agent-skills/skills/vue-best-practices/reference/composable-vs-utility-functions.md

title, impact, impactDescription, type, tags

title	impact	impactDescription	type	tags
Don't Wrap Utility Functions as Composables	MEDIUM	Wrapping stateless utility functions as composables adds unnecessary complexity without any benefit	best-practice	vue3 composables composition-api utilities patterns

Don't Wrap Utility Functions as Composables

Impact: MEDIUM - Not every function needs to be a composable. Composables are specifically for encapsulating stateful logic that uses Vue's reactivity system. Pure utility functions that just transform data or perform calculations should remain as regular JavaScript functions.

Wrapping utility functions as composables adds unnecessary abstraction, makes code harder to understand, and provides no benefits since there's no reactive state to manage.

Task Checklist

• Identify if the function manages reactive state or uses Vue lifecycle hooks
• Keep pure transformation/calculation functions as regular utilities
• Export utilities directly, not wrapped in a function that returns them
• Reserve the "use" prefix for actual composables

Incorrect:

// WRONG: These are just utility functions wrapped unnecessarily

// Adds no value - no reactive state
export function useFormatters() {
  const formatDate = (date) => {
    return new Intl.DateTimeFormat('en-US').format(date)
  }

  const formatCurrency = (amount) => {
    return new Intl.NumberFormat('en-US', {
      style: 'currency',
      currency: 'USD'
    }).format(amount)
  }

  const capitalize = (str) => {
    return str.charAt(0).toUpperCase() + str.slice(1)
  }

  return { formatDate, formatCurrency, capitalize }
}

// WRONG: Pure calculation, no reactive state
export function useMath() {
  const add = (a, b) => a + b
  const multiply = (a, b) => a * b
  const clamp = (value, min, max) => Math.min(Math.max(value, min), max)

  return { add, multiply, clamp }
}

// Usage adds ceremony for no benefit
const { formatDate, formatCurrency } = useFormatters()
const { clamp } = useMath()

Correct:

// CORRECT: Export as regular utility functions

// utils/formatters.js
export function formatDate(date) {
  return new Intl.DateTimeFormat('en-US').format(date)
}

export function formatCurrency(amount) {
  return new Intl.NumberFormat('en-US', {
    style: 'currency',
    currency: 'USD'
  }).format(amount)
}

export function capitalize(str) {
  return str.charAt(0).toUpperCase() + str.slice(1)
}

// utils/math.js
export function clamp(value, min, max) {
  return Math.min(Math.max(value, min), max)
}

// Usage - simple and direct
import { formatDate, formatCurrency } from '@/utils/formatters'
import { clamp } from '@/utils/math'

When to Use Composables vs Utilities

Use Composable When...	Use Utility When...
Managing reactive state (ref, reactive)	Pure data transformation
Using lifecycle hooks (onMounted, onUnmounted)	Stateless calculations
Setting up watchers (watch, watchEffect)	String/array manipulation
Creating computed properties	Formatting functions
Needs cleanup on component unmount	Validation functions
State changes over time	Mathematical operations

Examples: Composables vs Utilities

// COMPOSABLE: Has reactive state and lifecycle
export function useWindowSize() {
  const width = ref(window.innerWidth)
  const height = ref(window.innerHeight)

  function update() {
    width.value = window.innerWidth
    height.value = window.innerHeight
  }

  onMounted(() => window.addEventListener('resize', update))
  onUnmounted(() => window.removeEventListener('resize', update))

  return { width, height }
}

// UTILITY: Pure transformation, no state
export function parseQueryString(queryString) {
  return Object.fromEntries(new URLSearchParams(queryString))
}

// COMPOSABLE: Manages form state over time
export function useForm(initialValues) {
  const values = ref({ ...initialValues })
  const errors = ref({})
  const isDirty = computed(() =>
    JSON.stringify(values.value) !== JSON.stringify(initialValues)
  )

  function reset() {
    values.value = { ...initialValues }
    errors.value = {}
  }

  return { values, errors, isDirty, reset }
}

// UTILITY: Stateless validation
export function validateEmail(email) {
  return /^[^\s@]+@[^\s@]+\.[^\s@]+$/.test(email)
}

export function validateRequired(value) {
  return value !== null && value !== undefined && value !== ''
}

Mixed Pattern: Composable Using Utilities

It's perfectly fine for composables to use utility functions:

// utils/validators.js
export function validateEmail(email) {
  return /^[^\s@]+@[^\s@]+\.[^\s@]+$/.test(email)
}

// composables/useEmailInput.js
import { ref, computed } from 'vue'
import { validateEmail } from '@/utils/validators'

export function useEmailInput(initialValue = '') {
  const email = ref(initialValue)
  const isValid = computed(() => validateEmail(email.value))
  const error = computed(() =>
    email.value && !isValid.value ? 'Invalid email format' : null
  )

  return { email, isValid, error }
}

File Organization

src/
  composables/        # Stateful reactive logic
    useAuth.js
    useFetch.js
    useLocalStorage.js
  utils/              # Pure utility functions
    formatters.js
    validators.js
    math.js
    strings.js

Reference

• Vue.js Composables - What is a Composable
• Common Mistakes Creating Composition Functions