feat(nuxt): allow useAppConfig with specific key

docs/content/2.guide/2.features/10.app-config.md

@@ -23,14 +23,28 @@ export default defineAppConfig({
 })
 ```
 
-When adding `theme` to the `app.config`, Nuxt uses Vite or Webpack to bundle the code. We can universally access `theme` in both server and browser using [useAppConfig](/api/composables/use-app-config) composable.
+When adding `theme` to the `app.config`, Nuxt uses Vite or Webpack to bundle the code to both Server and client bundles.
+
+In order to access the runtime config value, we can  [useAppConfig](/api/composables/use-app-config) composable. Returned value is a reactive object.
 
 ```js
 const appConfig = useAppConfig()
 
 console.log(appConfig.theme)
 ```
 
+Or if you need to access a sub-key:
+
+```js
+const themeConfig = useAppConfig('theme')
+
+console.log(themeConfig)
+```
+
+::alert{type=warning}
+Destructuring `appConfig` causes it to loose reactivity. Always use `computed` or provide key to access a sub-key.
+::
+
 <!-- TODO: Document module author for extension -->
 
 ### Manually Typing App Config

docs/content/3.api/1.composables/use-app-config.md

@@ -8,9 +8,11 @@ Access [app config](/guide/features/app-config):
 **Usage:**
 
 ```js
+// Access whole app config (reactive object)
 const appConfig = useAppConfig()
 
-console.log(appConfig)
+// Access sub key of app-config (still reactive)
+const themeConfig = useAppConfig('theme')
 ```
 
 ::ReadMore{link="/guide/features/app-config"}

packages/nuxt/src/app/config.ts

@@ -1,17 +1,20 @@
 import type { AppConfig } from '@nuxt/schema'
-import { reactive } from 'vue'
+import { reactive, computed } from 'vue'
 import { useNuxtApp } from './nuxt'
 // @ts-ignore
 import __appConfig from '#build/app.config.mjs'
 
 // Workaround for vite HMR with virtual modules
 export const _getAppConfig = () => __appConfig as AppConfig
 
-export function useAppConfig (): AppConfig {
+export function useAppConfig (key?: string): AppConfig {
   const nuxtApp = useNuxtApp()
   if (!nuxtApp._appConfig) {
     nuxtApp._appConfig = reactive(__appConfig) as AppConfig
   }
+  if (key) {
+    return computed(() => nuxtApp._appConfig[key])
+  }
   return nuxtApp._appConfig
 }