Install Plugins Before Mounting the App

Rule

All plugins must be installed using app.use() BEFORE calling app.mount(). Installing plugins after the app is mounted can lead to reactivity issues, missing dependencies, and unexpected behavior.

Why This Matters

Hidden dependencies: Components may render before plugins they depend on are available, causing runtime errors.
Reactivity issues: Late plugin installation can cause subtle reactivity problems where provided values aren't properly reactive.
Initialization order: Many plugins (like vue-router, pinia) need to set up state before any component renders.
Ecosystem complexity: Adding plugins after mount can cause issues with Vue's internal ecosystem and hydration in SSR scenarios.

Bad Practice

import { createApp } from 'vue'
import App from './App.vue'
import router from './router'
import i18nPlugin from './plugins/i18n'

const app = createApp(App)

// Mounting first - plugins not yet available!
app.mount('#app')

// Installing after mount - TOO LATE!
app.use(router)
app.use(i18nPlugin, { locale: 'en' })

Good Practice

import { createApp } from 'vue'
import App from './App.vue'
import router from './router'
import { createPinia } from 'pinia'
import i18nPlugin from './plugins/i18n'

const app = createApp(App)

// Install all plugins BEFORE mounting
app.use(createPinia())
app.use(router)
app.use(i18nPlugin, { locale: 'en' })

// Mount LAST
app.mount('#app')

Plugin Installation Order

The order of app.use() calls can matter when plugins depend on each other:

const app = createApp(App)

// 1. State management first (other plugins might need it)
app.use(createPinia())

// 2. Router (may depend on state)
app.use(router)

// 3. Other plugins (may depend on router or state)
app.use(authPlugin)
app.use(i18nPlugin, { locale: 'en' })

// 4. Mount last
app.mount('#app')

Async Plugin Installation

If you need to perform async operations before mounting:

import { createApp } from 'vue'
import App from './App.vue'
import { loadPlugins } from './plugins'

async function bootstrap() {
  const app = createApp(App)

  // Await async plugin setup
  const i18nPlugin = await loadI18nMessages()

  // Install all plugins
  app.use(i18nPlugin)

  // Mount after everything is ready
  app.mount('#app')
}

bootstrap()

Duplicate Installation Protection

Vue's app.use() automatically prevents duplicate plugin installation:

app.use(myPlugin)
app.use(myPlugin) // This second call is ignored - no double installation

// This is handled internally by Vue, providing a safety net

Common Symptoms of Late Plugin Installation

inject() returns undefined unexpectedly
Router navigation guards not firing
Store state not reactive
Template errors about undefined global properties
Hydration mismatches in SSR

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