Vue 3 - Progressive JavaScript Framework

Overview

Vue 3 is a progressive framework for building user interfaces with emphasis on approachability, performance, and flexibility. It features the Composition API for better logic reuse, a powerful reactivity system, and single-file components (.vue files).

Key Features:

• Composition API: setup() with ref, reactive, computed, watch
• Reactivity System: Fine-grained reactive data tracking
• Single-File Components: Template, script, style in one file
• Vue Router: Official routing for SPAs
• Pinia: Modern state management (Vuex successor)
• TypeScript: First-class TypeScript support
• Vite: Lightning-fast development with HMR

Installation:

# Create new Vue 3 project (recommended)
npm create vue@latest my-app
cd my-app
npm install
npm run dev

# Or with Vite template
npm create vite@latest my-app -- --template vue-ts

Composition API Fundamentals

setup() Function

<script setup lang="ts">
// Modern <script setup> syntax (recommended)
import { ref, computed, onMounted } from 'vue';

// Reactive state
const count = ref(0);
const message = ref('Hello Vue 3');

// Computed values
const doubled = computed(() => count.value * 2);

// Methods
function increment() {
  count.value++;
}

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

<template>
  <div>
    <p>Count: {{ count }} (Doubled: {{ doubled }})</p>
    <button @click="increment">Increment</button>
  </div>
</template>

Reactive State with ref() and reactive()

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

// ref() - for primitives and objects (needs .value in script)
const count = ref(0);
const user = ref({ name: 'Alice', age: 30 });

console.log(count.value); // 0
console.log(user.value.name); // 'Alice'

// reactive() - for objects only (no .value needed)
const state = reactive({
  todos: [] as Todo[],
  filter: 'all',
  error: null as string | null
});

console.log(state.todos); // []
state.todos.push({ id: 1, text: 'Learn Vue', done: false });
</script>

<template>
  <!-- In template, .value is automatic for refs -->
  <p>Count: {{ count }}</p>
  <p>User: {{ user.name }}</p>
  <p>Todos: {{ state.todos.length }}</p>
</template>

Computed Properties

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

const firstName = ref('John');
const lastName = ref('Doe');

// Read-only computed
const fullName = computed(() => `${firstName.value} ${lastName.value}`);

// Writable computed
const fullNameWritable = computed({
  get() {
    return `${firstName.value} ${lastName.value}`;
  },
  set(value: string) {
    const parts = value.split(' ');
    firstName.value = parts[0];
    lastName.value = parts[1];
  }
});

// Complex computations
interface Todo {
  id: number;
  text: string;
  done: boolean;
}

const todos = ref<Todo[]>([
  { id: 1, text: 'Learn Vue', done: true },
  { id: 2, text: 'Build app', done: false }
]);

const completedTodos = computed(() =>
  todos.value.filter(t => t.done)
);

const activeTodos = computed(() =>
  todos.value.filter(t => !t.done)
);

const progress = computed(() =>
  todos.value.length > 0
    ? (completedTodos.value.length / todos.value.length) * 100
    : 0
);
</script>

<template>
  <div>
    <p>Full Name: {{ fullName }}</p>
    <p>Progress: {{ progress.toFixed(1) }}%</p>
    <p>Active: {{ activeTodos.length }} | Done: {{ completedTodos.length }}</p>
  </div>
</template>

Watchers and Side Effects

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

const count = ref(0);
const user = ref({ name: 'Alice', age: 30 });

// watch() - explicit dependencies
watch(count, (newVal, oldVal) => {
  console.log(`Count changed from ${oldVal} to ${newVal}`);
});

// Watch multiple sources
watch([count, user], ([newCount, newUser], [oldCount, oldUser]) => {
  console.log('Count or user changed');
});

// Watch object property (needs getter)
watch(
  () => user.value.name,
  (newName, oldName) => {
    console.log(`Name changed from ${oldName} to ${newName}`);
  }
);

// Deep watch for nested objects
watch(
  user,
  (newUser) => {
    console.log('User object changed deeply');
  },
  { deep: true }
);

// watchEffect() - automatic dependency tracking
watchEffect(() => {
  // Automatically watches count and user
  console.log(`Count: ${count.value}, User: ${user.value.name}`);
});

