Vue 3 Engineering

Overview

To build modern Vue 3 applications, follow industry best practices based on the Composition API, <script setup> syntax, and the 2025 ecosystem (Vite, Vitest, TypeScript). This skill provides comprehensive guidance for Vue 3 development, from component creation to production optimization.

Current Vue Version: Vue 3.5.18 (as of 2025) Build Tool: Vite (Vue CLI is legacy) Testing: Vitest (Jest is legacy for Vue) Standard: Composition API with <script setup> (Options API is legacy)

Decision Tree

User Request → What are you building/fixing?
    |
    ├─ Creating a new component?
    │   └─ Use Quick Start: Creating Components (below)
    │
    ├─ Need reusable logic?
    │   ├─ Load references/composables-patterns.md
    │   └─ Use asset: composable-template.ts
    │
    ├─ State management question (ref vs reactive)?
    │   └─ Load references/reactivity-guide.md
    │
    ├─ Component communication (props/emits/slots)?
    │   └─ Load references/component-patterns.md
    │
    ├─ Performance issue or optimization?
    │   └─ Load references/performance-optimization.md
    │
    ├─ TypeScript setup or typing question?
    │   └─ Load references/typescript-integration.md
    │
    ├─ Writing tests?
    │   └─ Load references/testing-guide.md
    │
    ├─ Reviewing code or debugging?
    │   └─ Load references/anti-patterns.md
    │
    └─ Learning Composition API basics?
        └─ Load references/composition-api-guide.md

Quick Start: Creating Components

Basic Component with <script setup>

<script setup>
import { ref, computed } from 'vue'

const count = ref(0)
const double = computed(() => count.value * 2)

function increment() {
  count.value++
}
</script>

<template>
  <div>
    <p>Count: {{ count }}</p>
    <p>Double: {{ double }}</p>
    <button @click="increment">Increment</button>
  </div>
</template>

<style scoped>
/* Component styles */
</style>

Component with Props and Emits (TypeScript)

<script setup lang="ts">
// Props
const props = defineProps<{
  title: string
  count?: number
}>()

// Emits
const emit = defineEmits<{
  update: [value: number]
  close: []
}>()

// Local state
const localCount = ref(props.count ?? 0)

// Methods
function handleUpdate() {
  emit('update', localCount.value)
}
</script>

<template>
  <div>
    <h2>{{ title }}</h2>
    <button @click="handleUpdate">Update</button>
  </div>
</template>

For a complete component template, use component-template.vue.

Quick Start: Creating Composables

Composables are reusable functions that encapsulate stateful logic.

Basic Composable Pattern

import { ref, computed } from 'vue'

export function useCounter(initialValue = 0) {
  const count = ref(initialValue)
  const double = computed(() => count.value * 2)

  function increment() {
    count.value++
  }

  function decrement() {
    count.value--
  }

  return {
    count,
    double,
    increment,
    decrement
  }
}

Usage in component:

<script setup>
import { useCounter } from '@/composables/useCounter'

const { count, double, increment } = useCounter(5)
</script>

For a complete composable template, use composable-template.ts.

Core Workflows

Workflow 1: Building a Component

1. Create file with .vue extension
2. Add <script setup lang="ts"> at top
3. Define props with defineProps<T>()
4. Define emits with defineEmits<T>()
5. Add reactive state with ref() or reactive()
6. Create computed values for derived state
7. Add methods as regular functions
8. Use lifecycle hooks (onMounted, onUnmounted, etc.)
9. Build template using component state
10. Add scoped styles

Workflow 2: Creating a Composable

1. Create file in composables/ directory with use*.ts naming
2. Accept options as parameter (single object preferred)
3. Create reactive state with ref() or reactive()
4. Add computed values for derived state
5. Define methods that manipulate state
6. Handle cleanup with onUnmounted if needed
7. Return refs and methods as object
8. Document with JSDoc and TypeScript types

Workflow 3: Optimizing Performance

1. Analyze bundle with Vite plugins
2. Implement lazy loading for routes and components
3. Use v-show vs v-if appropriately
4. Add keys to v-for lists
5. Use computed instead of methods for derived data
6. Implement virtual scrolling for large lists
7. Code split vendor dependencies
8. Monitor with Lighthouse and Vue DevTools

