docs: avoid $fetch in top-level <script setup> (#19357)

docs/1.getting-started/6.data-fetching.md

@@ -219,14 +219,24 @@ Using `<script setup lang="ts">` is the recommended way of declaring Vue compone
 ::ReadMore{link="/docs/api/utils/define-nuxt-component"}
 ::
 
-## Isomorphic `fetch` and `$fetch`
+## Using `$fetch` directly
 
-When we call `fetch` in the browser, user headers like `cookie` will be directly sent to the API. But during server-side-rendering, since the `fetch` request takes place 'internally' within the server, it doesn't include the user's browser cookies, nor does it pass on cookies from the fetch response.
+There are instances where you may need to directly call the API. Nuxt 3 provides a globally available `$fetch` method using [unjs/ofetch](https://github.com/unjs/ofetch) (in addition to `fetch`) with the same API as the [Fetch API](https://developer.mozilla.org/en-US/docs/Web/API/Fetch_API/Using_Fetch).
+
+Using `$fetch` has a number of benefits, including:
+
+It will handle 'smartly' making direct API calls if it's running on the server, or making a client-side call to your API if it's running on the client. (It can also handle calling third-party APIs.)
+
+Plus, it comes with convenience features including automatically parsing responses and stringifying data.
 
 ::ReadMore{link="/docs/api/utils/dollarfetch"}
 ::
 
-### Example: Pass Client Headers to the API
+### Isomorphic `$fetch` and `fetch` calls
+
+When we call `$fetch` in the browser, user headers like `cookie` will be directly sent to the API. But during server-side-rendering, since the `$fetch` request takes place 'internally' within the server, it doesn't include the user's browser cookies, nor does it pass on cookies from the fetch response.
+
+#### Example: Pass Client Headers to the API
 
 We can use [`useRequestHeaders`](/docs/api/composables/use-request-headers) to access and proxy cookies to the API from server-side.
 
@@ -248,7 +258,7 @@ Be very careful before proxying headers to an external API and just include head
 * `cf-connecting-ip`, `cf-ray`
 ::
 
-### Example: Pass Cookies From Server-side API Calls on SSR Response
+#### Example: Pass Cookies From Server-side API Calls on SSR Response
 
  If you want to pass on/proxy cookies in the other direction, from an internal request back to the client, you will need to handle this yourself.
 
@@ -274,6 +284,8 @@ onMounted(() => console.log(document.cookie))
 
 ## Best Practices
 
+### Minimize Payload
+
 The data returned by these composables will be stored inside the page payload. This means that every key returned that is not used in your component will be added to the payload.
 
 ::alert{icon=👉}
@@ -309,6 +321,39 @@ const { data: mountain } = await useFetch('/api/mountains/everest', { pick: ['ti
 </template>
 ```
 
+### Avoid double calls
+
+Calling `$fetch` in code that is executed on both server and client (such as in the top level of a `setup` function) will fetch the data twice - initially on the server and then again on the client during the hydration phase. This is because `$fetch` does not automatically serialize or transfer the data to the client.
+
+For example:
+
+**/pages/price.vue**: Isomorphic code below executes `$fetch` twice (initially on the server, then again on the client).
+
+```ts
+<script setup lang="ts">
+const price = $fetch('/api/price');
+</script>
+```
+
+**/server/api/product.get.ts**: Server only code below executes `$fetch` only once at the server side.
+
+```ts
+export default eventHandler(async (event: H3Event) => {
+  const price = $fetch('/api/price');
+  return { color: getColor(), price };
+});
+```
+
+If fetching twice isn't your intended behavior, to fetch only on the server side and transfer it to the client, wrap `$fetch` with `useAsyncData()` or use `useFetch()`.
+
+```ts
+<script setup lang="ts">
+const { data } = await useAsyncData('price', () => $fetch('/api/price'));
+// or
+const { data } = await useFetch('/api/price')
+</script>
+```
+
 ## Using Async Setup
 
 If you are using `async setup()`, the current component instance will be lost after the first `await`. (This is a Vue 3 limitation.) If you want to use multiple async operations, such as multiple calls to `useFetch`, you will need to use `<script setup>` or await them together at the end of setup.
@@ -342,13 +387,3 @@ export default defineComponent({
 </template>
 ```
 
-## Directly Calling an API Endpoint
-
-There are instances where you may need to directly call the API. Nuxt 3 provides a globally available `$fetch` method using [unjs/ofetch](https://github.com/unjs/ofetch) (in addition to `fetch`)
-with the same API as the [Fetch API](https://developer.mozilla.org/en-US/docs/Web/API/Fetch_API/Using_Fetch).
-
-Using `$fetch` has a number of benefits, including:
-
-It will handle 'smartly' making direct API calls if it's running on the server, or making a client-side call to your API if it's running on the client. (It can also handle calling third-party APIs.)
-
-Plus, it comes with convenience features including automatically parsing responses and stringifying data.