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app.vue

Nuxt 3 provides a central entry point to your app via ~/app.vue.
If you don’t have an app.vue file in your source directory, Nuxt will use its own default version.
This file is a great place to put any custom code that needs to be run once when your app starts up, as well as any components that are present on every page of your app. For example, if you only have one layout, you can move this to app.vue instead.

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Creating app.vue

Consider creating an app.vue file and including any logic that needs to run once at the top-level of your app:
app.vue
<template>
  <div>
    <NuxtPage />
  </div>
</template>

<script setup>
// This code runs once when the app starts
const config = useRuntimeConfig()
console.log('App started with config:', config.public)
</script>

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Layouts

If you are using layouts in your app for multiple pages, there is only a slight change required. In Nuxt 2, the <Nuxt> component is used within a layout to render the current page. In Nuxt 3, layouts use slots instead, so you will have to replace that component with a <slot />. This also allows advanced use cases with named and scoped slots. You will also need to change how you define the layout used by a page using the definePageMeta compiler macro. Layouts will be kebab-cased. So app/layouts/customLayout.vue becomes custom-layout when referenced in your page.

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Migration Steps

  1. Replace <Nuxt /> with <slot />
app/layouts/custom.vue
<template>
  <div id="app-layout">
    <main>
-     <Nuxt />
+     <slot />
    </main>
  </div>
</template>
  1. Use definePageMeta to select the layout used by your page
app/pages/index.vue
+ <script setup>
+ definePageMeta({
+   layout: 'custom'
+ })
+ </script>
- <script>
- export default {
-   layout: 'custom'
- }
  </script>
  1. Move ~/layouts/_error.vue to ~/error.vue
If you want to ensure that this page uses a layout, you can use <NuxtLayout> directly within error.vue:
error.vue
<template>
  <div>
    <NuxtLayout name="default">
      <h1>Error: {{ error.message }}</h1>
    </NuxtLayout>
  </div>
</template>

<script setup>
defineProps(['error'])
</script>

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Pages

Nuxt 3 ships with an optional vue-router integration triggered by the existence of a app/pages/ directory in your source directory. If you only have a single page, you may consider instead moving it to app.vue for a lighter build.

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Dynamic Routes

The format for defining dynamic routes in Nuxt 3 is slightly different from Nuxt 2, so you may need to rename some of the files within app/pages/.
  1. Where you previously used _id to define a dynamic route parameter, you now use [id].
  2. Where you previously used _.vue to define a catch-all route, you now use [...slug].vue.

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Dynamic Routes Example

- URL: /users
- Page: /pages/users/index.vue

- URL: /users/some-user-name
- Page: /pages/users/_user.vue
- Usage: params.user

- URL: /users/some-user-name/edit
- Page: /pages/users/_user/edit.vue
- Usage: params.user

- URL: /users/anything-else
- Page: /pages/users/_.vue
- Usage: params.pathMatch

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Nested Routes

In Nuxt 2, you will have defined any nested routes (with parent and child components) using <Nuxt> and <NuxtChild>. In Nuxt 3, these have been replaced with a single <NuxtPage> component.

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Nested Routes Example

<template>
  <div>
    <NuxtChild
      keep-alive
      :keep-alive-props="{ exclude: ['modal'] }"
      :nuxt-child-key="$route.slug"
    />
  </div>
</template>

<script>
export default {
  transition: 'page', // or { name: 'page' }
}
</script>

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Page Keys and Keep-alive Props

If you were passing a custom page key or keep-alive props to <Nuxt>, you will now use definePageMeta to set these options.

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Page and Layout Transitions

If you have been defining transitions for your page or layout directly in your component options, you will now need to use definePageMeta to set the transition. Since Vue 3, -enter and -leave CSS classes have been renamed. The style prop from <Nuxt> no longer applies to transition when used on <slot>, so move the styles to your -active class. Most of the syntax and functionality are the same for the global NuxtLink component. If you have been using the shortcut <NLink> format, you should update this to use <NuxtLink>. <NuxtLink> is now a drop-in replacement for all links, even external ones. 
<template>
  <div>
    <!-- Internal links -->
    <NuxtLink to="/about">About</NuxtLink>
    
    <!-- External links -->
    <NuxtLink to="https://nuxt.com" external>Nuxt</NuxtLink>
  </div>
</template>

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Programmatic Navigation

When migrating from Nuxt 2 to Nuxt 3, you will have to update how you programmatically navigate your users. In Nuxt 2, you had access to the underlying Vue Router with this.$router. In Nuxt 3, you can use the navigateTo() utility method which allows you to pass a route and parameters to Vue Router.
Make sure to always await on navigateTo or chain its result by returning from functions.
<script>
export default {
  methods: {
    navigate () {
      this.$router.push({
        path: '/search',
        query: {
          name: 'first name',
          type: '1',
        },
      })
    },
  },
}
</script>

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Migration Checklist

  1. Rename any pages with dynamic parameters to match the new format (_id → [id])
  2. Update <Nuxt> and <NuxtChild> to be <NuxtPage>
  3. Replace <Nuxt /> with <slot /> in layouts
  4. If you’re using the Composition API, migrate this.$route and this.$router to use useRoute and useRouter composables
  5. Update <NLink> to <NuxtLink>
  6. Move layout definitions from component options to definePageMeta