Agent Skills: Vue.js Patterns and Best Practices

Vue.js Patterns and Best Practices

Comprehensive guide for Vue.js 3 development using Composition API (<script setup>), covering component design, reactivity, state management, routing, testing, and SSR patterns. Nuxt-specific guidance is included where it differs from vanilla Vue.

When to Activate

Activate this skill when:

• The project uses Vue.js (any version), Nuxt, Vite + Vue, or Pinia.
• The user asks about Vue component architecture, composables, reactivity, or state management.
• Reviewing Vue Single-File Components (.vue files).
• Setting up Vue Router, Pinia stores, or Vite/Vitest configuration.
• Discussing Vue-specific performance, security, or SSR patterns.

1. Project Structure

Recommended Layout (Feature-First)

src/
├── api/              # API client and endpoint definitions
├── assets/           # Static assets (images, fonts, icons)
├── components/       # Shared/reusable components
│   ├── base/         # Base UI primitives (Button, Input, Modal)
│   └── features/     # Feature-specific shared components
├── composables/      # Reusable Composition API logic
├── layouts/          # Page layouts (optional)
├── pages/            # Route-level page components
├── router/           # Vue Router configuration
├── stores/           # Pinia stores
├── types/            # TypeScript type definitions
├── utils/            # Pure utility functions
└── App.vue           # Root component

File Naming

| Convention | When to Use | |-----------|-------------| | PascalCase.vue | All components (enforced by vue/multi-word-component-names) | | useCamelCase.ts | Composables | | camelCase.ts | Utilities, API clients, types | | kebab-case directories | Route segments, feature folders |

2. Component Architecture

Single-File Component Order

<script setup lang="ts">
// 1. Imports (vue → ecosystem → absolute → relative)
// 2. Props & Emits & Slots
// 3. Composables
// 4. Local state (ref/reactive)
// 5. Computed properties
// 6. Methods
// 7. Watchers
// 8. Lifecycle hooks
</script>

<template>
  <!-- Template content -->
</template>

<style scoped>
  /* Scoped styles */
</style>

Presentational vs Container

• Container components: Own data fetching, state, and side effects. Render presentational components.
• Presentational components: Receive props, emit events. No API calls, no store access. Pure rendering.

Props Best Practices

// Type-based props with defaults
interface Props {
  label: string;
  variant?: "primary" | "secondary";
  disabled?: boolean;
  items: Item[];
}

const props = withDefaults(defineProps<Props>(), {
  variant: "primary",
  disabled: false,
});

• Always provide type, and required/default where appropriate.
• Boolean props: isXxx, hasXxx, canXxx.
• Never mutate props — emit events instead.
• For v-model binding, use defineModel() (Vue 3.4+) or modelValue + update:modelValue.

Events

const emit = defineEmits<{
  submit: [];
  "update:modelValue": [value: string];
  select: [id: string, index: number];
}>();

• Use kebab-case in templates (@update:model-value).
• Use camelCase in script (emit("update:modelValue", val)).

3. Composables (Reusable Logic)

Structure

// composables/useDebounce.ts
export function useDebounce<T>(value: MaybeRef<T>, delay: number): Ref<T> {
  const debounced = ref(toValue(value)) as Ref<T>;

  let timer: ReturnType<typeof setTimeout>;
  watch(
    () => toValue(value),
    (newVal) => {
      clearTimeout(timer);
      timer = setTimeout(() => { debounced.value = newVal; }, delay);
    }
  );

  onUnmounted(() => clearTimeout(timer));
  return readonly(debounced);
}

Rules

• Must start with use prefix.
• Return reactive values (ref, computed, reactive), never plain primitives.
• Accept reactive inputs via MaybeRef / toRef() / toValue().
• Clean up side effects in onUnmounted or watcher onCleanup.
• No module-scope side effects.

vs Mixins

Composables replace Vue 2 mixins entirely:

• Mixins: Opaque data flow, source-of-truth collisions, name conflicts.
• Composables: Explicit imports, clear return values, composable and tree-shakable.