// Cleanup function
watchEffect((onCleanup) => {
  const timer = setTimeout(() => {
    console.log('Delayed effect');
  }, 1000);

  onCleanup(() => {
    clearTimeout(timer);
  });
});
</script>

Component Props and Events

Defining Props (TypeScript)

<script setup lang="ts">
// Type-safe props with defineProps
interface Props {
  title: string;
  count?: number;
  tags?: string[];
  user: {
    name: string;
    email: string;
  };
  disabled?: boolean;
}

// With defaults
const props = withDefaults(defineProps<Props>(), {
  count: 0,
  tags: () => [],
  disabled: false
});

// Access props
console.log(props.title);
console.log(props.count);
</script>

<template>
  <div>
    <h1>{{ title }}</h1>
    <p>Count: {{ count }}</p>
    <p>Tags: {{ tags.join(', ') }}</p>
  </div>
</template>

Emitting Events

<script setup lang="ts">
// Define emitted events with types
const emit = defineEmits<{
  update: [value: number];
  submit: [data: { name: string; email: string }];
  delete: [id: number];
}>();

function handleClick() {
  emit('update', 42);
}

function handleSubmit() {
  emit('submit', { name: 'Alice', email: 'alice@example.com' });
}
</script>

<template>
  <button @click="handleClick">Update</button>
  <button @click="handleSubmit">Submit</button>
</template>

v-model for Two-Way Binding

<!-- Child: CustomInput.vue -->
<script setup lang="ts">
// v-model creates 'modelValue' prop and 'update:modelValue' event
const props = defineProps<{
  modelValue: string;
  placeholder?: string;
}>();

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

function handleInput(event: Event) {
  const target = event.target as HTMLInputElement;
  emit('update:modelValue', target.value);
}
</script>

<template>
  <input
    :value="modelValue"
    @input="handleInput"
    :placeholder="placeholder"
  />
</template>

<!-- Parent.vue -->
<script setup lang="ts">
import { ref } from 'vue';
import CustomInput from './CustomInput.vue';

const searchQuery = ref('');
</script>

<template>
  <CustomInput v-model="searchQuery" placeholder="Search..." />
  <p>Searching for: {{ searchQuery }}</p>
</template>

Multiple v-model Bindings

<!-- Child: UserForm.vue -->
<script setup lang="ts">
defineProps<{
  firstName: string;
  lastName: string;
}>();

const emit = defineEmits<{
  'update:firstName': [value: string];
  'update:lastName': [value: string];
}>();
</script>

<template>
  <div>
    <input
      :value="firstName"
      @input="emit('update:firstName', ($event.target as HTMLInputElement).value)"
    />
    <input
      :value="lastName"
      @input="emit('update:lastName', ($event.target as HTMLInputElement).value)"
    />
  </div>
</template>

<!-- Parent.vue -->
<script setup lang="ts">
import { ref } from 'vue';
import UserForm from './UserForm.vue';

const first = ref('John');
const last = ref('Doe');
</script>

<template>
  <UserForm v-model:first-name="first" v-model:last-name="last" />
  <p>Full name: {{ first }} {{ last }}</p>
</template>

Template Syntax

Directives

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

const message = ref('Hello Vue');
const isActive = ref(true);
const hasError = ref(false);
const items = ref(['Apple', 'Banana', 'Cherry']);
const user = ref({ name: 'Alice', email: 'alice@example.com' });

const formData = reactive({
  username: '',
  agree: false,
  gender: 'male',
  interests: [] as string[]
});
</script>

<template>
  <!-- Text interpolation -->
  <p>{{ message }}</p>

  <!-- Raw HTML (careful with XSS!) -->
  <div v-html="'<strong>Bold</strong>'"></div>

  <!-- Attribute binding -->
  <div :id="'container-' + user.name"></div>
  <img :src="user.avatar" :alt="user.name" />

  <!-- Class binding -->
  <div :class="{ active: isActive, 'text-danger': hasError }"></div>
  <div :class="[isActive ? 'active' : '', hasError && 'error']"></div>

  <!-- Style binding -->
  <div :style="{ color: 'red', fontSize: '16px' }"></div>
  <div :style="{ color: isActive ? 'green' : 'gray' }"></div>