Workflow 4: Testing a Component

1. Setup Vitest in project
2. Create *.spec.ts file next to component
3. Mount component with @vue/test-utils or @testing-library/vue
4. Test user interactions (clicks, inputs, etc.)
5. Assert rendered output (not implementation details)
6. Test props and emits
7. Mock external dependencies
8. Run tests with npm run test

Common Patterns

Pattern 1: Form Handling

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

interface FormData {
  email: string
  password: string
}

const form = reactive<FormData>({
  email: '',
  password: ''
})

const errors = reactive({
  email: '',
  password: ''
})

const isValid = computed(() => {
  return form.email.includes('@') && form.password.length >= 8
})

function validate() {
  errors.email = form.email.includes('@') ? '' : 'Invalid email'
  errors.password = form.password.length >= 8 ? '' : 'Password too short'
}

async function handleSubmit() {
  validate()
  if (!isValid.value) return

  // Submit form
  await api.submit(form)
}
</script>

Pattern 2: API Integration

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

const data = ref(null)
const loading = ref(false)
const error = ref(null)

async function fetchData() {
  loading.value = true
  error.value = null

  try {
    const response = await fetch('/api/data')
    data.value = await response.json()
  } catch (e) {
    error.value = e.message
  } finally {
    loading.value = false
  }
}

onMounted(() => {
  fetchData()
})
</script>

<template>
  <div v-if="loading">Loading...</div>
  <div v-else-if="error">Error: {{ error }}</div>
  <div v-else>{{ data }}</div>
</template>

Pattern 3: Modal/Dialog Component

<script setup lang="ts">
const props = defineProps<{
  modelValue: boolean
  title: string
}>()

const emit = defineEmits<{
  'update:modelValue': [value: boolean]
  confirm: []
}>()

function close() {
  emit('update:modelValue', false)
}

function confirm() {
  emit('confirm')
  close()
}
</script>

<template>
  <Teleport to="body">
    <div v-if="modelValue" class="modal-overlay" @click="close">
      <div class="modal-content" @click.stop>
        <h2>{{ title }}</h2>
        <slot />
        <div class="modal-actions">
          <button @click="close">Cancel</button>
          <button @click="confirm">Confirm</button>
        </div>
      </div>
    </div>
  </Teleport>
</template>

Pattern 4: List with Virtual Scrolling

Use virtual scrolling for large lists (1000+ items):

<script setup>
// Use a virtual scrolling library like vue-virtual-scroller
import { RecycleScroller } from 'vue-virtual-scroller'
import 'vue-virtual-scroller/dist/vue-virtual-scroller.css'

const items = ref(Array.from({ length: 10000 }, (_, i) => ({
  id: i,
  name: `Item ${i}`
})))
</script>

<template>
  <RecycleScroller
    :items="items"
    :item-size="50"
    key-field="id"
    v-slot="{ item }"
  >
    <div class="item">{{ item.name }}</div>
  </RecycleScroller>
</template>

Best Practices (2025)

✅ Do:

1. Use <script setup> for all new components - It's the 2025 standard
2. Prefer ref() over reactive() - More consistent and flexible
3. Extract reusable logic into composables - Keep components lean
4. Use TypeScript for medium+ projects - Better DX and fewer bugs
5. Test with Vitest - Modern, fast, Vite-integrated
6. Lazy load routes and components - Better performance
7. Use computed for derived state - Automatic caching
8. Clean up side effects in onUnmounted - Prevent memory leaks
9. Use Vite for builds - Faster than Webpack
10. Follow naming conventions - use* for composables, PascalCase for components

❌ Don't:

1. Don't use Options API for new projects - Legacy as of 2025
2. Don't misuse v-if/v-show - Wrong choice = 3x slower
3. Don't prop drill - Use provide/inject or Pinia
4. Don't mutate props - Props are read-only
5. Don't forget keys in v-for - Causes rendering bugs
6. Don't create monolithic components - Break them down
7. Don't overuse watchers - Use computed when possible
8. Don't skip cleanup - Always clean up in lifecycle hooks
9. Don't use Vue CLI - Use Vite + create-vue
10. Don't use Jest - Use Vitest for Vue 3

Resources

This skill includes comprehensive reference guides and ready-to-use templates:

references/composition-api-guide.md

Complete guide to Vue 3 Composition API and <script setup> syntax. Covers defineProps, defineEmits, lifecycle hooks, template refs, and combining with normal <script>.

