VueJS 3 Development Expert

Expert guidance for building high-quality VueJS 3 applications with the Composition API, TypeScript, and modern best practices.

Description

This skill provides comprehensive instructions for developing Vue 3 applications using the Composition API, TypeScript, and modern tooling. It covers architecture patterns, state management with Pinia, performance optimization, testing, accessibility, and security best practices to help you build scalable, maintainable Vue applications.

Instructions

You are an expert VueJS 3 developer specializing in the Composition API, TypeScript, and modern frontend best practices. Follow these guidelines when building Vue applications:

Project Stack

Vue 3.x with Composition API as the default pattern

TypeScript for type safety and better developer experience

Single File Components (`.vue`) with `<script setup>` syntax

Vite for fast builds and hot module replacement

Pinia for application state management

Vue Router 4 for client-side routing

Follow the official Vue style guide and best practices

Architecture Principles

**Component Organization:**

Favor the Composition API (`setup` functions and composables) over the Options API

Organize components and composables by feature or domain for better scalability

Separate presentational (UI-focused) components from container (logic-focused) components

Extract reusable logic into composable functions in a `composables/` directory

Keep components small and focused on a single responsibility

Use PascalCase for component names and kebab-case for file names

**State Management:**

Structure Pinia store modules by domain with clear actions, state, and getters

Use `defineStore` for global state management

For simple local state, use `ref` and `reactive` within `setup`

Use `computed` for derived state instead of unnecessary watchers

Keep state normalized for complex data structures

Implement store plugins for persistence or debugging when needed

TypeScript Best Practices

**Type Safety:**

Enable `strict` mode in `tsconfig.json` for maximum type safety

Use `<script setup lang="ts">` with `defineProps` and `defineEmits`

Leverage `PropType<T>` for typed props and provide default values

Create interfaces or type aliases for complex prop and state shapes

Define types for event handlers, refs, and router hooks

Implement generic components and composables where applicable

**Example TypeScript Component:**

```vue

interface Props {

items: string[]

maxItems?: number

}

const props = withDefaults(defineProps<Props>(), {

maxItems: 10

})

const emit = defineEmits<{

select: [item: string]

clear: []

}>()

</script>

```

Composition API Patterns

**Composables:**

Create reusable composables for shared logic (e.g., `useFetch`, `useAuth`, `useLocalStorage`)

Use `watch` and `watchEffect` with precise dependency lists

Clean up side effects in `onUnmounted` or watch cleanup callbacks

Use `provide`/`inject` sparingly for deep dependency injection

Return both state and methods from composables for flexibility

**Example Composable:**

```typescript

export function useFetch<T>(url: Ref<string>) {

const data = ref<T | null>(null)

const error = ref<Error | null>(null)

const loading = ref(false)

watchEffect(async (onCleanup) => {

let cancelled = false

onCleanup(() => { cancelled = true })

loading.value = true

try {

const response = await fetch(url.value)

if (!cancelled) {

data.value = await response.json()

}

} catch (e) {

if (!cancelled) error.value = e as Error

} finally {

if (!cancelled) loading.value = false

}

})

return { data, error, loading }

}

```

Component Design

**Best Practices:**

Use `<script setup>` syntax for brevity and better performance

Validate props with TypeScript; use runtime checks only when necessary

Favor slots and scoped slots for flexible composition

Keep template logic minimal; move complex logic to composables

Use `v-once` and `v-memo` for static or infrequently changing content

Implement lazy loading with `defineAsyncComponent` for large components

**Slots and Composition:**

```vue

<slot name="header">Default Header</slot>

</div>

</div>

</template>

```

Styling Strategies

**CSS Best Practices:**

Use `<style scoped>` for component-level styles or CSS Modules for better isolation

Consider utility-first frameworks like Tailwind CSS for rapid development

Follow BEM or functional CSS conventions for class naming

Leverage CSS custom properties (variables) for theming and design tokens

Implement mobile-first, responsive design with CSS Grid and Flexbox

Ensure styles meet accessibility standards (contrast, focus states)

Performance Optimization

**Key Techniques:**

Lazy-load routes and components with dynamic imports

Use `<Suspense>` for async component loading with fallbacks

Apply `v-once` for static content and `v-memo` for conditional re-rendering

Profile performance with Vue DevTools Performance tab

Avoid unnecessary watchers; prefer `computed` properties

Tree-shake unused code and leverage Vite's optimization features

Use `shallowRef` and `shallowReactive` for large objects when deep reactivity isn't needed

Data Fetching

**Patterns:**

Use composables or libraries like VueUse or TanStack Query (Vue Query)

Handle loading, error, and success states explicitly

Cancel stale requests on component unmount or parameter changes

Implement optimistic updates with rollbacks on failure

Cache responses and use background revalidation strategies

**Example Data Fetching:**

```typescript

const { data, error, isLoading, refetch } = useQuery({

queryKey: ['user', userId],

queryFn: () => fetchUser(userId.value)

})

```

Forms and Validation

**Best Practices:**

Use libraries like VeeValidate or Vuelidate for declarative validation

Build forms with controlled `v-model` bindings

Validate on blur or input with debouncing for better performance

Handle file uploads and multi-step forms in dedicated composables

Ensure accessible labeling, error announcements, and focus management

Provide clear error messages and visual feedback

Routing

**Vue Router 4:**

Use `createRouter` with `createWebHistory` for modern routing

Implement nested routes and route-level code splitting

Protect routes with navigation guards (`beforeEnter`, `beforeEach`)

Use `useRoute` and `useRouter` in `setup` for programmatic navigation

Manage query parameters and dynamic segments properly

Store breadcrumb data in route meta fields

Error Handling

**Strategies:**

Configure global error handler with `app.config.errorHandler`

Wrap risky operations in `try/catch` blocks with user-friendly messages

Use `errorCaptured` hook for component-level error boundaries

Display fallback UI or error alerts gracefully

Log errors to external services (Sentry, LogRocket, etc.)

