fix: replaceEqualDeep correctly handles arrays that contain undefined

fix ported from query: TanStack/query#7376

packages/react-router/src/utils.ts

@@ -138,10 +138,9 @@ export function replaceEqualDeep<T>(prev: any, _next: T): T {
     for (let i = 0; i < nextSize; i++) {
       const key = array ? i : nextItems[i]
       if (
-        !array &&
+        ((!array && prevItems.includes(key)) || array) &&
         prev[key] === undefined &&
-        next[key] === undefined &&
-        prevItems.includes(key)
+        next[key] === undefined
       ) {
         copy[key] = undefined
         equalItems++

packages/react-router/tests/utils.test.ts

@@ -280,6 +280,20 @@ describe('replaceEqualDeep', () => {
 
     expect(current).toBe(next)
   })
+
+  it('should return the previous value when both values are an array of undefined', () => {
+    const current = [undefined]
+    const next = replaceEqualDeep(current, [undefined])
+
+    expect(next).toBe(current)
+  })
+
+  it('should return the previous value when both values are an array that contains undefined', () => {
+    const current = [{ foo: 1 }, undefined]
+    const next = replaceEqualDeep(current, [{ foo: 1 }, undefined])
+
+    expect(next).toBe(current)
+  })
 })
 
 describe('isPlainArray', () => {