From 7edd7ea841dafda71dd2e6eef348c8194a70ec90 Mon Sep 17 00:00:00 2001
From: ladybluenotes <hello@sarahgerrard.me>
Date: Thu, 10 Sep 2026 19:39:08 -0700
Subject: [PATCH] perf(history): avoid temporary arrays in hash parsing

---
 .changeset/history-hash-allocations.md        |  5 ++
 packages/history/src/index.ts                 | 16 +++--
 .../history/tests/createHashHistory.test.ts   | 72 +++++++++++++++++++
 3 files changed, 86 insertions(+), 7 deletions(-)
 create mode 100644 .changeset/history-hash-allocations.md

diff --git a/.changeset/history-hash-allocations.md b/.changeset/history-hash-allocations.md
new file mode 100644
index 00000000000..ba8755bb97e
--- /dev/null
+++ b/.changeset/history-hash-allocations.md
@@ -0,0 +1,5 @@
+---
+'@tanstack/history': patch
+---
+
+Avoid temporary arrays when reading hash-history locations.
diff --git a/packages/history/src/index.ts b/packages/history/src/index.ts
index b0e006c80da..6c80f201ca4 100644
--- a/packages/history/src/index.ts
+++ b/packages/history/src/index.ts
@@ -616,13 +616,15 @@ export function createHashHistory(opts?: { window?: any }): RouterHistory {
   return createBrowserHistory({
     window: win,
     parseLocation: () => {
-      const hashSplit = win.location.hash.split('#').slice(1)
-      const pathPart = hashSplit[0] ?? '/'
-      const searchPart = win.location.search
-      const hashEntries = hashSplit.slice(1)
-      const hashPart =
-        hashEntries.length === 0 ? '' : `#${hashEntries.join('#')}`
-      const hashHref = `${pathPart}${searchPart}${hashPart}`
+      const hash = win.location.hash
+      const pathStart = hash.indexOf('#') + 1
+      const hashIndex = hash.indexOf('#', pathStart)
+      const pathPart =
+        pathStart === 0
+          ? '/'
+          : hash.slice(pathStart, hashIndex === -1 ? undefined : hashIndex)
+      const hashPart = hashIndex === -1 ? '' : hash.slice(hashIndex)
+      const hashHref = `${pathPart}${win.location.search}${hashPart}`
       return parseHref(hashHref, win.history.state)
     },
     createHref: (href) =>
diff --git a/packages/history/tests/createHashHistory.test.ts b/packages/history/tests/createHashHistory.test.ts
index 621cefe1568..da3b4d68a1c 100644
--- a/packages/history/tests/createHashHistory.test.ts
+++ b/packages/history/tests/createHashHistory.test.ts
@@ -59,6 +59,78 @@ describe('createHashHistory', () => {
     history.destroy()
   })
 
+  test.each([
+    [
+      '/?shell=1#/hello#section#tail',
+      {
+        href: '/hello?shell=1#section#tail',
+        pathname: '/hello',
+        search: '?shell=1',
+        hash: '#section#tail',
+      },
+    ],
+    [
+      '/?shell=1#/hello?route=2#section',
+      {
+        href: '/hello?route=2?shell=1#section',
+        pathname: '/hello',
+        search: '?route=2?shell=1',
+        hash: '#section',
+      },
+    ],
+    [
+      '/#/hello##tail#',
+      {
+        href: '/hello##tail#',
+        pathname: '/hello',
+        search: '',
+        hash: '##tail#',
+      },
+    ],
+    [
+      '/#/hello%23nested?value=%23#anchor%23tail',
+      {
+        href: '/hello%23nested?value=%23#anchor%23tail',
+        pathname: '/hello%23nested',
+        search: '?value=%23',
+        hash: '#anchor%23tail',
+      },
+    ],
+    [
+      '/?shell=1#//evil.example/path#fragment',
+      {
+        href: '/evil.example/path?shell=1#fragment',
+        pathname: '/evil.example/path',
+        search: '?shell=1',
+        hash: '#fragment',
+      },
+    ],
+    [
+      '/?shell=1#',
+      {
+        href: '/?shell=1',
+        pathname: '/',
+        search: '?shell=1',
+        hash: '',
+      },
+    ],
+  ])('preserves the logical URL when reading %s', (href, expected) => {
+    const originalHref = window.location.href
+    const originalState = window.history.state
+    window.history.replaceState(null, '', href)
+    const history = createHashHistory()
+
+    try {
+      expect(history.location).toMatchObject(expected)
+      window.history.replaceState(window.history.state, '', '/shell')
+      window.history.pushState(window.history.state, '', href)
+      expect(history.location).toMatchObject(expected)
+    } finally {
+      history.destroy()
+      window.history.replaceState(originalState, '', originalHref)
+    }
+  })
+
   describe('parseLocation', () => {
     describe.each([
       ['/', { pathname: '/', search: '' }, 'neither search params nor hash'],