4. State Management

When to Use What

| Pattern | Use Case | |---------|----------| | ref() / reactive() | Local component state | | Props + Emits | Parent-child communication | | Provide / Inject | Theme, config, plugin API | | Pinia store | Global, shared, complex state | | Server state composable | API data with caching (wrap fetch/TanStack Query) |

Pinia Setup Store (Preferred)

// stores/useCartStore.ts
export const useCartStore = defineStore("cart", () => {
  const items = ref<CartItem[]>([]);
  const isLoading = ref(false);

  const totalPrice = computed(() =>
    items.value.reduce((sum, i) => sum + i.price * i.quantity, 0)
  );
  const itemCount = computed(() =>
    items.value.reduce((sum, i) => sum + i.quantity, 0)
  );

  async function addItem(productId: string) {
    isLoading.value = true;
    try {
      const item = await fetchProduct(productId);
      const existing = items.value.find(i => i.id === item.id);
      if (existing) existing.quantity++;
      else items.value.push({ ...item, quantity: 1 });
    } finally {
      isLoading.value = false;
    }
  }

  return { items, isLoading, totalPrice, itemCount, addItem };
});

• Use Setup Store syntax (not Options Store).
• Prefer actions for business-level mutations and $patch() for grouped updates.
• Every async action: handle loading + success + error.

5. Vue Router

Route Definitions

const routes = [
  {
    path: "/users/:id",
    name: "user-detail",
    component: () => import("@/pages/UserDetail.vue"), // lazy
    props: true, // pass params as props
    meta: { requiresAuth: true },
  },
];

Navigation Guards

router.beforeEach((to, from) => {
  const { isLoggedIn } = useAuthStore();
  if (to.meta.requiresAuth && !isLoggedIn) {
    return { name: "login", query: { redirect: to.fullPath } };
  }
});

Reactive Route Params

When a component stays mounted but route params change:

const route = useRoute();
const id = computed(() => route.params.id as string);
watch(id, (newId) => fetchItem(newId));

6. Template Patterns

Template Syntax

<!-- v-if/v-else-if/v-else -->
<div v-if="isLoading">Loading...</div>
<div v-else-if="error">Error: {{ error }}</div>
<div v-else>{{ content }}</div>

<!-- v-show for frequent toggles -->
<div v-show="isOpen">Toggled content</div>

<!-- v-for with stable keys -->
<div v-for="item in items" :key="item.id">{{ item.name }}</div>

<!-- Computed filtered list (not v-if + v-for on same element) -->
<div v-for="item in activeItems" :key="item.id">{{ item.name }}</div>

<!-- Event handling -->
<form @submit.prevent="handleSubmit">
  <button type="submit">Save</button>
</form>

<!-- v-model -->
<input v-model="name" />
<CustomInput v-model="value" v-model:title="title" />

7. Performance

| Technique | When to Use | |-----------|-------------| | v-memo | List items that rarely change | | v-once | Content rendered once and static forever | | shallowRef() | Large data structures replaced wholesale | | shallowReactive() | Only top-level properties are reactive | | v-show over v-if | Frequent visibility toggles | | <KeepAlive :max="10"> | Cache toggled views | | Lazy routes | () => import(...) for non-critical routes | | Suspense | Async component loading with fallback |

8. Testing

Stack

• Vitest for unit and component tests
• Vue Test Utils for mounting and interaction
• @pinia/testing for store mocking
• Playwright for E2E

Component Test Pattern

import { mount } from "@vue/test-utils";
import { createPinia, setActivePinia } from "pinia";
import UserCard from "./UserCard.vue";

beforeEach(() => { setActivePinia(createPinia()); });

it("renders and emits", async () => {
  const wrapper = mount(UserCard, {
    props: { user: { id: "1", name: "Alice" } },
  });
  expect(wrapper.text()).toContain("Alice");
  await wrapper.find("button").trigger("click");
  expect(wrapper.emitted("select")![0]).toEqual(["1"]);
});

9. Nuxt-Specific Patterns

Auto-Imports

Nuxt auto-imports ref, computed, watch, useFetch, useAsyncData, etc. Use them directly without importing. For non-Nuxt projects, always import explicitly.

