Difference between Asyncdata vs Fetch

Question

What is the exact difference between fetch and async data. The official documentation says the following:

asyncData

You may want to fetch data and render it on the server-side. Nuxt.js adds an asyncData method that lets you handle async operations before setting the component data.

asyncData is called every time before loading the component (only for page components). It can be called from the server-side or before navigating to the corresponding route. This method receives the context object as the first argument, you can use it to fetch some data and return the component data.

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Fetch

The fetch method is used to fill the store before rendering the page, it's like the asyncData method except it doesn't set the component data. The fetch method, if set, is called every time before loading the component (only for page components). It can be called from the server-side or before navigating to the corresponding route.

The fetch method receives the context object as the first argument, we can use it to fetch some data and fill the store. To make the fetch method asynchronous, return a Promise, nuxt.js will wait for the promise to be resolved before rendering the component.

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Fetch is been used to fill the store with data? But in asyncData is this also possible to commit trough a store? I don't understand why there are two methods for.

Both methods are running server-side on the initial load, after that when you navigate through the applicatie it runs client side.

Can someone explain me the advantage of use these methods above the other?

Thanks for help.

Piggy

Answer 1

Let me re-iterate few points as a pretext to what i'm going to say

• asyncData can set component level objects and access vuex store
• fetch cannot set component level objects but has access to vuex store
• Both asyncData & fetch will be triggered in server side during initial load
• After initial load, asyncData and fetch will be triggered when the corresponding page routes are invoked

1) if your design is

• Use vuex store as a central repository
• Access data from the vuex store for the entire application

then use fetch

2) if your design is

• Use vuex store as a central repository
• Have options to set component level objects
• Data fetched in a particular route is used only by a single component
• Need flexibility to have permission to either vuex store or set component level object

then use asyncData

Can someone explain me the advantage of use these methods above the other?

i don't see any drawbacks in using asyncData or fetch

Choosing asyncData or fetch totally depends on your architecture

Update for NuxtJS >= 2.12

Several points mentioned in the answer no longer apply when using newer NuxtJS versions (>= 2.12). Official RFC announcement here.

A good explanation of the new behaviour and differences between asyncData and the new fetch can be found in this post in the NuxtJS official blog.

As for choosing between both, I believe the original answer still applies:

i don't see any drawbacks in using asyncData or fetch

Choosing asyncData or fetch totally depends on your architecture

Darill

upvoted! can I call fetch manually, i have a universal mode nuxt app where on one page I must paginate a table fetching data from the server without changing the page, if I click next page button should I call fetch manually or how – Muezzin

@Muezzin my practical experience is, you cannot execute 'fetch' manually, behavior of 'fetch' is controlled by nuxtjs. if you need to trigger any events (for dataFetch) after pageLoad, better to write a customMethod for dataFetch and bind the 'nextPage button' to it – Darill

What about not using them at all, and using created or mounted hooks instead? – Basra

@PrimozRome created & mounted hooks always runs on the client side. on first request to the nuxtapp for a particular route asyncData & fetch will run on the server side. So with asyncData & fetch, you have the opportunity to load the data into client without any ajax request after page load in client side as you would do for created & mounted hooks – Darill

@Darill yes that is correct, thanks for explanation. I forgot to mention in my comment if using Nuxt.js app in SPA mode only. Then it should be the same using fetch() or created() hook, correct? – Basra

@PrimozRome yes correct, in spa whatever goes into asyncData or fetch will be invoked first then created hook gets invoked then mounted hook gets invoked. So asyncData or fetch gets invoked before the component gets created. – Darill

I want to fill the VUEX store with data in the fetch method. Is this data then available in the asyncData method? I cannot understand which one runs first... – Alonaalone

@Darill To clarify: are the created and mounted hooks called server side in nuxt? If so, the only reason to use asyncData would be timing. – Vichy

To emphasize: asyncData is only in 'pages' not in 'components.' Must use fetch in that one. Strangely, when making this mistake of using asyncData in a 'components' file, there is no warning or linting error. 🤷🏽‍♂️ – Lin

But doesn't fetch has access to component context with this? Then it can set data like this.someData = data; -> With the help of this context, fetch is able to mutate component’s data directly. – Abernon

> "yes correct, in spa whatever goes into asyncData or fetch will be invoked first then created hook gets invoked" - if you check update in nuxt 2.12 fetch() is called after create() – Abernon

a drawback of using fetch() is that you can't insert requested data at page headers – Acidify

Answer 2

TL;DR - use asyncData for stuff which must be loaded before rendering a page, use fetch for everything else.

