# Data Fetching

Nuxt provides `useFetch`, `useLazyFetch`, `useAsyncData` and `useLazyAsyncData` to handle data fetching within your application.

::alert{icon=👉}
**`useFetch`, `useLazyFetch`, `useAsyncData` and `useLazyAsyncData` only works during `setup` or `Lifecycle Hooks`**
::

## `useAsyncData`

Within your pages and components you can use `useAsyncData` to get access to data that resolves asynchronously.

### Usage

```js
useAsyncData(
  key: string,
  fn: () => Object,
  options?: { lazy: boolean, server: boolean }
)
```

* **key**: a unique key to ensure that data fetching can be properly de-duplicated across requests
* **fn** an asynchronous function that returns a value.
* **options**:
  * _lazy_: whether to resolve the async function after loading the route, instead of blocking navigation (defaults to `false`)
  * _default_: a factory function to set the default value of the data, before the async function resolves - particularly useful with the `lazy: true` option
  * _server_: whether to fetch the data on server-side (defaults to `true`)
  * _transform_: A function that can be used to alter fn result after resolving
  * _pick_: Only pick specified keys in this array from fn result

Under the hood, `lazy: false` uses `<Suspense>` to block the loading of the route before the data has been fetched. Consider using `lazy: true` and implementing a loading state instead for a snappier user experience.

### Example

```js [server/api/count.ts]
let counter = 0
export default () => {
  counter++
  return JSON.stringify(counter)
}
```

```vue [app.vue]
<script setup>
const { data } = await useAsyncData('count', () => $fetch('/api/count'))
</script>

<template>
  Page visits: {{ data }}
</template>
```

## `useLazyAsyncData`

This composable behaves identically to `useAsyncData` with the `lazy: true` option set. In other words, the async function does not block navigation. That means you will need to handle the situation where the data is `null` (or whatever value you have provided in a custom `default` factory function).

## `useFetch`

Within your pages and components you can use `useFetch` to get universally fetch from any URL.

This composable provides a convenient wrapper around `useAsyncData` and `$fetch` and automatically generates a key based on url and fetch options and infers API response type.

### Usage

```ts
useFetch(url: string, options?)
```

Available options:

* `key`: Provide a custom key
* Options from [ohmyfetch](https://github.com/unjs/ohmyfetch)
  * `method`: Request method
  * `params`: Query params
  * `baseURL`: Base URL for request
* Options from `useAsyncData`
  * `lazy`
  * `server`
  * `default`
  * `pick`
  * `transform`

### Example

```vue [app.vue]
<script setup>
const { data } = await useFetch('/api/count')
</script>

<template>
  Page visits: {{ data.count }}
</template>
```

## `useLazyFetch`

This composable behaves identically to `useFetch` with the `lazy: true` option set. In other words, the async function does not block navigation. That means you will need to handle the situation where the data is `null` (or whatever value you have provided in a custom `default` factory function).

## Best practices

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=👉}
**We strongly recommend you only select the keys that you will use in your component.**
::

Imagine that `/api/mountains/everest` returns the following object:

```json
{
  "title": "Mount Everest",
  "description": "Mount Everest is Earth's highest mountain above sea level, located in the Mahalangur Himal sub-range of the Himalayas. The China–Nepal border runs across its summit point",
  "height": "8,848 m",
  "countries": [
    "China",
    "Nepal"
  ],
  "continent": "Asia",
  "image": "https://upload.wikimedia.org/wikipedia/commons/thumb/f/f6/Everest_kalapatthar.jpg/600px-Everest_kalapatthar.jpg"
}
```

If you plan to only use `title` and `description` in your component, you can select the keys by chaining the result of `$fetch` or `pick` option:

```vue
<script setup>
const { data: mountain } = await useFetch('/api/mountains/everest', { pick: ['title', 'description'] })
</script>

<template>
  <h1>{{ mountain.title }}</h1>
  <p>{{ mountain.description }}</p>
</template>
```

## 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.

::alert{icon=👉}
Using `<script setup>` is recommended, as it removes the limitation of using top-level await. [Read more](https://v3.vuejs.org/api/sfc-script-setup.html#top-level-await)
::

```vue
<script>
export default defineComponent({
  async setup() {
    const [{ data: organization }, { data: repos }] = await Promise.all([
      useFetch(`https://api.github.com/orgs/nuxt`),
      useFetch(`https://api.github.com/orgs/nuxt/repos`)
    ])

    return {
      organization,
      repos
    }
  }
})
</script>

<template>
  <header>
    <h1>{{ organization.login }}</h1>
    <p>{{ organization.description }}</p>
  </header>
</template>
```