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.

In a nutshell:

• useFetch is the most straightforward way to handle data fetching in a component setup function.
• $fetch is great to make network requests based on user interaction.
• useAsyncData, combined with $fetch, offers more fine-grained control.

Both useFetch and useAsyncData share a common set of options and patterns that we will detail in the last sections.

Before that, it's imperative to know why these composables exist in the first place.

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, instead of letting the developer rely on $fetch calls alone.

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.

The payload is a JavaScript object accessible through useNuxtApp().payload. It is used on the client to avoid refetching the same data when the code is executed in the browser.

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

The useFetch composable is the most straightforward way to perform data fetching.

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

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

This composable is a wrapper around the useAsyncData composable and $fetch utility.

$fetch

Nuxt includes the ofetch library, and is auto-imported as the $fetch alias globally across your application. It's what useFetch uses behind the scenes.

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

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

useAsyncData

The useAsyncData composable is responsible for wrapping async logic and returning the result once it is resolved.

Indeed, 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.

The first argument of useAsyncData is the unique key used to cache the response of the second argument, the querying function. This argument can be ignored by directly passing the querying function. In that case, it will be auto-generated.

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

Since the autogenerated key only takes into account the file and line where useAsyncData is invoked, it is recommended to always create your own key to avoid unwanted behavior, if you are creating your own custom composable that is wrapping useAsyncData.

const id = ref(1)

const { data, error } = await useAsyncData(`user:${id.value}`, () => {
  return myGetFunction('users', { id: id.value })
})

The useAsyncData composable is a great way to wrap and wait for multiple useFetch to be done, and then retrieve the results of each.

const { data: discounts, pending } = await useAsyncData('cart-discount', async () => {
  const [coupons, offers] = await Promise.all([$fetch('/cart/coupons'), $fetch('/cart/offers')])

  return {
    coupons,
    offers
  }
})

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.

<script setup lang="ts">
const { pending, data: posts } = useFetch('/api/posts', {
  lazy: true
})
</script>

<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>

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. On initial load, the data will not be fetched before hydration is complete so you have to handle a pending state, though on subsequent client-side navigation the data will be awaited before loading the page.

Combined with the lazy option, this can be useful for data that is not needed on the first render (for example, non-SEO sensitive data).

/* This call is performed before hydration */
const { article } = await useFetch('api/article')

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

The useFetch composable is meant to be invoked in setup method or called directly at the top level of a function in lifecycle hooks, otherwise you should use $fetch method.

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 lang="ts">
/* 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.

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

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

The execute function is an alias for refresh that works in exactly the same way but is more semantic for cases when the fetch is not immediate.

Watch

To re-run your fetching function each time other reactive values in your application change, use the watch option. You can use it for one or multiple watchable elements.

const id = ref(1)

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

Note that watching a reactive value won't change the URL fetched. For example, this will keep fetching the same initial ID of the user because the URL is constructed at the moment the function is invoked.

const id = ref(1)

const { data, error, refresh } = await useFetch(`/api/users/${id.value}`, {
  watch: [id]
})

If you need to change the URL based on a reactive value, you may want to use a computed URL instead.

Computed URL

Sometimes you may need to compute an URL from reactive values, and refresh the data each time these change. Instead of juggling your way around, you can attach each param as a reactive value. Nuxt will automatically use the reactive value and re-fetch each time it changes.

const id = ref(null)

const { data, pending } = useLazyFetch('/api/user', {
  query: {
    user_id: id
  }
})

In the case of more complex URL construction, you may use a callback as a computed getter that returns the URL string.

Every time a dependency changes, the data will be fetched using the newly constructed URL. Combine this with not-immediate, and you can wait until the reactive element changes before fetching.

<script setup lang="ts">
const id = ref(null)

const { data, pending, status } = useLazyFetch(() => `/api/users/${id.value}`, {
  immediate: false
})
</script>

<template>
  <div>
    <!-- disable the input while fetching -->
    <input v-model="id" type="number" :disabled="pending"/>

    <div v-if="status === 'idle'">
      Type an user ID
    </div>
    
    <div v-else-if="pending">
      Loading ...
    </div>

    <div v-else>
      {{ data }}
    </div>
  </div>
</template>

If you need to force a refresh when other reactive values change, you can also watch other values.

Not immediate

The useFetch composable will start fetching data the moment is invoked. You may prevent this by setting immediate: false, for example, to wait for user interaction.

With that, you will need both the status to handle the fetch lifecycle, and execute to start the data fetch.

<script setup lang="ts">
const { data, error, execute, pending, status } = await useLazyFetch('/api/comments')
</script>

<template>
  <div v-if="status === 'idle'">
    <button @click="execute">Get data</button>
  </div>

  <div v-else-if="pending">
    Loading comments...
  </div>

  <div v-else>
    {{ data }}
  </div>
</template>

For finer control, the status variable can be:

• idle when the fetch hasn't started
• pending when a fetch has started but not yet completed
• error when the fetch fails
• success when the fetch is completed successfully

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 lang="ts">
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) => {
  /* Get the response from the server endpoint */
  const res = await $fetch.raw(url)
  /* Get the cookies from the response */
  const cookies = (res.headers.get('set-cookie') || '').split(',')
  /* Attach each cookie to our incoming Request */
  for (const cookie of cookies) {
    appendResponseHeader(event, 'set-cookie', cookie)
  }
  /* Return the data of the response */
  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>

Serializing Data From Server to Client

When using useAsyncData and useLazyAsyncData to transfer data fetched on server to the client (as well as anything else that utilizes the Nuxt payload), the payload is serialized with devalue. This allows us to transfer not just basic JSON but also to serialize and revive/deserialize more advanced kinds of data, such as regular expressions, Dates, Map and Set, ref, reactive, shallowRef, shallowReactive and NuxtError - and more.

It is also possible to define your own serializer/deserializer for types that are not supported by Nuxt. You can read more in the useNuxtApp docs.

Serializing Data From API Routes

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>