From f02ebba751e3fdd8e426b50297c71ba9a75c5740 Mon Sep 17 00:00:00 2001
From: Alexander Kireev <ak.chatman.media@gmail.com>
Date: Tue, 23 Jun 2026 03:19:54 +0700
Subject: [PATCH] =?UTF-8?q?fix:=20=F0=9F=90=9B=20useDeepCompareEffect=20th?=
 =?UTF-8?q?rows=20on=20null-prototype=20objects?=
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`isDeepEqual` delegated to `fast-deep-equal/react`, which assumes every
object inherits `valueOf`/`toString` from `Object.prototype` and calls them
unconditionally. Objects created with `Object.create(null)` have neither, so
the comparison threw `TypeError: a.valueOf is not a function` (closes #2700).

Inline the comparator and guard the `valueOf`/`toString` shortcuts behind a
real-method check so null-prototype objects fall through to the own-keys
comparison. Behaviour is otherwise identical (verified by a 200k-case fuzz
against the upstream module). The now-unused `fast-deep-equal` direct
dependency is dropped.
---
 package.json                       |  1 -
 src/misc/isDeepEqual.ts            | 81 +++++++++++++++++++++++++++++-
 tests/useDeepCompareEffect.test.ts | 26 ++++++++++
 3 files changed, 105 insertions(+), 3 deletions(-)

diff --git a/package.json b/package.json
index 6b82e7614a..ca5864ba56 100644
--- a/package.json
+++ b/package.json
@@ -50,7 +50,6 @@
     "@types/js-cookie": "^3.0.0",
     "@xobotyi/scrollbar-width": "^1.9.5",
     "copy-to-clipboard": "^3.3.1",
-    "fast-deep-equal": "^3.1.3",
     "fast-shallow-equal": "^1.0.0",
     "js-cookie": "^3.0.0",
     "nano-css": "^5.6.2",
diff --git a/src/misc/isDeepEqual.ts b/src/misc/isDeepEqual.ts
index 2cff67378a..186425e407 100644
--- a/src/misc/isDeepEqual.ts
+++ b/src/misc/isDeepEqual.ts
@@ -1,3 +1,80 @@
-import isDeepEqualReact from 'fast-deep-equal/react';
+// Based on `fast-deep-equal/react` (MIT, Evgeny Poberezkin), inlined so that
+// objects with a `null` prototype (e.g. created via `Object.create(null)`) can be
+// compared without throwing. The upstream module assumes every object inherits
+// `valueOf`/`toString` from `Object.prototype` and calls them unconditionally,
+// which throws `TypeError: a.valueOf is not a function` for null-prototype objects.
+// See https://github.com/streamich/react-use/issues/2700 and
+// https://github.com/epoberezkin/fast-deep-equal/issues/111.
+const isDeepEqual = (a: any, b: any): boolean => {
+  if (a === b) {
+    return true;
+  }
 
-export default isDeepEqualReact;
+  if (a && b && typeof a === 'object' && typeof b === 'object') {
+    if (a.constructor !== b.constructor) {
+      return false;
+    }
+
+    let length: number;
+    let i: number;
+
+    if (Array.isArray(a)) {
+      length = a.length;
+      if (length !== b.length) {
+        return false;
+      }
+      for (i = length; i-- !== 0; ) {
+        if (!isDeepEqual(a[i], b[i])) {
+          return false;
+        }
+      }
+      return true;
+    }
+
+    if (a.constructor === RegExp) {
+      return a.source === b.source && a.flags === b.flags;
+    }
+    // Guard the `valueOf`/`toString` shortcuts behind a real-method check so
+    // null-prototype objects fall through to the own-keys comparison below.
+    if (a.valueOf !== Object.prototype.valueOf && typeof a.valueOf === 'function') {
+      return a.valueOf() === b.valueOf();
+    }
+    if (a.toString !== Object.prototype.toString && typeof a.toString === 'function') {
+      return a.toString() === b.toString();
+    }
+
+    const keys = Object.keys(a);
+    length = keys.length;
+    if (length !== Object.keys(b).length) {
+      return false;
+    }
+
+    for (i = length; i-- !== 0; ) {
+      if (!Object.prototype.hasOwnProperty.call(b, keys[i])) {
+        return false;
+      }
+    }
+
+    for (i = length; i-- !== 0; ) {
+      const key = keys[i];
+
+      if (key === '_owner' && a.$$typeof) {
+        // React-specific: avoid traversing React elements' _owner, which contains
+        // circular references and is not needed when comparing the elements themselves.
+        continue;
+      }
+
+      if (!isDeepEqual(a[key], b[key])) {
+        return false;
+      }
+    }
+
+    return true;
+  }
+
+  // true if both NaN, false otherwise (`x !== x` is the canonical NaN check)
+  // eslint-disable-next-line no-self-compare
+  return a !== a && b !== b;
+};
+
+export default isDeepEqual;
diff --git a/tests/useDeepCompareEffect.test.ts b/tests/useDeepCompareEffect.test.ts
index d76897772a..05ddb9bfbc 100644
--- a/tests/useDeepCompareEffect.test.ts
+++ b/tests/useDeepCompareEffect.test.ts
@@ -39,3 +39,29 @@ it('should run clean-up provided on unmount', () => {
   unmount();
   expect(mockEffectCleanup).toHaveBeenCalledTimes(1);
 });
+
+it('should not throw when comparing null-prototype objects', () => {
+  const mockEffect = jest.fn();
+  const createNullProtoDep = (value: number) => {
+    const dep = Object.create(null);
+    dep.value = value;
+    return dep;
+  };
+
+  let dep = createNullProtoDep(1);
+  const { rerender, result } = renderHook(() => useDeepCompareEffect(mockEffect, [dep]));
+  expect(result.error).toBeUndefined();
+  expect(mockEffect).toHaveBeenCalledTimes(1);
+
+  // Same shape -> comparison must not throw and the effect must not re-run.
+  dep = createNullProtoDep(1);
+  rerender();
+  expect(result.error).toBeUndefined();
+  expect(mockEffect).toHaveBeenCalledTimes(1);
+
+  // Different value -> comparison must not throw and the effect should re-run.
+  dep = createNullProtoDep(2);
+  rerender();
+  expect(result.error).toBeUndefined();
+  expect(mockEffect).toHaveBeenCalledTimes(2);
+});