Data fetching

Nuxt comes with two composables and a built-in library to perform data-fetching in browser or server environments: useFetch, useAsyncData and $fetch .

Used together, they ensure cross-environment compatibility and efficient caching and avoid duplicate network calls.

useFetch is the most straightforward way to handle data fetching in a component setup function.

On the other hand, when wanting to make a network request based on user interaction, $fetch is almost always the right handler to go for.

If you need more fine-grained control, you can use useAsyncData and $fetch independently.

The two composables share a common set of options and patterns that we will detail in the last sections.

Why using specific composables?

When using a framework like Nuxt that can perform calls and render pages on both client and server environments, some challenges must be addressed. This is why Nuxt provides composables to wrap your queries.

Network calls duplication

The useFetch and useAsyncData composables ensure that once an API call is made on the server, the data is properly forwarded to the client in the payload. This JavaScript object is accessible through useNuxtApp().payload and is used on the client to avoid refetching the same data when the code is executed in the browser.

Effective caching

useFetch and useAsyncData both use a key to cache API responses and further reduce API calls. We will detail later how to invalidate this cache.

Suspense

Nuxt uses Vue’s <Suspense> component under the hood to prevent navigation before every async data is available to the view. The data fetching composables can help you leverage this feature and use what suits best on a per-calls basis.

useFetch

useFetch is the most straightforward way to perform data fetching. It is a wrapper around the useAsyncData composable and $fetch utility.

<script setup>
const { data: count } = await useFetch('/api/count')
</script>

<template>
  Page visits: {{ count }}
</template>

$fetch

The ofetch library is built on top of the fetch API and adds handy features to it:

• Works the same way in browser, Node or worker environments
• Automatic response parsing
• Error handling
• Auto-retry
• Interceptors

ofetch is auto-imported by Nuxt and used by the useFetch composable.

It can also be used in your whole application with the $fetch alias:

const users = await $fetch('/api/users').catch((error) => error.data)

useAsyncData

useFetch receives a URL and gets that data, whereas useAsyncData might have more complex logic. useFetch(url) is nearly equivalent to useAsyncData(url, () => $fetch(url)) - it's developer experience sugar for the most common use case.

There are some cases when using the useFetch composable is not appropriate, for example when a CMS or a third-party provide their own query layer. In this case, you can use useAsyncData to wrap your calls and still keep the benefits provided by the composable:

const { data, error } = await useAsyncData('users', () => myGetFunction('users'))

Options

useAsyncData and useFetch return the same object type and accept a common set of options as their last argument. They can help you control the composables behavior, such as navigation blocking, caching or execution.

Lazy

By default, data fetching composables will wait for the resolution of their asynchronous function before navigating to a new page by using Vue’s Suspense. This feature can be ignored on client-side navigation with the lazy option. In that case, you will have to manually handle loading state using the pending value.

<template>
  <!-- you will need to handle a loading state -->
  <div v-if="pending">
    Loading ...
  </div>
  <div v-else>
    <div v-for="post in posts">
      <!-- do something -->
    </div>
  </div>
</template>
<script setup>
const { pending, data: posts } = useFetch('/api/posts', {
  lazy: true
})
</script>

You can alternatively use useLazyFetch and useLazyAsyncData as convenient methods to perform the same.

const { pending, data: posts } = useLazyFetch('/api/posts')

Client-only fetching

By default, data fetching composables will perform their asynchronous function on both client and server environments. Set the server option to false to only perform the call on the client-side. Combined with the lazy option, this can be useful for data that are not needed on the first render (for example, non-SEO sensitive data).

/* This call will only be performed on the client */
const { pending, data: posts } = useFetch('/api/comments', {
  lazy: true,
  server: false
})

Minimize payload size

The pick option helps you to minimize the payload size stored in your HTML document by only selecting the fields that you want returned from the composables.

<script setup>
/* only pick the fields used in your template */
const { data: mountain } = await useFetch('/api/mountains/everest', { pick: ['title', 'description'] })
</script>

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

If you need more control or map over several objects, you can use the transform function to alter the result of the query.

const { data: mountains } = await useFetch('/api/mountains', { 
  transform: (mountains) => {
    return mountains.map(mountain => ({ title: mountain.title, description: mountain.description }))
  }
})

Caching and refetching

Keys

useFetch and useAsyncData use keys to prevent refetching the same data.

