feat(extendLocale): new composable

Resolves #3729

docs/content/1.getting-started/7.i18n/1.nuxt.md

@@ -60,7 +60,7 @@ import { fr } from '@nuxt/ui-pro/locale'
 
 ### Custom locale
 
-You also have the option to add your own locale using `defineLocale`:
+You can create your own locale using the `defineLocale` composable:
 
 ::module-only
 
@@ -125,6 +125,65 @@ Look at the `code` parameter, there you need to pass the iso code of the languag
 
 ::
 
+### Extend locale :badge{label="Soon" class="align-text-top"}
+
+You can customize an existing locale by overriding its `messages` or `code` using the `extendLocale` composable:
+
+::module-only
+
+#ui
+:::div
+
+```vue [app.vue]
+<script setup lang="ts">
+import { en } from '@nuxt/ui/locale'
+
+const locale = extendLocale(en, {
+  code: 'en-GB',
+  messages: {
+    commandPalette: {
+      placeholder: 'Search a component...'
+    }
+  }
+})
+</script>
+
+<template>
+  <UApp :locale="locale">
+    <NuxtPage />
+  </UApp>
+</template>
+```
+
+:::
+
+#ui-pro
+:::div
+
+```vue [app.vue]
+<script setup lang="ts">
+import { en } from '@nuxt/ui-pro/locale'
+
+const locale = extendLocale(en, {
+  code: 'en-GB',
+  messages: {
+    commandPalette: {
+      placeholder: 'Search a component...'
+    }
+  }
+})
+</script>
+
+<template>
+  <UApp :locale="locale">
+    <NuxtPage />
+  </UApp>
+</template>
+```
+
+:::
+::
+
 ### Dynamic locale
 
 To dynamically switch between languages, you can use the [Nuxt I18n](https://i18n.nuxtjs.org/) module.

docs/content/1.getting-started/7.i18n/2.vue.md

@@ -60,7 +60,7 @@ import { fr } from '@nuxt/ui-pro/locale'
 
 ### Custom locale
 
-You also have the option to add your locale using `defineLocale`:
+You can create your own locale using the `defineLocale` composable:
 
 ::module-only
 
@@ -127,6 +127,67 @@ Look at the `code` parameter, there you need to pass the iso code of the languag
 
 ::
 
+### Extend locale :badge{label="Soon" class="align-text-top"}
+
+You can customize an existing locale by overriding its `messages` or `code` using the `extendLocale` composable:
+
+::module-only
+
+#ui
+:::div
+
+```vue [App.vue]
+<script setup lang="ts">
+import { en } from '@nuxt/ui/locale'
+import { extendLocale } from '@nuxt/ui/composables/defineLocale.js'
+
+const locale = extendLocale(en, {
+  code: 'en-GB',
+  messages: {
+    commandPalette: {
+      placeholder: 'Search a component...'
+    }
+  }
+})
+</script>
+
+<template>
+  <UApp :locale="locale">
+    <RouterView />
+  </UApp>
+</template>
+```
+
+:::
+
+#ui-pro
+:::div
+
+```vue [App.vue]
+<script setup lang="ts">
+import { en } from '@nuxt/ui-pro/locale'
+import { extendLocale } from '@nuxt/ui/composables/defineLocale.js'
+
+const locale = extendLocale(en, {
+  code: 'en-GB',
+  messages: {
+    commandPalette: {
+      placeholder: 'Search a component...'
+    }
+  }
+})
+</script>
+
+<template>
+  <UApp :locale="locale">
+    <RouterView />
+  </UApp>
+</template>
+```
+
+:::
+::
+
 ### Dynamic locale
 
 To dynamically switch between languages, you can use the [Vue I18n](https://vue-i18n.intlify.dev/) plugin.

src/runtime/composables/defineLocale.ts

@@ -1,5 +1,6 @@
 import { defu } from 'defu'
 import type { Locale, Direction } from '../types/locale'
+import type { DeepPartial } from '../types/utils'
 
 interface DefineLocaleOptions<M> {
   name: string
@@ -12,3 +13,8 @@ interface DefineLocaleOptions<M> {
 export function defineLocale<M>(options: DefineLocaleOptions<M>): Locale<M> {
   return defu<DefineLocaleOptions<M>, [{ dir: Direction }]>(options, { dir: 'ltr' })
 }
+
+/* @__NO_SIDE_EFFECTS__ */
+export function extendLocale<M>(locale: Locale<M>, options: Partial<DefineLocaleOptions<DeepPartial<M>>>): Locale<M> {
+  return defu<Locale<M>, [DefineLocaleOptions<M>]>(options, locale)
+}

src/runtime/types/utils.ts

@@ -1,6 +1,10 @@
 import type { VNode } from 'vue'
 import type { AcceptableValue as _AcceptableValue } from 'reka-ui'
 
+export type DeepPartial<T> = {
+  [P in keyof T]?: T[P] extends object ? DeepPartial<T[P]> : T[P] | undefined
+}
+
 export type DynamicSlotsKeys<Name extends string | undefined, Suffix extends string | undefined = undefined> = (
   Name extends string
     ? Suffix extends string