  <!-- Conditional rendering -->
  <p v-if="isActive">Active</p>
  <p v-else-if="hasError">Error</p>
  <p v-else>Inactive</p>

  <!-- v-show (toggles display CSS) -->
  <p v-show="isActive">Visible when active</p>

  <!-- List rendering -->
  <ul>
    <li v-for="(item, index) in items" :key="index">
      {{ index + 1 }}. {{ item }}
    </li>
  </ul>

  <!-- Object iteration -->
  <div v-for="(value, key) in user" :key="key">
    {{ key }}: {{ value }}
  </div>

  <!-- Event handling -->
  <button @click="isActive = !isActive">Toggle</button>
  <button @click.prevent="handleSubmit">Submit</button>
  <input @keyup.enter="handleSearch" />

  <!-- Form binding -->
  <input v-model="formData.username" />
  <input type="checkbox" v-model="formData.agree" />
  <input type="radio" v-model="formData.gender" value="male" />
  <input type="radio" v-model="formData.gender" value="female" />
  <select v-model="formData.interests" multiple>
    <option>Reading</option>
    <option>Gaming</option>
    <option>Coding</option>
  </select>
</template>

Event Modifiers

<template>
  <!-- Prevent default -->
  <form @submit.prevent="handleSubmit">
    <button type="submit">Submit</button>
  </form>

  <!-- Stop propagation -->
  <div @click="handleOuter">
    <button @click.stop="handleInner">Click me</button>
  </div>

  <!-- Capture mode -->
  <div @click.capture="handleCapture">...</div>

  <!-- Self (only if event.target is the element itself) -->
  <div @click.self="handleSelf">...</div>

  <!-- Once (trigger at most once) -->
  <button @click.once="handleOnce">Click once</button>

  <!-- Key modifiers -->
  <input @keyup.enter="handleEnter" />
  <input @keyup.esc="handleEscape" />
  <input @keyup.ctrl.s="handleSave" />
  <input @keyup.shift.t="handleShiftT" />

  <!-- Mouse button modifiers -->
  <div @click.left="handleLeftClick"></div>
  <div @click.right="handleRightClick"></div>
  <div @click.middle="handleMiddleClick"></div>
</template>

Lifecycle Hooks

<script setup lang="ts">
import {
  onBeforeMount,
  onMounted,
  onBeforeUpdate,
  onUpdated,
  onBeforeUnmount,
  onUnmounted,
  onErrorCaptured
} from 'vue';

// Before component is mounted
onBeforeMount(() => {
  console.log('Component about to mount');
});

// After component is mounted (DOM is ready)
onMounted(() => {
  console.log('Component mounted');
  // Good place for API calls, DOM manipulation
  fetchData();
});

// Before component updates due to reactive changes
onBeforeUpdate(() => {
  console.log('Component about to update');
});

// After component updates
onUpdated(() => {
  console.log('Component updated');
  // Careful: can cause infinite loops if you update state here
});

// Before component unmounts
onBeforeUnmount(() => {
  console.log('Component about to unmount');
  // Clean up subscriptions, timers, etc.
});

// After component unmounts
onUnmounted(() => {
  console.log('Component unmounted');
});

// Error handling
onErrorCaptured((err, instance, info) => {
  console.error('Error captured:', err, info);
  return false; // Prevent propagation
});

async function fetchData() {
  const response = await fetch('/api/data');
  const data = await response.json();
  console.log(data);
}
</script>

Provide/Inject (Dependency Injection)

<!-- Parent.vue -->
<script setup lang="ts">
import { ref, provide } from 'vue';
import type { InjectionKey } from 'vue';

interface Theme {
  primary: string;
  secondary: string;
}

// Create typed injection key
export const ThemeKey: InjectionKey<Theme> = Symbol('theme');

const theme = ref<Theme>({
  primary: '#007bff',
  secondary: '#6c757d'
});

// Provide to all descendants
provide(ThemeKey, theme.value);
provide('userPermissions', ['read', 'write']);
</script>

<!-- Child.vue (any depth) -->
<script setup lang="ts">
import { inject } from 'vue';
import { ThemeKey } from './Parent.vue';

// Inject with type safety
const theme = inject(ThemeKey);
const permissions = inject<string[]>('userPermissions', []);

