feat: composer type definition extending (**experimental feature**) (#838)

Author: kazupon

packages/vue-i18n-core/src/composer.ts

@@ -1081,6 +1081,30 @@ export interface ComposerNumberFormatting<
   ): string
 }
 
+/**
+ * The type custom definition of Composer
+ *
+ * @remarks
+ *
+ * The interface that can extend Composer.
+ *
+ * The type defined by 3rd party (e.g. nuxt/i18n)
+ *
+ * @example
+ * ```ts
+ * // vue-i18n.d.ts (`.d.ts` file at your app)
+ *
+ * declare module 'vue-i18n' {
+ *   interface ComposerCustom {
+ *     localeCodes: string[]
+ *   }
+ * }
+ * ```
+ *
+ * @VueI18nComposition
+ */
+export interface ComposerCustom {} // eslint-disable-line @typescript-eslint/no-empty-interface
+
 /**
  * Composer interfaces
  *
@@ -1103,7 +1127,7 @@ export interface Composer<
       ? Locale
       : ResourceLocales
     : OptionLocale | ResourceLocales
-> {
+> extends ComposerCustom {
   /**
    * @remarks
    * Instance ID.

test-d/vue-i18n/composer.test-d.ts

@@ -14,6 +14,12 @@ import {
 import { SchemaParams, LocaleParams } from '../../packages/core-base/src'
 import { ResourceSchema, MyDatetimeScehma, MyNumberSchema } from '../schema'
 
+declare module '../../packages/vue-i18n-core/src/composer' {
+  interface ComposerCustom {
+    localeCode: string[]
+  }
+}
+
 // loose options
 const looseOptions = {
   locale: 'en',
@@ -133,6 +139,7 @@ expectType<typeof looseComposer.messages.value.en>(
 expectType<{ japan: string }>(
   looseComposer.getLocaleMessage<{ japan: string }>('japan')
 )
+expectType<string[]>(looseComposer.localeCode)
 looseComposer.setLocaleMessage('en', {
   foo: 'foo',
   nest: {