docs: adjust example and additional instructions of useNuxtData (#30570)

docs/3.api/2.composables/use-nuxt-data.md

@@ -9,11 +9,27 @@ links:
 ---
 
 ::note
-`useNuxtData` gives you access to the current cached value of [`useAsyncData`](/docs/api/composables/use-async-data) , `useLazyAsyncData`, [`useFetch`](/docs/api/composables/use-fetch) and [`useLazyFetch`](/docs/api/composables/use-lazy-fetch) with explicitly provided key.
+`useNuxtData` gives you access to the current cached value of [`useAsyncData`](/docs/api/composables/use-async-data) , [`useLazyAsyncData`](/docs/api/composables/use-lazy-async-data), [`useFetch`](/docs/api/composables/use-fetch) and [`useLazyFetch`](/docs/api/composables/use-lazy-fetch) with explicitly provided key.
 ::
 
 ## Usage
 
+The `useNuxtData` composable is used to access the current cached value of data-fetching composables such as `useAsyncData`, `useLazyAsyncData`, `useFetch`, and `useLazyFetch`. By providing the key used during the data fetch, you can retrieve the cached data and use it as needed.
+
+This is particularly useful for optimizing performance by reusing already-fetched data or implementing features like Optimistic Updates or cascading data updates.
+
+To use `useNuxtData`, ensure that the data-fetching composable (`useFetch`, `useAsyncData`, etc.) has been called with an explicitly provided key.
+
+## Params
+
+- `key`: The unique key that identifies the cached data. This key should match the one used during the original data fetch.
+
+## Return Values
+
+- `data`: A reactive reference to the cached data associated with the provided key. If no cached data exists, the value will be `null`. This `Ref` automatically updates if the cached data changes, allowing seamless reactivity in your components.
+
+## Example
+
 The example below shows how you can use cached data as a placeholder while the most recent data is being fetched from the server.
 
 ```vue [pages/posts.vue]
@@ -26,13 +42,15 @@ const { data } = await useFetch('/api/posts', { key: 'posts' })
 ```vue [pages/posts/[id\\].vue]
 <script setup lang="ts">
 // Access to the cached value of useFetch in posts.vue (parent route)
-const { id } = useRoute().params
 const { data: posts } = useNuxtData('posts')
-const { data } = useLazyFetch(`/api/posts/${id}`, {
-  key: `post-${id}`,
+
+const route = useRoute()
+
+const { data } = useLazyFetch(`/api/posts/${route.params.id}`, {
+  key: `post-${route.params.id}`,
   default() {
     // Find the individual post from the cache and set it as the default value.
-    return posts.value.find(post => post.id === id)
+    return posts.value.find(post => post.id === route.params.id)
   }
 })
 </script>
@@ -40,7 +58,9 @@ const { data } = useLazyFetch(`/api/posts/${id}`, {
 
 ## Optimistic Updates
 
-We can leverage the cache to update the UI after a mutation, while the data is being invalidated in the background.
+The example below shows how implementing Optimistic Updates can be achieved using useNuxtData.
+
+Optimistic Updates is a technique where the user interface is updated immediately, assuming a server operation will succeed. If the operation eventually fails, the UI is rolled back to its previous state.
 
 ```vue [pages/todos.vue]
 <script setup lang="ts">
@@ -52,28 +72,34 @@ const { data } = await useAsyncData('todos', () => $fetch('/api/todos'))
 ```vue [components/NewTodo.vue]
 <script setup lang="ts">
 const newTodo = ref('')
-const previousTodos = ref([])
+let previousTodos = []
 
 // Access to the cached value of useAsyncData in todos.vue
 const { data: todos } = useNuxtData('todos')
 
-const { data } = await useFetch('/api/addTodo', {
+const addTodo = async () => {
+  return $fetch('/api/addTodo', {
     method: 'post',
     body: {
       todo: newTodo.value
     },
     onRequest () {
-    previousTodos.value = todos.value // Store the previously cached value to restore if fetch fails.
+      // Store the previously cached value to restore if fetch fails.
+      previousTodos = todos.value
 
-    todos.value.push(newTodo.value) // Optimistically update the todos.
+      // Optimistically update the todos.
+      todos.value = [...todos.value, newTodo.value]
     },
-  onRequestError () {
-    todos.value = previousTodos.value // Rollback the data if the request failed.
+    onResponseError () {
+      // Rollback the data if the request failed.
+      todos.value = previousTodos
     },
     async onResponse () {
-    await refreshNuxtData('todos') // Invalidate todos in the background if the request succeeded.
+      // Invalidate todos in the background if the request succeeded.
+      await refreshNuxtData('todos')
     }
   })
+}
 </script>
 ```