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agent-skills/skills/vue-best-practices/reference/mount-return-value.md
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title, impact, impactDescription, type, tags
title impact impactDescription type tags
mount() Returns Component Instance, Not App Instance MEDIUM Using mount() return value for app configuration silently fails capability
vue3
createApp
mount
api

mount() Returns Component Instance, Not App Instance

Impact: MEDIUM - The .mount() method returns the root component instance, not the application instance. Attempting to chain app configuration methods after mount() will fail or produce unexpected behavior.

This is a subtle API detail that catches developers who assume mount() returns the app for continued chaining.

Task Checklist

  • Never chain app configuration methods after mount()
  • If you need both instances, store them separately
  • Use the component instance for accessing root component state or methods
  • Use the app instance for configuration, plugins, and global registration

Incorrect:

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

// WRONG: Assuming mount returns app instance
const app = createApp(App).mount('#app')

// This fails! app is actually the root component instance
app.use(router)  // TypeError: app.use is not a function
app.config.errorHandler = fn  // app.config is undefined
// WRONG: Trying to save both in one line
const { app, component } = createApp(App).mount('#app')  // Doesn't work this way

Correct:

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

// Store app instance separately
const app = createApp(App)

// Configure the app
app.use(router)
app.config.errorHandler = (err) => console.error(err)

// Store component instance if needed
const rootComponent = app.mount('#app')

// Now you have access to both:
// - app: the application instance (for config, plugins)
// - rootComponent: the root component instance (for state, methods)
// If you only need the app configured and mounted (most common case):
createApp(App)
  .use(router)
  .use(pinia)
  .mount('#app')  // Return value (component instance) discarded - that's fine

When You Need the Root Component Instance

const app = createApp(App)
const vm = app.mount('#app')

// Access root component's exposed state/methods
console.log(vm.someExposedProperty)
vm.someExposedMethod()

// In Vue 3 with <script setup>, use defineExpose to expose:
// <script setup>
// import { ref } from 'vue'
// const count = ref(0)
// defineExpose({ count })
// </script>

Reference