packages/query-core/src/utils.ts (1)

273-286: Micro-opt: hoist hasOwn and avoid double reads of b[key]

Hoist hasOwn and reuse bItem to shave a couple of lookups in tight loops.

Apply:

- let equalItems = 0
+ const hasOwn = Object.prototype.hasOwnProperty
+ let equalItems = 0
@@
- if (
- (array || Object.prototype.hasOwnProperty.call(a, key)) &&
+ if (
+ (array || hasOwn.call(a, key)) &&

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packages/query-core/src/utils.ts (1)

271-283: Cache bItem to avoid repeated property lookups inside the hot loop.

Reads b[key] twice per iteration. Caching improves locality and aligns with the PR’s stated perf goals.

 for (let i = 0; i < bSize; i++) {
 const key = array ? i : bItems[i]
 const aItem = a[key]
+ const bItem = b[key]
 if (
- (array || Object.prototype.hasOwnProperty.call(a, key)) &&
- aItem === undefined &&
- b[key] === undefined
+ (array || Object.prototype.hasOwnProperty.call(a, key)) &&
+ aItem === undefined &&
+ bItem === undefined
 ) {
 copy[key] = undefined
 equalItems++
 } else {
- const value = replaceEqualDeep(aItem, b[key])
+ const value = replaceEqualDeep(aItem, bItem)
 copy[key] = value
 if (value === aItem && aItem !== undefined) {
 equalItems++
 }
 }
 }

packages/query-core/src/utils.ts (1)

261-268: Early-return and pre-allocation look good; trim an unnecessary allocation.

aItems is only used to compute aSize. Avoid creating it to reduce work/memory.

- const aItems = array ? a : Object.keys(a)
- const aSize = aItems.length
+ const aSize = array ? a.length : Object.keys(a).length

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packages/query-core/src/utils.ts (1)

248-248: Good fix: prototype-safe own-property check and cached reads.

Resolved the earlier "shadowed hasOwnProperty" concern and reduced repeated property access by caching aItem/bItem. Looks solid.

Also applies to: 319-327

packages/query-core/src/utils.ts (3)

255-256: Prefer unknown over any in the implementation signature.

Keeping the overload replaceEqualDeep<T>(a: unknown, b: T): T is great. For the implementation, returning unknown avoids leaking any while staying compatible with the overloads.

-export function replaceEqualDeep(a: unknown, b: unknown): any {
+export function replaceEqualDeep(a: unknown, b: unknown): unknown {

---

311-345: Object path: tight loop and O(1) own-checks LGTM.

• for..in over b avoids materializing key arrays.
• Equality counting gated by hasOwn.call(a, k) is correct.
• Recursion preserves references where possible.

One note: symbol keys aren’t considered (consistent with JSON-only constraint). If that’s intentional, consider documenting it near replaceData to preempt surprises.

---

368-370: Sparse/augmented arrays won’t be treated as "plain arrays". Please confirm.

isPlainArray returns false for sparse arrays or arrays with extra enumerable props (length !== Object.keys(value).length). That changes behavior vs a naive Array.isArray check and will fall back to "return b" instead of structural sharing. If intentional, add tests for:

• sparse arrays (holes)
• arrays with extra enumerable props

I can draft these tests if desired.

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packages/query-core/src/utils.ts (1)

248-248: Prototype-safe own-property helper: LGTM

Using a cached reference and calling via .call avoids shadowing risks. This addresses the earlier review concern.

packages/query-core/src/utils.ts (1)

322-324: Clarify dense-array intent in isPlainArray

Minor: add a short doc comment noting that sparse arrays or arrays with extra enumerable props return false.

+/** True only for dense Arrays with no extra enumerable properties (sparse arrays excluded). */
 export function isPlainArray(value: unknown): value is Array<unknown> {
 return Array.isArray(value) && value.length === Object.keys(value).length
 }

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