From ea0bcb86902fddc60f805b4bee10a392122d379a Mon Sep 17 00:00:00 2001
From: ladybluenotes <hello@sarahgerrard.me>
Date: Thu, 10 Sep 2026 20:01:22 -0700
Subject: [PATCH] perf(history): avoid temporary arrays in memory history

---
 .changeset/history-forward-entries.md         |  5 +++
 packages/history/src/index.ts                 | 10 +++---
 .../history/tests/createMemoryHistory.test.ts | 32 +++++++++++++++++++
 3 files changed, 42 insertions(+), 5 deletions(-)
 create mode 100644 .changeset/history-forward-entries.md

diff --git a/.changeset/history-forward-entries.md b/.changeset/history-forward-entries.md
new file mode 100644
index 00000000000..a4517e4c6a5
--- /dev/null
+++ b/.changeset/history-forward-entries.md
@@ -0,0 +1,5 @@
+---
+'@tanstack/history': patch
+---
+
+Avoid temporary arrays when replacing forward memory-history entries.
diff --git a/packages/history/src/index.ts b/packages/history/src/index.ts
index b0e006c80da..ae89e4aae08 100644
--- a/packages/history/src/index.ts
+++ b/packages/history/src/index.ts
@@ -663,13 +663,13 @@ export function createMemoryHistory(
     getLength: () => entries.length,
     pushState: (path, state) => {
       // Removes all subsequent entries after the current index to start a new branch
+      index = index < entries.length - 1 ? index + 1 : entries.length
+      states[index] = state
+      entries[index] = path
       if (index < entries.length - 1) {
-        entries.splice(index + 1)
-        states.splice(index + 1)
+        entries.length = index + 1
+        states.length = index + 1
       }
-      states.push(state)
-      entries.push(path)
-      index = Math.max(entries.length - 1, 0)
     },
     replaceState: (path, state) => {
       states[index] = state
diff --git a/packages/history/tests/createMemoryHistory.test.ts b/packages/history/tests/createMemoryHistory.test.ts
index 11a0c269b34..c3162ab7de1 100644
--- a/packages/history/tests/createMemoryHistory.test.ts
+++ b/packages/history/tests/createMemoryHistory.test.ts
@@ -70,6 +70,38 @@ describe('createMemoryHistory', () => {
     expect(history.location.pathname).toBe('/b')
   })
 
+  test('discards forward entries and their state when pushing a new branch', () => {
+    const initialEntries = ['/']
+    const history = createMemoryHistory({ initialEntries })
+    history.push('/kept', { marker: 'kept' })
+    const keptState = history.location.state
+    history.push('/discarded-first', { marker: 'discarded-first' })
+    history.push('/discarded-last', { marker: 'discarded-last' })
+    history.go(-2)
+    history.push('/new', { marker: 'new' })
+
+    expect(initialEntries).toEqual(['/', '/kept', '/new'])
+    expect(history.length).toBe(3)
+    expect(history.location.pathname).toBe('/new')
+    expect(history.location.state).toMatchObject({
+      __TSR_index: 2,
+      marker: 'new',
+    })
+    const newState = history.location.state
+
+    history.forward()
+    expect(history.location.pathname).toBe('/new')
+    expect(history.location.state).toBe(newState)
+
+    history.back()
+    expect(history.location.pathname).toBe('/kept')
+    expect(history.location.state).toBe(keptState)
+
+    history.forward()
+    expect(history.location.pathname).toBe('/new')
+    expect(history.location.state).toBe(newState)
+  })
+
   test('length', () => {
     const history = createMemoryHistory()
     expect(history.length).toBe(1)