agent-skills/skills/vue-best-practices/reference/plugin-prefer-provide-inject-over-global-properties.md

Prefer provide/inject Over Global Properties in Plugins

Rule

When creating Vue plugins, prefer using app.provide() to make plugin functionality available to components instead of attaching properties to app.config.globalProperties.

Why This Matters

1. globalProperties don't work in setup(): Properties attached to globalProperties are only accessible via this in Options API. They are NOT available in the Composition API's setup() function.

2. Type safety: provide/inject integrates better with TypeScript and requires less type augmentation boilerplate.

3. Testability: Injected dependencies are easier to mock in tests compared to global properties.

4. Code clarity: Explicit inject() calls make dependencies visible, while global properties can appear "magic".

5. Scoping: provide/inject follows Vue's component hierarchy, making it easier to provide different values to different parts of your app.

Bad Practice

// plugins/i18n.ts
export default {
  install(app, options) {
    // Attaching to globalProperties - only works with Options API
    app.config.globalProperties.$translate = (key: string) => {
      return key.split('.').reduce((o, i) => o?.[i], options)
    }
  }
}

// In component - requires type augmentation for TypeScript
// Also DOES NOT work in <script setup>
export default {
  mounted() {
    console.log(this.$translate('greeting.hello'))
  }
}

Good Practice

// plugins/i18n.ts
import type { InjectionKey, App } from 'vue'

export interface I18nOptions {
  [key: string]: string | I18nOptions
}

export interface I18n {
  translate: (key: string) => string
  options: I18nOptions
}

export const i18nKey: InjectionKey<I18n> = Symbol('i18n')

export default {
  install(app: App, options: I18nOptions) {
    const translate = (key: string): string => {
      return key.split('.').reduce((o, i) => o?.[i], options) as string ?? key
    }

    // Use provide for Composition API compatibility
    app.provide(i18nKey, { translate, options })
  }
}

// In component - works in setup() and has full type safety
<script setup lang="ts">
import { inject } from 'vue'
import { i18nKey } from '@/plugins/i18n'

const i18n = inject(i18nKey)
console.log(i18n?.translate('greeting.hello'))
</script>

Hybrid Approach

If you must support both APIs (e.g., for backwards compatibility), provide both:

export default {
  install(app: App, options: I18nOptions) {
    const i18n = {
      translate: (key: string) => /* ... */
    }

    // For Composition API
    app.provide(i18nKey, i18n)

    // For Options API (use sparingly)
    app.config.globalProperties.$i18n = i18n
  }
}

TypeScript Type Augmentation (if using globalProperties)

If you must use globalProperties, you need proper type augmentation:

// types/vue.d.ts
export {}

declare module 'vue' {
  interface ComponentCustomProperties {
    $translate: (key: string) => string
  }
}

Important: The file MUST contain export {} or another top-level export/import. Without it, the augmentation will OVERWRITE types instead of augmenting them.

References

• Vue.js Plugins Documentation
• Vue.js Provide/Inject
• TypeScript with Options API