Use Global Properties Sparingly in Plugins

Rule

When using app.config.globalProperties in Vue plugins, use them sparingly and with clear naming conventions. Excessive global properties lead to confusion, naming conflicts, and debugging difficulties.

Why This Matters

1. Implicit dependencies: Global properties make component dependencies invisible, making code harder to understand and maintain.

1. Naming collisions: Multiple plugins may try to use the same property name (e.g., $http, $api), causing silent overwrites.

1. Debugging difficulty: When issues arise, tracing back to which plugin provides a global property is challenging.

1. IDE limitations: Global properties may not have proper autocomplete or type checking without careful configuration.

1. Testing complexity: Global state is harder to mock and isolate in unit tests.

Bad Practice

// Too many global properties from various plugins
app.config.globalProperties.$http = axios
app.config.globalProperties.$api = apiClient
app.config.globalProperties.$auth = authService
app.config.globalProperties.$translate = i18n.translate
app.config.globalProperties.$format = formatters
app.config.globalProperties.$utils = utilities
app.config.globalProperties.$config = appConfig
app.config.globalProperties.$logger = logger

// In component - where did all these come from?
export default {
  mounted() {
    this.$logger.info('Mounted')
    const data = await this.$http.get(this.$config.apiUrl)
    this.$api.process(this.$utils.transform(data))
  }
}

Good Practice

// Use provide/inject for most functionality
export default {
  install(app, options) {
    // Provide services via injection
    app.provide('api', apiClient)
    app.provide('auth', authService)
    app.provide('i18n', i18n)

    // Reserve globalProperties for truly global template helpers
    // that are used extensively in templates across the app
    app.config.globalProperties.$t = i18n.translate  // Common convention
  }
}

// In component - explicit dependencies
<script setup>
import { inject } from 'vue'

const api = inject('api')
const auth = inject('auth')
</script>

<template>
  <!-- $t is acceptable for common template-only usage -->
  <h1>{{ $t('welcome') }}</h1>
</template>

Naming Conventions

If you do use globalProperties:

1. Use $ prefix: This is the Vue convention and avoids conflicts with component data/methods
2. Use unique prefixes for your library: e.g., $myLib_translate for third-party plugins
3. Document all global properties: Keep a central registry of what each plugin provides

// Good: namespaced to avoid conflicts
app.config.globalProperties.$myPlugin = {
  translate: (key) => /* ... */,
  format: (value) => /* ... */
}

// Usage
{{ $myPlugin.translate('key') }}

Auditing Global Properties

You can inspect all global properties for debugging:

console.log(app.config.globalProperties)

When Global Properties Are Acceptable

1. Template-only utilities used very frequently (like $t for translations)
2. Legacy migration when transitioning from Vue 2
3. Libraries that need Options API compatibility (but prefer also providing inject)

References

• Vue.js Plugins Documentation
• Vue.js Global Properties