vue-patterns

name	vue-patterns
description	Vue.js 3 Composition API patterns, component architecture, reactivity best practices, Pinia state management, Vue Router navigation, and Nuxt SSR patterns. Activates for Vue, Nuxt, Vite, or Pinia projects.
origin	ECC

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