Client Utilities

Pure functions for formatting, validation, transformation, and parsing.

Quick Reference

Rules

Pure functions:

• Same input → same output
• No side effects
• No external state mutation
• No API calls, no refs, no reactive

When NOT to use utils:

• Stateful logic → use composables
• Vue-specific → use composables
• Component logic → keep in component
• API calls → use queries

Structure

// utils/formatters.ts
export function formatCurrency(amount: number, currency = 'USD'): string {
  return new Intl.NumberFormat('en-US', {
    style: 'currency',
    currency,
  }).format(amount)
}

export function formatRelativeTime(date: Date): string {
  const rtf = new Intl.RelativeTimeFormat('en', { numeric: 'auto' })
  const diff = date.getTime() - Date.now()
  const days = Math.floor(diff / (1000 * 60 * 60 * 24))
  return rtf.format(days, 'day')
}

Naming: Descriptive verbs (formatCurrency, validateEmail, parseQuery) Organization: Group by category (formatters.ts, validators.ts) Exports: Named exports only

Examples by Category

Formatters:

// utils/formatters.ts
export function formatBytes(bytes: number): string { ... }
export function formatPhone(phone: string): string { ... }

Validators:

// utils/validators.ts
export function isValidEmail(email: string): boolean {
  return /^[^\s@]+@[^\s@][^\s.@]*\.[^\s@]+$/.test(email)
}

export function isValidUrl(url: string): boolean {
  try { new URL(url); return true }
  catch { return false }
}

Transformers:

// utils/transformers.ts
export function slugify(text: string): string {
  return text.toLowerCase()
    .replace(/[^\w\s-]/g, '')
    .replace(/\s+/g, '-')
}

export function truncate(text: string, length: number): string {
  return text.length > length ? `${text.slice(0, length)}...` : text
}

Parsers:

// utils/parsers.ts
export function parseQuery(search: string): Record<string, string> {
  return Object.fromEntries(new URLSearchParams(search))
}

export function parseJSON<T>(json: string, fallback: T): T {
  try { return JSON.parse(json) }
  catch { return fallback }
}

Common Mistakes

Side effects (not pure):

// ❌ Wrong - mutates external state
let count = 0
export function increment() {
  count++
  return count
}

// ✅ Correct - pure
export function add(a: number, b: number): number {
  return a + b
}

Using utils for stateful logic:

// ❌ Wrong - should be composable
export function useCounter() { ... }

// ✅ Correct - pure transformation
export function formatCount(count: number): string { ... }

Organization

Flat for small projects:

utils/
├── formatters.ts
├── validators.ts
└── transformers.ts

Nested for large projects:

utils/
├── formatters/
│   ├── date.ts
│   ├── currency.ts
│   └── index.ts
└── validators/
    ├── email.ts
    └── index.ts