fix: clone settings locale data (#96)

* fix: clone settings locale data

* fix: address locale data review feedback

* docs: clarify locale data clone comment

Author: astandrik

src/settings/settings.test.ts

@@ -23,4 +23,15 @@ describe('settings', () => {
         settings.setLocale(LOCALE);
         expect(settings.getLocale()).toEqual(LOCALE);
     });
+
+    it('should clone locale data with function values', () => {
+        settings.setLocale('en');
+
+        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);
+    });
 });

src/settings/settings.ts

@@ -5,6 +5,23 @@ 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;
+    }
+
+    // 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;
+}
+
 class Settings implements PublicSettings {
     // 'en' - preloaded locale in dayjs
     private loadedLocales = new Set(['en']);
@@ -50,7 +67,7 @@ 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) {