perf(history): avoid generating unused location keys

.changeset/lazy-locations-listen.md

@@ -0,0 +1,5 @@
+---
+'@tanstack/history': patch
+---
+
+Avoid generating unused history keys for locations with existing state.

packages/history/src/index.ts

@@ -697,8 +697,10 @@ export function parseHref(
   const sanitizedHref = normalizeHref(href)
   const hashIndex = sanitizedHref.indexOf('#')
   const searchIndex = sanitizedHref.indexOf('?')
-
-  const addedKey = createRandomKey()
+  if (!state) {
+    const key = createRandomKey()
+    state = { [stateIndexKey]: 0, key, __TSR_key: key }
+  }
 
   return {
     href: sanitizedHref,
@@ -720,7 +722,7 @@ export function parseHref(
             hashIndex === -1 ? undefined : hashIndex,
           )
         : '',
-    state: state || { [stateIndexKey]: 0, key: addedKey, __TSR_key: addedKey },
+    state,
   }
 }
 

packages/history/tests/parseHref.test.ts

@@ -21,6 +21,30 @@ describe('parseHref', () => {
     expect(parsed.hash).toEqual('#qux')
   })
 
+  test.each([
+    { __TSR_index: 2 },
+    { __TSR_index: 2, key: 'legacy-key', __TSR_key: 'current-key' },
+  ])('preserves supplied state %j', (state) => {
+    Object.freeze(state)
+    expect(parseHref('/foo', state).state).toBe(state)
+  })
+
+  test('creates independent initial states with matching history keys', () => {
+    const first = parseHref('/foo', undefined)
+    const second = parseHref('/bar', undefined)
+
+    for (const location of [first, second]) {
+      expect(location.state).toEqual({
+        __TSR_index: 0,
+        key: expect.any(String),
+        __TSR_key: location.state.key,
+      })
+    }
+
+    first.state.__TSR_index = 1
+    expect(second.state.__TSR_index).toBe(0)
+  })
+
   describe('open redirect prevention', () => {
     test('strips CR characters to prevent open redirect', () => {
       // If \r (CR) is in the href, it should be stripped