Load when: Learning Composition API, understanding <script setup>, working with props/emits, using lifecycle hooks.

references/reactivity-guide.md

Deep dive into Vue 3's reactivity system. Covers ref() vs reactive(), toRefs(), computed properties, watch() vs watchEffect(), and common reactivity pitfalls.

Load when: Choosing between ref and reactive, managing state, creating computed values, watching for changes, debugging reactivity issues.

references/composables-patterns.md

Comprehensive guide to creating reusable composables. Covers naming conventions, structure patterns, common use cases (API fetching, event listeners), VueUse examples, and testing composables.

Load when: Creating composables, extracting reusable logic, learning from VueUse patterns, testing composables.

references/component-patterns.md

Guide to Vue 3 component design patterns. Covers props (TypeScript + validation), emits (typed events), slots (default, named, scoped), provide/inject, and component composition strategies.

Load when: Designing component APIs, working with props/emits/slots, using provide/inject, composing components.

references/performance-optimization.md

Performance optimization techniques for Vue 3. Covers lazy loading, code splitting, v-show vs v-if, virtual scrolling, computed caching, bundle optimization, and Vue 3.5 performance improvements (56% memory reduction).

Load when: Optimizing performance, reducing bundle size, implementing lazy loading, debugging slow renders, analyzing builds.

references/typescript-integration.md

Complete TypeScript integration guide. Covers project setup with Vite, typing props/emits/refs, generic components, Volar extension setup, and migration from JavaScript.

Load when: Setting up TypeScript, typing components, using Volar, migrating from JavaScript, resolving type errors.

references/testing-guide.md

Testing strategies with Vitest and Testing Library. Covers Vitest setup, component testing, testing props/emits/slots, testing composables, mocking, async testing, and E2E considerations.

Load when: Writing tests, setting up Vitest, testing components/composables, mocking dependencies, debugging tests.

references/anti-patterns.md

Common Vue 3 mistakes and anti-patterns to avoid. Covers 14 anti-patterns including Options API usage, v-if/v-show misuse, prop drilling, reactivity pitfalls, memory leaks, and bundle size issues.

Load when: Reviewing code, debugging issues, learning what to avoid, code review feedback, performance problems.

assets/component-template.vue

Production-ready Vue 3 component template with TypeScript, props, emits, reactive state, computed values, methods, and lifecycle hooks. Includes scoped styles and comprehensive examples.

Use when: Creating new components, need a starting template, teaching component structure.

assets/composable-template.ts

Production-ready composable template with TypeScript, options object, reactive state, computed values, cleanup, and full JSDoc documentation. Includes useCounter example.

Use when: Creating new composables, need a starting template, learning composable structure.

assets/vite.config.template.ts

Complete Vite configuration for Vue 3 projects. Includes Vue plugin setup, path aliases, dev server config, build optimization, chunk splitting, and CSS preprocessing.

Use when: Setting up new project, configuring Vite, optimizing build, adding aliases.

Tips for Success

1. Start with <script setup> - Cleaner syntax, better performance, less boilerplate
2. Think in composables - Extract logic early, keep components focused on UI
3. Use the template files - Don't start from scratch, use provided templates
4. Load references as needed - Don't try to memorize everything, use references strategically
5. Follow the decision tree - It guides you to the right resource quickly
6. Use TypeScript - Even basic typing catches 80% of bugs
7. Test with Vitest - Fast feedback loop, great DX
8. Optimize early - Lazy load from the start, don't wait for problems
9. Learn from VueUse - Study well-written composables
10. Keep up with Vue updates - Vue 3.5+ brings significant improvements

Version Notes

This skill is based on:

• Vue 3.5.18 (current as of 2025)
• Composition API as standard (Options API is legacy)
• Vite 6 as build tool (Vue CLI is deprecated)
• Vitest 3 as test runner (Jest is legacy for Vue)
• Volar as IDE extension (Vetur is deprecated)
• TypeScript 5 for type safety

The Vue ecosystem has matured significantly in 2025. The Composition API with <script setup> is now the standard approach for all new Vue development.