Key differences:

Availability

• asyncData is only available on page components
• fetch can be used on any component (including page components)

Loading

• asyncData blocks the page transition until it resolves. This means the data properties returned are guaranteed to be available on the component. But it also means users may have to wait longer before seeing content.
• fetch exposes a $fetchState.pending property and it's up to you how to handle that

Error handling

• if an error is thrown in asyncData the page is not rendered
• fetch exposes a $fetchState.error property and it's up to you how to handle that

Allegedly

That said, in a SSG site we "should" use asyncData for retrieving data for a page that will not change during navigation and, for example, set the dynamic meta tags in the head method (if using fetch(), head() could not receive the data in time). We should use fetch() when we want more control on the data received, for example if there is a button to trigger the fetch() again while showing a placeholder with $fetchState.pending. Am I thinking it right? – Lunate

@StefanoFranceschetto makes sense – Maccarthy

The section on "loading" really clarifies things for me. – Schmuck

Answer 3

One point I'd like to make that I don't see mentioned above (at least, not clearly). asyncData automatically MERGES the data into your page's data() object. Fetch does not. With fetch, it's up to you to do with the data as you please.

Duleba

Answer 4

The 1st

Because of the different nature of asyncData and fetch there is one important advantage in case of asyncData - Nuxt waits for the asyncData hook to be finished before navigating to the next page.

Taking from here:

Unlike fetch, the promise returned by the asyncData hook is resolved during route transition. This means that no "loading placeholder" is visible during client-side transitions (although the loading bar can be used to indicate a loading state to the user). Nuxt will instead wait for the asyncData hook to be finished before navigating to the next page or display the error page).

What does it mean on practise?

Lets imagine you have the next structure of layout:

• Header
• Content
• Footer

In case of using fetch when you open a new page, you may see for few seconds just Header and Footer (because data for Content is downloading). In case of using asyncData you avoid this problem and see a new page with Header + Content + Footer (however the disadvantage of this method is that you need to wait the same few seconds to download Content data).

The 2nd

I see in different places in web that when you want to store something in vuex, you need to use fetch - this is incorrect.

In the code below (taken from my project) you can find implementation of asyncData and fetch that both store data into vuex.

<script>
import { mapActions, mapMutations, mapState } from 'vuex'

export default {
  name: 'PagesBlog',

  async asyncData ({ store }) {
    if (!store.state.global.blogAuthors.length) {
      store.commit('global/blogAuthorsSet', await blogAuthorsDownload())
    }

    await store.dispatch('global/blogsDownloadAndSet')
  },

  async fetch () {
    if (!this.blogAuthors.length) {
      this.blogAuthorsSet(await blogAuthorsDownload())
    }

    await this.blogsDownloadAndSet()
  },

  computed: {
    ...mapState('global', [
      'blogAuthors'
    ])
  },

  methods: {
    ...mapActions('global', [
      'blogsDownloadAndSet'
    ]),

    ...mapMutations('global', [
      'blogAuthorsSet'
    ])
  }
</script>

Summary

1. If you have some important data (either visible to user or not visible to user, but needed for some hidden calculation) - use asyncData.

2. If you want to see page with all information (for example when you have Header + Content + Footer) - use asyncData.

3. If you have some data that can be loaded a bit later - use fetch.

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Fetch Hook and Nuxt Lifecycle

Howarth

Answer 5

I. fetch and asyncData are processed on the server-side.

II. can see the difference in the way to use them:

a) fetch: change store data

<script>
export default {
  async fetch ({ store, params }) {
    await store.dispatch('GET_STARS');
  }
}
</script>

b) asyncData: change context (component data)

<script>
export default {
  asyncData (context) {
    return { project: 'nuxt' }
  }
}
</script>

Melonymelos

this is incorrect example. While asyncData indeed can change local/component data, you can use it to save data in store as well. Simply replace "fetch" by "asyncData" in your example, that's it. – Howarth