Nuxt/docs/3.api/1.composables/use-nuxt-app.md

useNuxtApp

useNuxtApp is a built-in composable that provides a way to access shared runtime context of Nuxt, which is available on both client and server side. It helps you access the Vue app instance, runtime hooks, runtime config variables and internal states, such as ssrContext and payload.

You can use useNuxtApp() within composables, plugins and components.

<script setup>
  const nuxtApp = useNuxtApp()
</script>

Methods

provide (name, value)

nuxtApp is a runtime context that you can extend using Nuxt plugins. Use the provide function to create Nuxt plugins to make values and helper methods available in your Nuxt application across all composables and components.

provide function accepts name and value parameters.

Example:

const nuxtApp = useNuxtApp()
nuxtApp.provide('hello', (name) => `Hello ${name}!`)

// Prints "Hello name!"
console.log(nuxtApp.$hello('name'))

As you can see in the example above, $hello has become the new and custom part of nuxtApp context and it is available in all places where nuxtApp is accessible.

hook(name, cb)

Hooks available in nuxtApp allows you to customize the runtime aspects of your Nuxt application. You can use runtime hooks in Vue.js composables and Nuxt plugins to hook into the rendering lifecycle.

hook function is useful for adding custom logic by hooking into the rendering lifecycle at a specific point. hook function is mostly used when creating Nuxt plugins.

See Runtime Hooks for available runtime hooks called by Nuxt.

export default defineNuxtPlugin((nuxtApp) => {
  nuxtApp.hook('page:start', () => {
    /* your code goes here */
  })
  nuxtApp.hook('vue:error', (..._args) => {
    console.log('vue:error')
    // if (process.client) {
    //   console.log(..._args)
    // }
  })
})

callhook(name, ...args)

callHook returns a promise when called with any of the existing hooks.

await nuxtApp.callHook('my-plugin:init')

Properties

useNuxtApp() exposes the following properties that you can use to extend and customize your app and share state, data and variables.

vueApp

vueApp is the global Vue.js application instance that you can access through nuxtApp. Some useful methods:

• component() - Registers a global component if passing both a name string and a component definition, or retrieves an already registered one if only the name is passed.
• directive() - Registers a global custom directive if passing both a name string and a directive definition, or retrieves an already registered one if only the name is passed(example).
• use() - Installs a Vue.js Plugin (example).

:ReadMore{link="https://vuejs.org/api/application.html#application-api"}

ssrContext

ssrContext is generated during server-side rendering and it is only available on the server side. Nuxt exposes the following properties through ssrContext:

• url (string) - Current request url.
• event (unjs/h3 request event) - Access to req and res objects for the current request.
• payload (object) - NuxtApp payload object.

payload

payload exposes data and state variables from server side to client side. The following keys will be available on the client after they have been passed from the server side:

• serverRendered (boolean) - Indicates if response is server-side-rendered.
• data (object) - When you fetch the data from an API endpoint using either useFetch or useAsyncData, resulting payload can be accessed from the payload.data. This data is cached and helps you prevent fetching the same data in case an identical request is made more than once.

export default defineComponent({
  async setup() {
    const { data } = await useAsyncData('count', () => $fetch('/api/count'))
  }
})

After fetching the value of count using useAsyncData in the example above, if you access payload.data, you will see { count: 1 } recorded there. The value of count is updated whenever the page count increases.

When accessing the same payload.data from ssrcontext, you can access the same value on the server side as well.

• state (object) - When you use useState composable in Nuxt to set shared state, this state data is accessed through payload.state.[name-of-your-state].

export const useColor = () => useState<string>('color', () => 'pink')

export default defineNuxtPlugin((nuxtApp) => {
  if (process.server) {
    const color = useColor()
  }
})

It is also possible to use more advanced types, such as ref, reactive, shallowRef, shallowReactive and NuxtError.

You can also add your own types, with a special plugin helper:

  /**
   * This kind of plugin runs very early in the Nuxt lifecycle, before we revive the payload.
   * You will not have access to the router or other Nuxt-injected properties.
   */
export default definePayloadPlugin((nuxtApp) => {
  definePayloadReducer('BlinkingText', data => data === '<blink>' && '_')
  definePayloadReviver('BlinkingText', () => '<blink>')
})

isHydrating

Use nuxtApp.isHydrating (boolean) to check if the Nuxt app is hydrating on the client side.

Example:

export default defineComponent({
  setup (_props, { slots, emit }) {
    const nuxtApp = useNuxtApp()
    onErrorCaptured((err) => {
      if (process.client && !nuxtApp.isHydrating) {
        // ...
      }
    })
  }
})