 remain after this review.

---

📝 Walkthrough

Walkthrough

Memory history now replaces forward entries without temporary arrays. A regression test verifies branch truncation, state retention, and navigation. A patch changeset records the update.

Changes

Memory history updates

Layer / File(s)	Summary
Forward branch replacement and validation packages/history/src/index.ts, packages/history/tests/createMemoryHistory.test.ts, .changeset/history-forward-entries.md	createMemoryHistory writes new entries directly and truncates forward history. Tests verify discarded entries, retained states, and navigation. The changeset declares a patch release.

Priority: ⬇️ Low

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

Suggested reviewers: sheraff

Merge Risk: ⚪ Minimal · up to ea0bc

The optimized memory-history branch replacement preserves the tested navigation and state behavior, with no current merge-blocking risk identified.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description check	✅ Passed	The description follows the repository template. It explains the change and motivation, reports performance measurements, completes the checklist, and identifies the generated changeset for the publis…
Title check	✅ Passed	The title clearly and concisely identifies the main change: avoiding temporary arrays in memory history. It matches the implementation and performance objective.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 1 functions across 2 files. (1 skipped: 1 …
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 perf/history-truncate-forward-entries

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[nx-cloud]

View your CI Pipeline Execution ↗ for commit ea0bcb8

Command	Status	Duration	Result
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---

☁️ Nx Cloud last updated this comment at 2026-09-11 03:14:38 UTC

[github-actions]

🚀 Changeset Version Preview

1 package(s) bumped directly, 23 bumped as dependents.

🟩 Patch bumps

Package	Version	Reason
@tanstack/history	1.162.3 → 1.162.4	Changeset
@tanstack/react-router	1.170.35 → 1.170.36	Dependent
@tanstack/react-start	1.168.52 → 1.168.53	Dependent
@tanstack/react-start-client	1.168.33 → 1.168.34	Dependent
@tanstack/react-start-rsc	0.1.51 → 0.1.52	Dependent
@tanstack/react-start-server	1.167.40 → 1.167.41	Dependent
@tanstack/router-cli	1.167.35 → 1.167.36	Dependent
@tanstack/router-core	1.171.29 → 1.171.30	Dependent
@tanstack/router-generator	1.167.35 → 1.167.36	Dependent
@tanstack/router-plugin	1.168.37 → 1.168.38	Dependent
@tanstack/router-vite-plugin	1.167.37 → 1.167.38	Dependent
@tanstack/solid-router	1.170.33 → 1.170.34	Dependent
@tanstack/solid-start	1.168.50 → 1.168.51	Dependent
@tanstack/solid-start-client	1.168.32 → 1.168.33	Dependent
@tanstack/solid-start-server	1.167.39 → 1.167.40	Dependent
@tanstack/start-client-core	1.170.29 → 1.170.30	Dependent
@tanstack/start-plugin-core	1.171.42 → 1.171.43	Dependent
@tanstack/start-server-core	1.169.34 → 1.169.35	Dependent
@tanstack/start-static-server-functions	1.167.34 → 1.167.35	Dependent
@tanstack/start-storage-context	1.167.31 → 1.167.32	Dependent
@tanstack/vue-router	1.170.32 → 1.170.33	Dependent
@tanstack/vue-start	1.168.49 → 1.168.50	Dependent
@tanstack/vue-start-client	1.167.35 → 1.167.36	Dependent
@tanstack/vue-start-server	1.167.39 → 1.167.40	Dependent

[github-actions]

Bundle Size Benchmarks

This pull request does not affect bundle size in any measured scenario.

[pkg-pr-new]

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commit: ea0bcb8

[codspeed-hq]

Merging this PR will not alter performance

✅ 180 untouched benchmarks

---

_{Comparing perf/history-truncate-forward-entries (ea0bcb8) with main (6494e75)}