Nuxt Data

const { data, error, status } = await useFetch('/api/users')

// Check error
if (error.value) {
  console.error('Error:', error.value.message)
  console.error('Status:', error.value.statusCode)
}

// Status values: 'idle' | 'pending' | 'success' | 'error'
if (status.value === 'error') {
  showError(error.value)
}

const { data, refresh, execute } = await useFetch('/api/users', {
  immediate: false  // Don't fetch on mount
})

// Fetch manually
await execute()

// Refresh (re-fetch)
await refresh()

// Refresh with new params
await refresh({ dedupe: true })

// Nuxt 4 default: Shallow reactivity
const { data } = await useFetch('/api/user')
data.value.name = 'New Name'  // Won't trigger reactivity!

// Enable deep reactivity for mutations
const { data } = await useFetch('/api/user', {
  deep: true
})
data.value.name = 'New Name'  // Now works!

// Or refresh instead of mutating
const { data, refresh } = await useFetch('/api/user')
await $fetch('/api/user', { method: 'PATCH', body: { name: 'New Name' } })
await refresh()  // Re-fetch updated data

const { data } = await useFetch('/api/users', {
  key: 'users-list',           // Custom cache key
  dedupe: 'cancel',            // Cancel duplicate requests
  getCachedData: (key, nuxtApp) => {
    // Return cached data if valid
    return nuxtApp.payload.data[key]
  }
})

// useLazyFetch - Navigation happens immediately, data loads in background
const { data, pending } = useLazyFetch('/api/users')

// useLazyAsyncData
const { data, pending } = useLazyAsyncData('users', () => $fetch('/api/users'))

// In event handlers (not during SSR)
const submitForm = async () => {
  const result = await $fetch('/api/submit', {
    method: 'POST',
    body: formData.value
  })
}

// In server routes
export default defineEventHandler(async (event) => {
  const externalData = await $fetch('https://api.example.com/data')
  return externalData
})

// Simple counter
const count = useState('count', () => 0)

// Complex object
const settings = useState('settings', () => ({
  theme: 'light',
  notifications: true,
  language: 'en'
}))

// Typed state
interface User {
  id: string
  name: string
  email: string
}
const user = useState<User | null>('user', () => null)

// composables/useCart.ts
interface CartItem {
  id: string
  name: string
  price: number
  quantity: number
}

export const useCart = () => {
  const items = useState<CartItem[]>('cart-items', () => [])

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

  const itemCount = computed(() =>
    items.value.reduce((sum, item) => sum + item.quantity, 0)
  )

  const addItem = (product: Omit<CartItem, 'quantity'>) => {
    const existing = items.value.find(i => i.id === product.id)
    if (existing) {
      existing.quantity++
    } else {
      items.value.push({ ...product, quantity: 1 })
    }
  }

  const removeItem = (id: string) => {
    items.value = items.value.filter(i => i.id !== id)
  }

  const updateQuantity = (id: string, quantity: number) => {
    const item = items.value.find(i => i.id === id)
    if (item) {
      item.quantity = Math.max(0, quantity)
      if (item.quantity === 0) removeItem(id)
    }
  }

  const clearCart = () => {
    items.value = []
  }

  return { items, total, itemCount, addItem, removeItem, updateQuantity, clearCart }
}

bun add pinia @pinia/nuxt

// nuxt.config.ts
export default defineNuxtConfig({
  modules: ['@pinia/nuxt']
})

// stores/auth.ts
import { defineStore } from 'pinia'

export const useAuthStore = defineStore('auth', {
  state: () => ({
    user: null as User | null,
    token: null as string | null
  }),

  getters: {
    isAuthenticated: (state) => !!state.user,
    userName: (state) => state.user?.name ?? 'Guest'
  },

  actions: {
    async login(email: string, password: string) {
      const { user, token } = await $fetch('/api/auth/login', {
        method: 'POST',
        body: { email, password }
      })
      this.user = user
      this.token = token
    },

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

// Usage in components
const authStore = useAuthStore()
await authStore.login('user@example.com', 'password')
console.log(authStore.userName)

// WRONG - Creates new instance every time!
export const useAuth = () => {
  const user = ref(null)  // Not shared
  return { user }
}

// CORRECT
export const useAuth = () => {
  const user = useState('auth-user', () => null)
  return { user }
}

// WRONG
const { data } = await useFetch('/api/users')
console.log(data.value.length)  // Crashes if error!

// CORRECT
const { data, error } = await useFetch('/api/users')
if (error.value) {
  showToast({ type: 'error', message: error.value.message })
  return
}
console.log(data.value.length)

// WRONG - Causes hydration mismatch!
const { data } = await useFetch('/api/users', {
  transform: (users) => users.sort(() => Math.random() - 0.5)
})

// CORRECT
const { data } = await useFetch('/api/users', {
  transform: (users) => users.sort((a, b) => a.name.localeCompare(b.name))
})

// WRONG - v4 uses shallow refs by default
const { data } = await useFetch('/api/user')
data.value.name = 'New Name'  // Won't trigger reactivity!

// CORRECT - Option 1: Enable deep
const { data } = await useFetch('/api/user', { deep: true })
data.value.name = 'New Name'

// CORRECT - Option 2: Replace entire value
data.value = { ...data.value, name: 'New Name' }

// CORRECT - Option 3: Refresh after mutation
await $fetch('/api/user', { method: 'PATCH', body: { name: 'New Name' } })
await refresh()