agent-skills/skills/vue-best-practices/reference/composable-readonly-state.md

title, impact, impactDescription, type, tags

title	impact	impactDescription	type	tags
Return State as Readonly with Explicit Update Methods	MEDIUM	Exposing mutable state directly allows uncontrolled mutations scattered throughout the codebase	best-practice	vue3 composables composition-api readonly encapsulation state-management

Return State as Readonly with Explicit Update Methods

Impact: MEDIUM - When a composable manages state that should only be modified in controlled ways, return the state as readonly and provide explicit methods for updates. This prevents scattered, uncontrolled mutations and makes state changes traceable and predictable.

Exposing raw refs allows any consumer to modify state directly, leading to bugs that are hard to track because mutations can happen anywhere in the codebase.

Task Checklist

• Use readonly() to wrap state that shouldn't be directly modified
• Provide explicit methods for all valid state transitions
• Document the intended ways to update state
• Consider returning shallowReadonly() for performance with large objects

Incorrect:

// WRONG: State is fully mutable by any consumer
export function useCart() {
  const items = ref([])
  const total = computed(() =>
    items.value.reduce((sum, item) => sum + item.price * item.quantity, 0)
  )

  return { items, total }  // Anyone can mutate items directly!
}

// Consumer code - mutations scattered everywhere
const { items, total } = useCart()

// In component A
items.value.push({ id: 1, name: 'Widget', price: 10, quantity: 1 })

// In component B - different mutation pattern
items.value = items.value.filter(item => item.id !== 1)

// In component C - direct modification
items.value[0].quantity = 5

// Hard to track: where did this item come from? Why did quantity change?

Correct:

import { ref, computed, readonly } from 'vue'

export function useCart() {
  const items = ref([])

  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)
  )

  // Explicit, controlled mutations
  function addItem(product, quantity = 1) {
    const existing = items.value.find(item => item.id === product.id)
    if (existing) {
      existing.quantity += quantity
    } else {
      items.value.push({ ...product, quantity })
    }
  }

  function removeItem(productId) {
    const index = items.value.findIndex(item => item.id === productId)
    if (index > -1) {
      items.value.splice(index, 1)
    }
  }

  function updateQuantity(productId, quantity) {
    const item = items.value.find(item => item.id === productId)
    if (item) {
      item.quantity = Math.max(0, quantity)
      if (item.quantity === 0) {
        removeItem(productId)
      }
    }
  }

  function clearCart() {
    items.value = []
  }

  return {
    // State is readonly - can't be mutated directly
    items: readonly(items),
    total,
    itemCount,
    // Only these methods can modify state
    addItem,
    removeItem,
    updateQuantity,
    clearCart
  }
}

// Consumer code - controlled mutations only
const { items, total, addItem, removeItem, updateQuantity } = useCart()

// items.value.push(...)  // TypeScript error: readonly!
// items.value = []       // TypeScript error: readonly!

// Correct way - through explicit methods
addItem({ id: 1, name: 'Widget', price: 10 })
updateQuantity(1, 3)
removeItem(1)

Pattern: Internal vs External State

Keep internal state private, expose readonly view:

export function useAuth() {
  // Internal, fully mutable
  const _user = ref(null)
  const _token = ref(null)
  const _isLoading = ref(false)
  const _error = ref(null)

  async function login(credentials) {
    _isLoading.value = true
    _error.value = null

    try {
      const response = await api.login(credentials)
      _user.value = response.user
      _token.value = response.token
    } catch (e) {
      _error.value = e.message
      throw e
    } finally {
      _isLoading.value = false
    }
  }

  function logout() {
    _user.value = null
    _token.value = null
  }

  return {
    // Readonly views of internal state
    user: readonly(_user),
    isAuthenticated: computed(() => !!_user.value),
    isLoading: readonly(_isLoading),
    error: readonly(_error),
    // Methods for state changes
    login,
    logout
  }
}

When to Use readonly vs Not

Use readonly	Don't Use readonly
State with specific update rules	Simple two-way binding state
Shared state between components	Form input values
State that needs validation on change	Local component state
When debugging mutation sources matters	When consumers need full control

// Form input - consumers SHOULD mutate directly
export function useForm(initial) {
  const values = ref({ ...initial })
  return { values }  // No readonly - it's meant to be mutated
}

// Counter with min/max - needs controlled mutations
export function useCounter(min = 0, max = 100) {
  const _count = ref(min)

  function increment() {
    if (_count.value < max) _count.value++
  }

  function decrement() {
    if (_count.value > min) _count.value--
  }

  return {
    count: readonly(_count),
    increment,
    decrement
  }
}

Performance: shallowReadonly

For large objects, use shallowReadonly to avoid deep readonly conversion:

export function useLargeDataset() {
  const data = ref([/* thousands of items */])

  return {
    // shallowReadonly - only top level is readonly
    // Nested properties are still technically mutable
    // but the ref itself can't be reassigned
    data: shallowReadonly(data)
  }
}

Reference

• Vue.js Reactivity API - readonly
• 13 Vue Composables Tips