# Nuxt Composables Pattern

> Fetch data and coordinate async state across server and client using Nuxt's built-in composables

## When to Use

- You need to fetch data in a Nuxt page or component with SSR/SSG support
- You want to avoid duplicate requests (server fetches, client rehydrates)
- You are choosing between `useFetch`, `useAsyncData`, and `useLazyFetch`
- You need access to the Nuxt application instance (`useNuxtApp`) for plugins or runtime config

## Instructions

1. Use `useFetch` as the default for HTTP requests — it wraps `useAsyncData` + `$fetch` and handles SSR deduplication automatically:

```typescript
// pages/posts/[id].vue
const { data: post, pending, error } = await useFetch(`/api/posts/${route.params.id}`);
```

2. Use `useAsyncData` when the async operation is not a simple HTTP call (e.g., database query via a server composable, computed key based on reactive state):

```typescript
const { data: user } = await useAsyncData(
  'user-profile', // unique cache key — required
  () => fetchUserProfile(userId.value),
  { watch: [userId] } // re-fetch when userId changes
);
```

3. Use `useLazyFetch` / `useLazyAsyncData` to defer data loading — the page renders immediately and `pending` is true until data arrives:

```typescript
// Good for non-critical data below the fold
const { data: comments, pending } = useLazyFetch('/api/comments');
```

4. Always provide a unique string key to `useAsyncData`. Keys collide across components — use route params or namespacing to avoid cache pollution.

5. Use `$fetch` directly only in event handlers (button clicks, form submissions) — not at the top of `setup()`, which would bypass SSR deduplication:

```typescript
// Good: event handler
async function submitForm() {
  await $fetch('/api/contact', { method: 'POST', body: formData.value });
}

// Bad: top-level setup — use useFetch instead
const data = await $fetch('/api/posts'); // runs on both server AND client
```

6. Transform responses inline with the `transform` option to avoid storing raw API shapes:

```typescript
const { data: posts } = await useFetch('/api/posts', {
  transform: (raw) => raw.items.map((p) => ({ id: p.id, title: p.title })),
});
```

7. Access Nuxt internals with `useNuxtApp()`:

```typescript
const { $toast, $i18n, ssrContext } = useNuxtApp();
```

## Details

Nuxt's data fetching composables solve a fundamental SSR problem: if you use plain `fetch()` or `axios` in `setup()`, the request fires on the server during SSR and again on the client during hydration — doubling your API calls. `useFetch` and `useAsyncData` solve this by serializing the server-fetched data into the HTML payload and rehydrating it on the client without a second network request.

**Key differences:**

| Composable         | Use case                          | Blocks navigation |
| ------------------ | --------------------------------- | ----------------- |
| `useFetch`         | Simple HTTP calls                 | Yes (await)       |
| `useAsyncData`     | Custom async logic, reactive keys | Yes (await)       |
| `useLazyFetch`     | Non-blocking HTTP                 | No                |
| `useLazyAsyncData` | Non-blocking custom async         | No                |
| `$fetch`           | Event handlers only               | N/A               |

**Cache key design:**

`useAsyncData` keys are global within a Nuxt app instance. Two components using `useAsyncData('posts', ...)` will share data. This is often desired (shared cache) but causes bugs when the same key is used with different fetch functions. Pattern: `<entity>-<id>` or `<page>-<section>`.

**Error handling:**

```typescript
const { data, error } = await useFetch('/api/posts');
if (error.value) {
  throw createError({ statusCode: 500, message: 'Failed to load posts' });
}
```

**Refreshing data:**

```typescript
const { data, refresh } = await useFetch('/api/posts');
// Later:
await refresh();
```

## Source

https://nuxt.com/docs/getting-started/data-fetching

## Process

1. Read the instructions and examples in this document.
2. Apply the patterns to your implementation, adapting to your specific context.
3. Verify your implementation against the details and edge cases listed above.

## Harness Integration

- **Type:** knowledge — this skill is a reference document, not a procedural workflow.
- **No tools or state** — consumed as context by other skills and agents.

## Success Criteria

- The patterns described in this document are applied correctly in the implementation.
- Edge cases and anti-patterns listed in this document are avoided.
