form-validation

Form Validation

Schema-first validation using Zod as the single source of truth for both runtime validation and TypeScript types.

Quick Start

import { z } from 'zod';

// 1. Define schema (validation + types in one place)
const schema = z.object({
  email: z.string().min(1, 'Required').email('Invalid email'),
  age: z.number().positive().optional()
});

// 2. Infer TypeScript types (never manually define)
type FormData = z.infer<typeof schema>;

// 3. Use with form library
import { zodResolver } from '@hookform/resolvers/zod';
const { register } = useForm<FormData>({
  resolver: zodResolver(schema)
});

Core Principle: Reward Early, Punish Late

This is the optimal validation timing pattern backed by UX research:

Event	Show Valid (✓)	Show Invalid (✗)	Why
On input	✅ Immediately	❌ Never	Don't yell while typing
On blur	✅ Immediately	✅ Yes	User finished, show errors
During correction	✅ Immediately	✅ Real-time	Let them fix quickly

Implementation

// React Hook Form
useForm({
  mode: 'onBlur',           // First validation on blur (punish late)
  reValidateMode: 'onChange' // Re-validate on change (real-time correction)
});

// TanStack Form
useForm({
  validators: {
    onBlur: schema,          // Validate on blur
    onChange: schema         // Re-validate on change (after touched)
  }
});

Zod Schema Patterns

Basic Types

import { z } from 'zod';

// Strings
z.string()                          // Any string
z.string().min(1, 'Required')       // Non-empty (better than .nonempty())
z.string().email('Invalid email')
z.string().url('Invalid URL')
z.string().uuid('Invalid ID')
z.string().regex(/^\d{5}$/, 'Invalid ZIP')

// Numbers
z.number()                          // Any number
z.number().positive('Must be positive')
z.number().int('Must be whole number')
z.number().min(0).max(100)

// Booleans
z.boolean()
z.literal(true)                     // Must be exactly true

// Enums
z.enum(['admin', 'user', 'guest'])

// Arrays
z.array(z.string())
z.array(z.string()).min(1, 'Select at least one')

// Objects
z.object({
  name: z.string(),
  email: z.string().email()
})