From e29d1e5e65e14860d127dbf270dde4f7b1584053 Mon Sep 17 00:00:00 2001
From: Anton Standrik <astandrik@yandex-team.ru>
Date: Mon, 22 Jun 2026 17:13:44 +0300
Subject: [PATCH 1/3] fix: clone settings locale data

---
 src/settings/settings.test.ts |  9 +++++++++
 src/settings/settings.ts      | 16 +++++++++++++++-
 2 files changed, 24 insertions(+), 1 deletion(-)

diff --git a/src/settings/settings.test.ts b/src/settings/settings.test.ts
index 4cfca9b..4aaff3b 100644
--- a/src/settings/settings.test.ts
+++ b/src/settings/settings.test.ts
@@ -23,4 +23,13 @@ describe('settings', () => {
         settings.setLocale(LOCALE);
         expect(settings.getLocale()).toEqual(LOCALE);
     });
+
+    it('should clone locale data with function values', () => {
+        const localeData = settings.getLocaleData();
+        const nextLocaleData = settings.getLocaleData();
+
+        expect(typeof localeData.ordinal).toBe('function');
+        expect(localeData).not.toBe(nextLocaleData);
+        expect(localeData.formats).not.toBe(nextLocaleData.formats);
+    });
 });
diff --git a/src/settings/settings.ts b/src/settings/settings.ts
index 0b532be..ca199c2 100644
--- a/src/settings/settings.ts
+++ b/src/settings/settings.ts
@@ -5,6 +5,20 @@ import {normalizeTimeZone} from '../timeZone';
 import {localeLoaders} from './locales';
 import type {Locale, Parser, PublicSettings, UpdateLocaleConfig} from './types';
 
+function cloneLocaleData<T>(value: T): T {
+    if (!value || typeof value !== 'object') {
+        return value;
+    }
+
+    if (Array.isArray(value)) {
+        return value.map((item) => cloneLocaleData(item)) as T;
+    }
+
+    return Object.fromEntries(
+        Object.entries(value).map(([key, item]) => [key, cloneLocaleData(item)]),
+    ) as T;
+}
+
 class Settings implements PublicSettings {
     // 'en' - preloaded locale in dayjs
     private loadedLocales = new Set(['en']);
@@ -50,7 +64,7 @@ class Settings implements PublicSettings {
             throw new Error('There is something really wrong happening. Locale data is absent.');
         }
 
-        return structuredClone(localeObject) as Locale;
+        return cloneLocaleData(localeObject) as Locale;
     }
 
     setLocale(locale: string) {

From 589af2ae33bdff1306ee3564ada67df2adb9ed7a Mon Sep 17 00:00:00 2001
From: Anton Standrik <astandrik@yandex-team.ru>
Date: Wed, 24 Jun 2026 10:46:53 +0300
Subject: [PATCH 2/3] fix: address locale data review feedback

---
 src/settings/settings.test.ts | 2 ++
 src/settings/settings.ts      | 1 +
 2 files changed, 3 insertions(+)

diff --git a/src/settings/settings.test.ts b/src/settings/settings.test.ts
index 4aaff3b..6d94c2b 100644
--- a/src/settings/settings.test.ts
+++ b/src/settings/settings.test.ts
@@ -25,6 +25,8 @@ describe('settings', () => {
     });
 
     it('should clone locale data with function values', () => {
+        settings.setLocale('en');
+
         const localeData = settings.getLocaleData();
         const nextLocaleData = settings.getLocaleData();
 
diff --git a/src/settings/settings.ts b/src/settings/settings.ts
index ca199c2..0d281fe 100644
--- a/src/settings/settings.ts
+++ b/src/settings/settings.ts
@@ -14,6 +14,7 @@ function cloneLocaleData<T>(value: T): T {
         return value.map((item) => cloneLocaleData(item)) as T;
     }
 
+    // Dayjs locale data is JSON-like with function fields; this is not a general structuredClone replacement.
     return Object.fromEntries(
         Object.entries(value).map(([key, item]) => [key, cloneLocaleData(item)]),
     ) as T;

From 27b66696686db82910d9e4e804029a322e2b36ca Mon Sep 17 00:00:00 2001
From: Anton Standrik <astandrik@yandex-team.ru>
Date: Wed, 24 Jun 2026 11:01:26 +0300
Subject: [PATCH 3/3] docs: clarify locale data clone comment

---
 src/settings/settings.ts | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/src/settings/settings.ts b/src/settings/settings.ts
index 0d281fe..3659b90 100644
--- a/src/settings/settings.ts
+++ b/src/settings/settings.ts
@@ -14,7 +14,9 @@ function cloneLocaleData<T>(value: T): T {
         return value.map((item) => cloneLocaleData(item)) as T;
     }
 
-    // Dayjs locale data is JSON-like with function fields; this is not a general structuredClone replacement.
+    // Dayjs locale data contains function fields such as ordinal.
+    // Clone array/plain-object containers so callers can safely mutate the result,
+    // while keeping locale functions callable by reference.
     return Object.fromEntries(
         Object.entries(value).map(([key, item]) => [key, cloneLocaleData(item)]),
     ) as T;