// With default value
const config = inject('config', { debug: false });
</script>

<template>
  <div :style="{ color: theme?.primary }">
    Themed content
  </div>
</template>

Vue Router Integration

Basic Setup

// router/index.ts
import { createRouter, createWebHistory } from 'vue-router';
import type { RouteRecordRaw } from 'vue-router';
import Home from '@/views/Home.vue';
import About from '@/views/About.vue';

const routes: RouteRecordRaw[] = [
  {
    path: '/',
    name: 'Home',
    component: Home
  },
  {
    path: '/about',
    name: 'About',
    component: About
  },
  {
    path: '/user/:id',
    name: 'User',
    component: () => import('@/views/User.vue'), // Lazy loading
    props: true // Pass route params as props
  },
  {
    path: '/dashboard',
    name: 'Dashboard',
    component: () => import('@/views/Dashboard.vue'),
    meta: { requiresAuth: true }
  },
  {
    path: '/:pathMatch(.*)*',
    name: 'NotFound',
    component: () => import('@/views/NotFound.vue')
  }
];

const router = createRouter({
  history: createWebHistory(import.meta.env.BASE_URL),
  routes
});

export default router;

Navigation and Route Access

<script setup lang="ts">
import { useRouter, useRoute } from 'vue-router';
import { computed } from 'vue';

const router = useRouter();
const route = useRoute();

// Access route params
const userId = computed(() => route.params.id);
const querySearch = computed(() => route.query.search);

// Programmatic navigation
function goToUser(id: number) {
  router.push({ name: 'User', params: { id } });
}

function goToAbout() {
  router.push('/about');
}

function goBack() {
  router.back();
}

function replaceRoute() {
  router.replace({ name: 'Home' }); // No history entry
}
</script>

<template>
  <nav>
    <!-- Declarative navigation -->
    <RouterLink to="/">Home</RouterLink>
    <RouterLink :to="{ name: 'About' }">About</RouterLink>
    <RouterLink :to="{ name: 'User', params: { id: 123 } }">
      User 123
    </RouterLink>

    <!-- Active link styling -->
    <RouterLink
      to="/dashboard"
      active-class="active"
      exact-active-class="exact-active"
    >
      Dashboard
    </RouterLink>
  </nav>

  <button @click="goToUser(456)">Go to User 456</button>
  <button @click="goBack">Back</button>

  <p>Current user ID: {{ userId }}</p>
  <p>Search query: {{ querySearch }}</p>

  <!-- Render matched component -->
  <RouterView />
</template>

Navigation Guards

// router/index.ts
import { createRouter } from 'vue-router';

const router = createRouter({
  // ... routes
});

// Global before guard
router.beforeEach((to, from, next) => {
  const isAuthenticated = checkAuth();

  if (to.meta.requiresAuth && !isAuthenticated) {
    next({ name: 'Login', query: { redirect: to.fullPath } });
  } else {
    next();
  }
});

// Global after hook
router.afterEach((to, from) => {
  document.title = `${to.meta.title || 'App'} - My App`;
});

// Per-route guard
const routes = [
  {
    path: '/admin',
    component: Admin,
    beforeEnter: (to, from, next) => {
      if (isAdmin()) {
        next();
      } else {
        next('/unauthorized');
      }
    }
  }
];

<!-- Component guard -->
<script setup lang="ts">
import { onBeforeRouteLeave, onBeforeRouteUpdate } from 'vue-router';

// Confirm before leaving
onBeforeRouteLeave((to, from) => {
  if (hasUnsavedChanges.value) {
    const answer = window.confirm('You have unsaved changes. Leave anyway?');
    return answer;
  }
});

// React to route changes (same component, different params)
onBeforeRouteUpdate((to, from) => {
  console.log(`Route updated from ${from.params.id} to ${to.params.id}`);
  fetchData(to.params.id);
});
</script>

Pinia State Management

Store Definition

// stores/counter.ts
import { defineStore } from 'pinia';
import { ref, computed } from 'vue';

// Composition API style (recommended)
export const useCounterStore = defineStore('counter', () => {
  // State
  const count = ref(0);
  const name = ref('Counter Store');

  // Getters (computed)
  const doubleCount = computed(() => count.value * 2);
  const isPositive = computed(() => count.value > 0);