useAsyncData / useFetch

const { data: user, pending, error, refresh } = await useAsyncData(
  "user", // unique key for caching
  () => $fetch(`/api/users/${id}`),
);

const { data: posts } = await useFetch("/api/posts", {
  query: { page: 1 },
  key: "posts-page-1", // dedupes requests
});

Server Routes

// server/api/users/[id].ts
export default defineEventHandler(async (event) => {
  const { id } = await getValidatedRouterParams(event, z.object({
    id: z.string().uuid(),
  }).parse);
  // ... fetch and return
});

Runtime Config

// nuxt.config.ts
export default defineNuxtConfig({
  runtimeConfig: {
    // server-only
    apiSecret: "",
    // public (exposed to client)
    public: {
      apiBase: "https://api.example.com",
    },
  },
});

10. Vue 3.5+ New APIs

Reactive Props Destructure

Vue 3.5 stabilized reactive props destructure — destructured variables from defineProps() are automatically reactive:

// Vue 3.5+: destructured props are reactive (no need for toRefs)
const { count = 0, msg = "hello" } = defineProps<{
  count?: number;
  msg?: string;
}>();

// Limitation: cannot watch destructured prop directly
watch(() => count, (newVal) => { ... }); // PASS getter required

useTemplateRef()

Replace name-matched plain refs with useTemplateRef() for template references:

import { useTemplateRef } from "vue";
const inputEl = useTemplateRef<HTMLInputElement>("input");
// "input" matches the ref="input" attribute in template, not the variable name

Supports dynamic ref IDs: useTemplateRef(dynamicRefId).

onWatcherCleanup()

Globally importable watcher cleanup API (Vue 3.5+). It must be called synchronously inside the watcher callback:

import { watch, onWatcherCleanup } from "vue";

watch(userId, async (newId) => {
  const controller = new AbortController();
  onWatcherCleanup(() => controller.abort());
  // ... fetch with signal
});

useId()

SSR-stable unique ID generation for form elements and accessibility:

import { useId } from "vue";
const id = useId();

defer Teleport

<Teleport defer> allows teleporting to targets rendered in the same cycle:

<Teleport defer to="#container">Content</Teleport>
<div id="container"></div>

Lazy Hydration (SSR)

defineAsyncComponent() now supports hydrate strategy:

import { defineAsyncComponent, hydrateOnVisible } from "vue";
const AsyncComp = defineAsyncComponent({
  loader: () => import("./Comp.vue"),
  hydrate: hydrateOnVisible(),
});

Anti-Patterns

| Anti-Pattern | Why It's Wrong | The Fix | |-------------|---------------|---------| | Destructuring defineProps() (Vue < 3.5) | Captures snapshot, loses reactivity | Access via props.xxx or use toRefs() | | watch() on destructured prop (Vue 3.5+) | Compile-time error — destructured props can't be watched directly | Use getter wrapper: watch(() => count, ...) | | v-if + v-for on same element | Ambiguous execution order | Use computed filtered array | | v-for key = index | Broken state on reorder | Use stable database IDs | | Mutating props | Violates one-way data flow | Emit events or use v-model | | v-html with user content | XSS vulnerability | Sanitize with DOMPurify | | Mixins in Vue 3 | Opaque, collision-prone | Replace with composables | | Module-scope side effects in composable | Shared across instances | Scope in onMounted + onUnmounted | | reactive() for replaceable state | Replacement breaks reactivity | Use ref() instead | | Watcher without cleanup | Memory leaks, race conditions | Use onCleanup or onWatcherCleanup() (Vue 3.5+) | | Options API in new Vue 3 code | Ecosystem move to Composition API | Use <script setup> | | Plain ref for template references | No dynamic ref support, name-matching fragile | Use useTemplateRef() (Vue 3.5+) |

Related Skills

• accessibility — ARIA, semantic HTML, focus management
• frontend-patterns — Cross-framework frontend architecture
• typescript — TypeScript best practices applied to Vue projects
• coding-standards — General code quality standards