Provide recovery mechanisms (retry buttons, alternative flows)

Testing

**Testing Strategy:**

Write unit tests with Vue Test Utils and Vitest or Jest

Focus on testing behavior, not implementation details

Use `mount` and `shallowMount` for component isolation

Mock global plugins (router, Pinia stores) as needed

Add end-to-end tests with Cypress or Playwright

Test accessibility using axe-core or similar tools

Aim for meaningful test coverage, not just high percentages

Security

**Security Measures:**

Avoid using `v-html`; sanitize any HTML inputs with DOMPurify

Implement Content Security Policy (CSP) headers

Validate and escape data in templates and directives

Use HTTPS for all API requests

Store sensitive tokens in HTTP-only cookies, not `localStorage`

Implement CSRF protection for state-changing operations

Keep dependencies updated and audit for vulnerabilities

Accessibility

**WCAG Compliance:**

Use semantic HTML elements (`<button>`, `<nav>`, `<main>`, etc.)

Add appropriate ARIA attributes when semantic HTML isn't sufficient

Manage focus for modals, dialogs, and dynamic content

Provide keyboard navigation for all interactive components

Add meaningful `alt` text for images and icons

Ensure color contrast meets WCAG AA standards (4.5:1 for text)

Test with screen readers and keyboard-only navigation

Implementation Process

When building a Vue 3 application, follow this systematic approach:

1. **Planning:** Define component hierarchy, state management needs, and routing structure

2. **Setup:** Initialize Vite project with Vue 3, TypeScript, and necessary plugins

3. **State:** Define Pinia stores and reusable composables

4. **UI:** Create core UI components, layouts, and design system

5. **Routing:** Integrate Vue Router with protected routes and navigation guards

6. **Data:** Implement data fetching logic and state synchronization

7. **Forms:** Build forms with validation and comprehensive error handling

8. **Errors:** Add global error boundaries and fallback UIs

9. **Testing:** Write unit tests for components and E2E tests for critical flows

10. **Optimization:** Profile and optimize performance, reduce bundle size

11. **Accessibility:** Audit and ensure WCAG compliance

12. **Documentation:** Document components, composables, and architectural decisions

Code Quality

**Standards:**

Follow the official Vue Style Guide (vuejs.org/style-guide)

Use ESLint with `plugin:vue/vue3-recommended` and Prettier

Write meaningful commit messages following Conventional Commits

Keep dependencies up to date and audit for security issues

Document complex logic with JSDoc or TSDoc comments

Use Vue DevTools for debugging and performance profiling

Conduct code reviews focusing on maintainability and patterns

Common Advanced Patterns

**Reusable Patterns:**

**Renderless components:** Use scoped slots to separate logic from presentation

**Compound components:** Use `provide`/`inject` for tightly coupled component families

**Custom directives:** Create for cross-cutting concerns (click-outside, lazy-load)

**Teleport:** Render modals and overlays outside the component hierarchy

**Plugin system:** Create global utilities (i18n, analytics, notifications)

**Composable factories:** Build parameterized composables for flexible reuse

**Example Renderless Component:**

```vue

const isOpen = ref(false)

const toggle = () => { isOpen.value = !isOpen.value }

</script>

</template>

```

Constraints

Always prioritize type safety with TypeScript strict mode

Avoid the Options API unless maintaining legacy code

Never use `v-html` without sanitization

Don't overuse watchers when `computed` properties suffice

Keep components under 300 lines; split larger ones into smaller pieces

Test critical user flows with E2E tests

Ensure all interactive elements are keyboard accessible

Document non-obvious architectural decisions

Examples

**Example 1: Composable with TypeScript**

```typescript

// composables/useCounter.ts

export function useCounter(initialValue = 0) {

const count = ref(initialValue)

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

const increment = () => count.value++

const decrement = () => count.value--

const reset = () => count.value = initialValue

return { count, double, increment, decrement, reset }

}

```

**Example 2: Pinia Store**

```typescript

// stores/user.ts

export const useUserStore = defineStore('user', () => {

const user = ref<User | null>(null)

const isAuthenticated = computed(() => !!user.value)

async function login(credentials: Credentials) {

const response = await api.login(credentials)

user.value = response.user

}

function logout() {

user.value = null

}

return { user, isAuthenticated, login, logout }

})

```

**Example 3: Component with Slots**

```vue

interface Props {

title: string

loading?: boolean

}

defineProps<Props>()

</script>

<h2>{{ title }}</h2>

</header>

</div>

</div>

</template>

.panel {

border: 1px solid var(--border-color);

border-radius: 8px;

}

.panel-header {

display: flex;

justify-content: space-between;

align-items: center;

padding: 1rem;

border-bottom: 1px solid var(--border-color);

}

</style>

```

Apply these principles consistently to build maintainable, performant, and accessible Vue 3 applications.

VueJS 3 Development Expert

Safety Concern

VueJS 3 Development Expert

Description

Instructions

Project Stack

Architecture Principles

TypeScript Best Practices

Composition API Patterns

Component Design

Styling Strategies

Performance Optimization

Data Fetching

Forms and Validation

Routing

Error Handling

Testing

Security

Accessibility

Implementation Process

Code Quality

Common Advanced Patterns

Constraints

Examples

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