  // Actions
  function increment() {
    count.value++;
  }

  function decrement() {
    count.value--;
  }

  async function fetchCount() {
    const response = await fetch('/api/count');
    const data = await response.json();
    count.value = data.count;
  }

  return {
    count,
    name,
    doubleCount,
    isPositive,
    increment,
    decrement,
    fetchCount
  };
});

// Options API style (alternative)
export const useUserStore = defineStore('user', {
  state: () => ({
    user: null as User | null,
    token: ''
  }),

  getters: {
    isLoggedIn: (state) => state.user !== null,
    fullName: (state) => state.user ? `${state.user.firstName} ${state.user.lastName}` : ''
  },

  actions: {
    async login(email: string, password: string) {
      const response = await fetch('/api/login', {
        method: 'POST',
        body: JSON.stringify({ email, password })
      });
      const data = await response.json();
      this.user = data.user;
      this.token = data.token;
    },

    logout() {
      this.user = null;
      this.token = '';
    }
  }
});

Using Stores in Components

<script setup lang="ts">
import { useCounterStore } from '@/stores/counter';
import { useUserStore } from '@/stores/user';
import { storeToRefs } from 'pinia';

const counterStore = useCounterStore();
const userStore = useUserStore();

// Get reactive refs from store
const { count, doubleCount } = storeToRefs(counterStore);
const { user, isLoggedIn } = storeToRefs(userStore);

// Actions can be destructured directly (they're not reactive)
const { increment, decrement } = counterStore;

// Access state directly
console.log(counterStore.count);

// Modify state directly
counterStore.count++;

// Or use $patch for multiple changes
counterStore.$patch({
  count: 10,
  name: 'Updated Counter'
});

// Reset state
counterStore.$reset();
</script>

<template>
  <div>
    <p>Count: {{ count }} (Double: {{ doubleCount }})</p>
    <button @click="increment">+</button>
    <button @click="decrement">-</button>

    <div v-if="isLoggedIn">
      <p>Welcome, {{ user?.firstName }}!</p>
      <button @click="userStore.logout()">Logout</button>
    </div>
  </div>
</template>

Store Composition (Accessing Other Stores)

// stores/cart.ts
import { defineStore } from 'pinia';
import { ref, computed } from 'vue';
import { useUserStore } from './user';

export const useCartStore = defineStore('cart', () => {
  const items = ref<CartItem[]>([]);
  const userStore = useUserStore();

  const total = computed(() =>
    items.value.reduce((sum, item) => sum + item.price * item.quantity, 0)
  );

  const canCheckout = computed(() =>
    userStore.isLoggedIn && items.value.length > 0
  );

  async function checkout() {
    if (!canCheckout.value) return;

    await fetch('/api/checkout', {
      method: 'POST',
      headers: {
        Authorization: `Bearer ${userStore.token}`
      },
      body: JSON.stringify({ items: items.value })
    });

    items.value = [];
  }

  return { items, total, canCheckout, checkout };
});

Composables (Reusable Logic)

Custom Composables

// composables/useFetch.ts
import { ref, type Ref } from 'vue';

interface UseFetchOptions {
  immediate?: boolean;
}

export function useFetch<T>(url: string, options: UseFetchOptions = {}) {
  const data = ref<T | null>(null) as Ref<T | null>;
  const error = ref<Error | null>(null);
  const loading = ref(false);

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

    try {
      const response = await fetch(url);
      if (!response.ok) throw new Error(response.statusText);
      data.value = await response.json();
    } catch (e) {
      error.value = e as Error;
    } finally {
      loading.value = false;
    }
  }

  if (options.immediate) {
    execute();
  }

  return { data, error, loading, execute };
}

// composables/useLocalStorage.ts
import { ref, watch, type Ref } from 'vue';

export function useLocalStorage<T>(key: string, defaultValue: T): Ref<T> {
  const storedValue = localStorage.getItem(key);
  const data = ref<T>(
    storedValue ? JSON.parse(storedValue) : defaultValue
  ) as Ref<T>;

  watch(
    data,
    (newValue) => {
      localStorage.setItem(key, JSON.stringify(newValue));
    },
    { deep: true }
  );

