fix: update updateLocale plugin to merge nested object properties instead of replacing (#3012)

When updating a locale with a partial nested object (e.g., only one key
in `formats`), the plugin now merges the new values into the existing
object rather than replacing the entire object. This preserves other
keys that were not included in the update.

Arrays and non-object values continue to be replaced entirely.

Fixes #1118

Co-authored-by: Test User <test@example.com>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: 3 people

src/plugin/updateLocale/index.js

@@ -5,7 +5,15 @@ export default (option, Dayjs, dayjs) => {
     if (!localeConfig) return
     const customConfigKeys = customConfig ? Object.keys(customConfig) : []
     customConfigKeys.forEach((c) => {
-      localeConfig[c] = customConfig[c]
+      if (localeConfig[c] && customConfig[c] && typeof localeConfig[c] === 'object' && typeof customConfig[c] === 'object'
+        && !Array.isArray(localeConfig[c])) {
+        localeConfig[c] = {
+          ...localeConfig[c],
+          ...customConfig[c]
+        }
+      } else {
+        localeConfig[c] = customConfig[c]
+      }
     })
     return localeConfig // eslint-disable-line consistent-return
   }

test/plugin/updateLocale.test.js

@@ -69,6 +69,35 @@ describe('Update locale', () => {
       .toEqual(moment().format(formatString))
   })
 
+  it('Partial update to nested object (formats)', () => {
+    dayjs.locale('en')
+    // First, get the original formats
+    const originalLocale = dayjs.Ls.en
+    const originalLT = originalLocale.formats && originalLocale.formats.LT
+
+    // Update only L format
+    dayjs.updateLocale('en', {
+      formats: {
+        L: 'DD/MM/YYYY'
+      }
+    })
+
+    const updatedLocale = dayjs.Ls.en
+    // The updated key should have the new value
+    expect(updatedLocale.formats.L).toBe('DD/MM/YYYY')
+    // Other keys in formats should be preserved
+    expect(updatedLocale.formats.LT).toBe(originalLT)
+  })
+
+  it('Non-object values should still be replaced entirely', () => {
+    const newMonths = new Array(12).fill('newMonth')
+    dayjs.updateLocale('en', {
+      months: newMonths
+    })
+    const updatedLocale = dayjs.Ls.en
+    expect(updatedLocale.months).toEqual(newMonths)
+  })
+
   it('Update invalid date string', () => {
     const locale = 'en'
     const localeSetting = { invalidDate: 'bad date' }