Vue 3 Composition API

Full Reference: See advanced.md for WebSocket composable, provide/inject plugin pattern, Socket.IO integration, and room management.

Deep Knowledge: Use mcp__documentation__fetch_docs with technology: vue for comprehensive documentation.

When NOT to Use This Skill

Skip this skill when:

• Working with Vue 2 Options API (use legacy Vue docs)
• Building React applications (use frontend-react)
• Using Angular framework (use angular)
• Working with Svelte (use svelte)
• Dealing with server-side only logic (no framework needed)

Component Structure

<script setup lang="ts">
import { ref, computed, onMounted } from 'vue'

interface Props {
  title: string
  count?: number
}

const props = defineProps<Props>()
const emit = defineEmits<{
  update: [value: string]
}>()

const localState = ref('')
const doubled = computed(() => props.count * 2)

onMounted(() => {
  console.log('Component mounted')
})
</script>

<template>
  <div>
    <h1>{{ title }}</h1>
    <p>{{ doubled }}</p>
  </div>
</template>

Reactivity System

Composables Pattern

// useCounter.ts
export function useCounter(initial = 0) {
  const count = ref(initial)
  const increment = () => count.value++
  const decrement = () => count.value--
  return { count, increment, decrement }
}

Key Concepts

• <script setup> is recommended syntax
• Use ref for primitives, reactive for objects
• v-model for two-way binding
• Slots for content distribution

Anti-Patterns

Quick Troubleshooting

Production Readiness

Security Best Practices

<script setup lang="ts">
// NEVER use v-html with user input without sanitization
import DOMPurify from 'dompurify'

const props = defineProps<{ userContent: string }>()
const sanitizedContent = computed(() => DOMPurify.sanitize(props.userContent))
</script>

<template>
  <!-- BAD -->
  <div v-html="userContent" />

  <!-- GOOD -->
  <div v-html="sanitizedContent" />
</template>

// Validate external URLs
const isValidUrl = (url: string): boolean => {
  try {
    const parsed = new URL(url)
    return ['http:', 'https:'].includes(parsed.protocol)
  } catch {
    return false
  }
}

// Never expose secrets in client code
// Use runtime config or server routes instead
const config = useRuntimeConfig()
// config.public.* is safe for client
// config.* (without public) stays server-side

Error Handling

<script setup lang="ts">
import { onErrorCaptured } from 'vue'

// Component-level error boundary
onErrorCaptured((error, instance, info) => {
  // Log to error tracking service
  logError(error, { component: instance?.$options.name, info })

  // Return false to prevent error propagation
  return false
})
</script>

// Global error handler (main.ts)
const app = createApp(App)

app.config.errorHandler = (error, instance, info) => {
  console.error('Global error:', error)
  // Send to error tracking (Sentry, etc.)
  captureException(error, { extra: { info } })
}

app.config.warnHandler = (msg, instance, trace) => {
  // Log warnings in development
  if (import.meta.env.DEV) console.warn(msg, trace)
}

Performance Optimization

<script setup lang="ts">
import { defineAsyncComponent, shallowRef } from 'vue'

// Lazy load heavy components
const HeavyChart = defineAsyncComponent({
  loader: () => import('./HeavyChart.vue'),
  loadingComponent: LoadingSpinner,
  delay: 200,
  errorComponent: ErrorDisplay,
})

// Use shallowRef for large objects that don't need deep reactivity
const largeDataset = shallowRef<DataItem[]>([])

// Computed with getter/setter for derived state
const filteredItems = computed(() =>
  items.value.filter(item => item.active)
)
</script>

<template>
  <!-- Use v-once for static content -->
  <header v-once>
    <h1>{{ appTitle }}</h1>
  </header>

  <!-- Use v-memo for expensive list items -->
  <div v-for="item in list" :key="item.id" v-memo="[item.id, item.updated]">
    <ExpensiveComponent :data="item" />
  </div>

  <!-- Virtual scrolling for large lists -->
  <VirtualList :items="largeDataset" :item-height="50" />
</template>

Accessibility (a11y)

<template>
  <!-- Use semantic HTML -->
  <button @click="handleClick">Submit</button>

  <!-- ARIA for dynamic content -->
  <div role="alert" aria-live="polite" v-if="error">
    {{ error }}
  </div>

  <!-- Focus management -->
  <dialog ref="dialogRef" @vue:mounted="dialogRef?.focus()">
    <h2 id="dialog-title">Confirm Action</h2>
    <div aria-labelledby="dialog-title">...</div>
  </dialog>
</template>

Testing Setup

// Component testing with Vue Test Utils
import { mount } from '@vue/test-utils'
import { describe, it, expect, vi } from 'vitest'

describe('UserForm', () => {
  it('emits submit with form data', async () => {
    const wrapper = mount(UserForm)

    await wrapper.find('input[name="email"]').setValue('test@example.com')
    await wrapper.find('form').trigger('submit')

    expect(wrapper.emitted('submit')).toBeTruthy()
    expect(wrapper.emitted('submit')[0]).toEqual([{ email: 'test@example.com' }])
  })
})

Monitoring Metrics

Build Optimization

// vite.config.ts
export default defineConfig({
  build: {
    rollupOptions: {
      output: {
        manualChunks: {
          vue: ['vue', 'vue-router', 'pinia'],
          ui: ['@headlessui/vue', '@vueuse/core'],
        },
      },
    },
    sourcemap: true,
  },
})

Checklist

• Global error handler configured
• No sensitive data in client state
• DOMPurify for v-html content
• Async components for code splitting
• shallowRef for large non-reactive data
• v-memo for expensive list rendering
• Virtual scrolling for long lists
• Semantic HTML and ARIA labels
• Core Web Vitals monitored
• Bundle size optimized
• Error reporting service integrated

Reference Documentation

Deep Knowledge: Use mcp__documentation__fetch_docs with technology: vue for comprehensive documentation.

• Reactivity Cheatsheet
• Composables Patterns