  return data;
}

// composables/useMouse.ts
import { ref, onMounted, onUnmounted } from 'vue';

export function useMouse() {
  const x = ref(0);
  const y = ref(0);

  function update(event: MouseEvent) {
    x.value = event.pageX;
    y.value = event.pageY;
  }

  onMounted(() => {
    window.addEventListener('mousemove', update);
  });

  onUnmounted(() => {
    window.removeEventListener('mousemove', update);
  });

  return { x, y };
}

Using Composables

<script setup lang="ts">
import { useFetch } from '@/composables/useFetch';
import { useLocalStorage } from '@/composables/useLocalStorage';
import { useMouse } from '@/composables/useMouse';

interface User {
  id: number;
  name: string;
  email: string;
}

const { data: user, loading, error, execute } = useFetch<User>(
  '/api/user/123',
  { immediate: true }
);

const settings = useLocalStorage('app-settings', {
  theme: 'dark',
  language: 'en'
});

const { x, y } = useMouse();
</script>

<template>
  <div>
    <div v-if="loading">Loading...</div>
    <div v-else-if="error">Error: {{ error.message }}</div>
    <div v-else-if="user">
      <h1>{{ user.name }}</h1>
      <p>{{ user.email }}</p>
    </div>

    <p>Theme: {{ settings.theme }}</p>
    <button @click="settings.theme = settings.theme === 'dark' ? 'light' : 'dark'">
      Toggle Theme
    </button>

    <p>Mouse: {{ x }}, {{ y }}</p>
  </div>
</template>

Testing with Vitest

Component Testing

// Counter.test.ts
import { mount } from '@vue/test-utils';
import { describe, it, expect } from 'vitest';
import Counter from '@/components/Counter.vue';

describe('Counter', () => {
  it('renders initial count', () => {
    const wrapper = mount(Counter);
    expect(wrapper.text()).toContain('Count: 0');
  });

  it('increments count on button click', async () => {
    const wrapper = mount(Counter);

    await wrapper.find('button').trigger('click');

    expect(wrapper.text()).toContain('Count: 1');
  });

  it('accepts initial count prop', () => {
    const wrapper = mount(Counter, {
      props: { initialCount: 10 }
    });

    expect(wrapper.text()).toContain('Count: 10');
  });

  it('emits update event', async () => {
    const wrapper = mount(Counter);

    await wrapper.find('button').trigger('click');

    expect(wrapper.emitted('update')).toBeTruthy();
    expect(wrapper.emitted('update')![0]).toEqual([1]);
  });
});

Testing with Pinia

// UserProfile.test.ts
import { mount } from '@vue/test-utils';
import { createPinia, setActivePinia } from 'pinia';
import { beforeEach, describe, it, expect } from 'vitest';
import UserProfile from '@/components/UserProfile.vue';
import { useUserStore } from '@/stores/user';

describe('UserProfile', () => {
  beforeEach(() => {
    setActivePinia(createPinia());
  });

  it('displays user name when logged in', () => {
    const userStore = useUserStore();
    userStore.user = { id: 1, firstName: 'Alice', lastName: 'Smith' };

    const wrapper = mount(UserProfile);

    expect(wrapper.text()).toContain('Alice Smith');
  });

  it('shows login prompt when not logged in', () => {
    const wrapper = mount(UserProfile);
    expect(wrapper.text()).toContain('Please log in');
  });
});

TypeScript Best Practices

Component Props with Interface

<script setup lang="ts">
interface User {
  id: number;
  name: string;
  email: string;
  role: 'admin' | 'user' | 'guest';
}

interface Props {
  user: User;
  showEmail?: boolean;
  onUpdate?: (user: User) => void;
}

const props = withDefaults(defineProps<Props>(), {
  showEmail: true
});

// Type-safe emits
const emit = defineEmits<{
  update: [user: User];
  delete: [userId: number];
}>();

function handleUpdate() {
  emit('update', props.user);
}
</script>

Generic Components

<script setup lang="ts" generic="T">
interface Props<T> {
  items: T[];
  keyFn: (item: T) => string | number;
  renderItem: (item: T) => string;
}

const props = defineProps<Props<T>>();
</script>

<template>
  <ul>
    <li v-for="item in items" :key="keyFn(item)">
      {{ renderItem(item) }}
    </li>
  </ul>
</template>