• useFetch uses the provided URL as a key. Alternatively, a key value can be provided in the options object passed as a last argument.
• useAsyncData uses its first argument as a key if it is a string. If the first argument is the handler function that performs the query, then a key that is unique to the file name and line number of the instance of useAsyncData will be generated for you.

Refresh and execute

If you want to fetch or refresh data manually, use the execute or refresh function provided by the composables. (execute is an alias for refresh that works in exactly the same way but is more semantic for cases when immediate: false).

<script setup>
const { data, error, execute, refresh } = await useFetch('/api/users')
</script>

<template>
<div>
  <p>{{ data }}</p>
  <button @click="refresh">Refresh data</button>
</div>
</template>

Watch

To re-run your fetching function each time other reactive values in your application change, use the watch option.

const { data, error, refresh } = await useFetch('/api/users', {
  /* Changing the id will trigger a refetch */
  watch: [id]
})

const id = ref(1)

Passing Headers and cookies

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.

Pass Client Headers to the API

We can use useRequestHeaders to access and proxy cookies to the API from server-side.

The example below adds the request headers to an isomorphic $fetch call to ensure that the API endpoint has access to the same cookie header originally sent by the user.

<script setup>
const headers = useRequestHeaders(['cookie'])
const { data } = await useFetch('/api/me', { headers })
</script>

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.

import { appendResponseHeader, H3Event } from 'h3'

export const fetchWithCookie = async (event: H3Event, url: string) => {
  const res = await $fetch.raw(url)
  const cookies = (res.headers.get('set-cookie') || '').split(',')
  for (const cookie of cookies) {
    appendResponseHeader(event, 'set-cookie', cookie)
  }
  return res._data
}

<script setup lang="ts">
// This composable will automatically pass cookies to the client
const event = useRequestEvent()
const result = await fetchWithCookie(event, '/api/with-cookie')
onMounted(() => console.log(document.cookie))
</script>

Options API support

Nuxt 3 provides a way to perform asyncData fetching within the Options API. You must wrap your component definition within defineNuxtComponent for this to work.

<script>
export default defineNuxtComponent({
  /* Use the fetchKey option to provide a unique key */
  fetchKey: 'hello',
  async asyncData () {
    return {
      hello: await $fetch('/api/hello')
    }
  }
})
</script>

Serialization

When fetching data from the server directory, the response is serialized using JSON.stringify. However, since serialization is limited to only JavaScript primitive types, Nuxt does its best to convert the return type of $fetch and useFetch to match the actual value.

Example

export default defineEventHandler(() => {
  return new Date()
})

<script setup lang="ts">
// Type of `data` is inferred as string even though we returned a Date object
const { data } = await useFetch('/api/foo')
</script>

Custom serializer function

To customize the serialization behavior, you can define a toJSON function on your returned object. If you define a toJSON method, Nuxt will respect the return type of the function and will not try to convert the types.

export default defineEventHandler(() => {
  const data = {
    createdAt: new Date(),

    toJSON() {
      return {
        createdAt: {
          year: this.createdAt.getFullYear(),
          month: this.createdAt.getMonth(),
          day: this.createdAt.getDate(),
        },
      }
    },
  }
  return data
})

<script setup lang="ts">
// Type of `data` is inferred as
// {
//   createdAt: {
//     year: number
//     month: number
//     day: number
//   }
// }
const { data } = await useFetch('/api/bar')
</script>

Using an alternative serializer

Nuxt does not currently support an alternative serializer to JSON.stringify. However, you can return your payload as a normal string and utilize the toJSON method to maintain type safety.

In the example below, we use superjson as our serializer.

import superjson from 'superjson'

export default defineEventHandler(() => {
  const data = {
    createdAt: new Date(),

    // Workaround the type conversion
    toJSON() {
      return this
    }
  }

  // Serialize the output to string, using superjson
  return superjson.stringify(data) as unknown as typeof data
})

<script setup lang="ts">
import superjson from 'superjson'

// `date` is inferred as { createdAt: Date } and you can safely use the Date object methods
const { data } = await useFetch('/api/superjson', {
  transform: (value) => {
    return superjson.parse(value as unknown as string)
  },
})
</script>