Performance Optimization

Virtual Scrolling for Large Lists

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

const items = ref(Array.from({ length: 10000 }, (_, i) => ({
  id: i,
  text: `Item ${i}`
})));

const containerHeight = 400;
const itemHeight = 40;
const scrollTop = ref(0);

const visibleCount = Math.ceil(containerHeight / itemHeight);
const startIndex = computed(() => Math.floor(scrollTop.value / itemHeight));
const endIndex = computed(() => startIndex.value + visibleCount);
const visibleItems = computed(() =>
  items.value.slice(startIndex.value, endIndex.value)
);

const offsetY = computed(() => startIndex.value * itemHeight);
const totalHeight = computed(() => items.value.length * itemHeight);

function handleScroll(event: Event) {
  scrollTop.value = (event.target as HTMLElement).scrollTop;
}
</script>

<template>
  <div
    class="virtual-list"
    :style="{ height: containerHeight + 'px', overflow: 'auto' }"
    @scroll="handleScroll"
  >
    <div :style="{ height: totalHeight + 'px', position: 'relative' }">
      <div :style="{ transform: `translateY(${offsetY}px)` }">
        <div
          v-for="item in visibleItems"
          :key="item.id"
          :style="{ height: itemHeight + 'px' }"
        >
          {{ item.text }}
        </div>
      </div>
    </div>
  </div>
</template>

Lazy Loading Components

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

// Lazy load heavy component
const HeavyComponent = defineAsyncComponent(() =>
  import('@/components/HeavyComponent.vue')
);

// With loading/error states
const AsyncComponent = defineAsyncComponent({
  loader: () => import('@/components/AsyncComponent.vue'),
  loadingComponent: LoadingSpinner,
  errorComponent: ErrorDisplay,
  delay: 200, // Show loading after 200ms
  timeout: 3000 // Error if takes > 3s
});
</script>

<template>
  <Suspense>
    <template #default>
      <HeavyComponent />
    </template>
    <template #fallback>
      <LoadingSpinner />
    </template>
  </Suspense>
</template>

Migration Guide

From Vue 2 to Vue 3

data() { return {} }

ref()

reactive()

computed: {}

computed(() => {})

watch: {}

watch()

watchEffect()

mounted()

onMounted()

this.$emit()

emit()

props: {}

defineProps<>()

$listeners

$attrs

From React to Vue 3

useState(0)

ref(0)

useMemo(() => x * 2, [x])

computed(() => x.value * 2)

useEffect(() => {}, [x])

watch(x, () => {})

useEffect(() => {}, [])

onMounted()

useCallback

props.name

props.name

setState(prev => prev + 1)

count.value++

Best Practices

1. Use Composition API over Options API for better type inference and composition
2. Prefer

   ref()

   for primitives,

   reactive()

   for objects or just use

   ref()

   everywhere
3. Use

   computed()

   for derived state instead of methods
4. Destructure props early with

   defineProps()

   for type safety
5. Use

   <script setup>

   for less boilerplate and better performance
6. Key your v-for loops with unique IDs for proper reactivity
7. Use Pinia over Vuex for better TypeScript support and devtools
8. Lazy load routes and heavy components for faster initial load
9. Use composables to extract and reuse logic across components
10. Enable Vue DevTools for debugging reactivity and component tree

Resources

• Vue 3 Docs: https://vuejs.org/guide/introduction.html
• Vue Router: https://router.vuejs.org/
• Pinia: https://pinia.vuejs.org/
• Vite: https://vitejs.dev/
• Vue DevTools: https://devtools.vuejs.org/
• Awesome Vue: https://github.com/vuejs/awesome-vue

Summary

• Vue 3 features Composition API with

  setup()

  ,

  ref()

  ,

  reactive()

  ,

  computed()

  ,

  watch()

• Single-File Components (.vue) combine template, script, and style
• TypeScript first-class support with

  defineProps<>()

  and

  defineEmits<>()

• Vue Router for client-side routing with lazy loading and guards
• Pinia modern state management with Composition API style
• Vite lightning-fast development with HMR
• Composables extract and reuse logic across components
• Progressive adopt incrementally from simple to complex