react-hook-form-zod

SKILL.md

React Hook Form + Zod Validation

Status: Production Ready ✅
Last Updated: 2025-11-20
Dependencies: None (standalone)
Latest Versions: react-hook-form@7.66.1, zod@4.1.12, @hookform/resolvers@5.2.2

Quick Start (10 Minutes)

1. Install Packages

npm install react-hook-form@7.66.1 zod@4.1.12 @hookform/resolvers@5.2.2

Why These Packages:

react-hook-form: Performant, flexible form library with minimal re-renders

zod: TypeScript-first schema validation with type inference

@hookform/resolvers: Adapter to connect Zod (and other validators) to React Hook Form

2. Create Your First Form

import { useForm } from 'react-hook-form'
import { zodResolver } from '@hookform/resolvers/zod'
import { z } from 'zod'

// 1. Define validation schema
const loginSchema = z.object({
  email: z.string().email('Invalid email address'),
  password: z.string().min(8, 'Password must be at least 8 characters'),
})

// 2. Infer TypeScript type from schema
type LoginFormData = z.infer<typeof loginSchema>

function LoginForm() {
  // 3. Initialize form with zodResolver
  const {
    register,
    handleSubmit,
    formState: { errors, isSubmitting },
  } = useForm<LoginFormData>({
    resolver: zodResolver(loginSchema),
    defaultValues: {
      email: '',
      password: '',
    },
  })

  // 4. Handle form submission
  const onSubmit = async (data: LoginFormData) => {
    // Data is guaranteed to be valid here
    console.log('Valid data:', data)
    // Make API call, etc.
  }

  return (
    <form onSubmit={handleSubmit(onSubmit)}>
      <div>
        <label htmlFor="email">Email</label>
        <input id="email" type="email" {...register('email')} />
        {errors.email && (
          <span role="alert" className="error">
            {errors.email.message}
          </span>
        )}
      </div>

      <div>
        <label htmlFor="password">Password</label>
        <input id="password" type="password" {...register('password')} />
        {errors.password && (
          <span role="alert" className="error">
            {errors.password.message}
          </span>
        )}
      </div>

      <button type="submit" disabled={isSubmitting}>
        {isSubmitting ? 'Logging in...' : 'Login'}
      </button>
    </form>
  )
}

CRITICAL:

Always set defaultValues to prevent "uncontrolled to controlled" warnings

Use zodResolver(schema) to connect Zod validation

Type form with z.infer<typeof schema> for full type safety

Validate on both client AND server (never trust client validation alone)

3. Add Server-Side Validation

// server/api/login.ts
import { z } from 'zod'

// SAME schema on server
const loginSchema = z.object({
  email: z.string().email('Invalid email address'),
  password: z.string().min(8, 'Password must be at least 8 characters'),
})

export async function loginHandler(req: Request) {
  try {
    // Parse and validate request body
    const data = loginSchema.parse(await req.json())

    // Data is type-safe and validated
    // Proceed with authentication logic
    return { success: true }
  } catch (error) {
    if (error instanceof z.ZodError) {
      // Return validation errors to client
      return { success: false, errors: error.flatten().fieldErrors }
    }
    throw error
  }
}

Why Server Validation:

Client validation can be bypassed (inspect element, Postman, curl)

Server validation is your security layer

Same Zod schema = single source of truth

Type safety across frontend and backend

Core Concepts

useForm Hook Anatomy

const {
  register,           // Register input fields
  handleSubmit,       // Wrap onSubmit handler
  watch,              // Watch field values
  formState,          // Form state (errors, isValid, isDirty, etc.)
  setValue,           // Set field value programmatically
  getValues,          // Get current form values
  reset,              // Reset form to defaults
  trigger,            // Trigger validation manually
  control,            // Control object for Controller/useController
} = useForm<FormData>({
  resolver: zodResolver(schema),  // Validation resolver
  mode: 'onSubmit',               // When to validate (onSubmit, onChange, onBlur, all)
  defaultValues: {},              // Initial values (REQUIRED for controlled inputs)
})

useForm Options:

Option
Description
Default

resolver
Validation resolver (e.g., zodResolver)
undefined

mode
When to validate ('onSubmit', 'onChange', 'onBlur', 'all')
'onSubmit'

reValidateMode
When to re-validate after error
'onChange'

defaultValues
Initial form values
{}

shouldUnregister
Unregister inputs when unmounted
false

criteriaMode
Return all errors or first error only
'firstError'

Form Validation Modes:

onSubmit - Validate on submit (best performance, less responsive)

onChange - Validate on every change (live feedback, more re-renders)

onBlur - Validate when field loses focus (good balance)

all - Validate on submit, blur, and change (most responsive, highest cost)

Zod Schema Definition

import { z } from 'zod'

// Primitives
const stringSchema = z.string()
const numberSchema = z.number()
const booleanSchema = z.boolean()
const dateSchema = z.date()

// With validation
const emailSchema = z.string().email('Invalid email')
const ageSchema = z.number().min(18, 'Must be 18+').max(120, 'Invalid age')
const usernameSchema = z.string().min(3).max(20).regex(/^[a-zA-Z0-9_]+$/)

// Objects
const userSchema = z.object({
  name: z.string(),
  email: z.string().email(),
  age: z.number().int().positive(),
})

// Arrays
const tagsSchema = z.array(z.string())
const usersSchema = z.array(userSchema)

// Optional and Nullable
const optionalField = z.string().optional()       // string | undefined
const nullableField = z.string().nullable()       // string | null
const nullishField = z.string().nullish()         // string | null | undefined

// Default values
const withDefault = z.string().default('default value')

// Unions
const statusSchema = z.union([
  z.literal('active'),
  z.literal('inactive'),
  z.literal('pending'),
])
// Shorthand for literals
const statusEnum = z.enum(['active', 'inactive', 'pending'])

// Nested objects
const addressSchema = z.object({
  street: z.string(),
  city: z.string(),
  zipCode: z.string().regex(/^\d{5}$/),
})

const profileSchema = z.object({
  name: z.string(),
  address: addressSchema,  // Nested object
})

// Custom error messages
const passwordSchema = z.string()
  .min(8, { message: 'Password must be at least 8 characters' })
  .regex(/[A-Z]/, { message: 'Password must contain uppercase letter' })
  .regex(/[0-9]/, { message: 'Password must contain number' })

Type Inference:

const userSchema = z.object({
  name: z.string(),
  age: z.number(),
})

// Automatically infer TypeScript type
type User = z.infer<typeof userSchema>
// Result: { name: string; age: number }

Zod Refinements (Custom Validation)

// Simple refinement
const passwordConfirmSchema = z.object({
  password: z.string().min(8),
  confirmPassword: z.string(),
}).refine((data) => data.password === data.confirmPassword, {
  message: "Passwords don't match",
  path: ['confirmPassword'], // Error will appear on confirmPassword field
})

// Multiple refinements
const signupSchema = z.object({
  username: z.string(),
  email: z.string().email(),
  age: z.number(),
})
  .refine((data) => data.username !== data.email.split('@')[0], {
    message: 'Username cannot be your email prefix',
    path: ['username'],
  })
  .refine((data) => data.age >= 18, {
    message: 'Must be 18 or older',
    path: ['age'],
  })

// Async refinement (for API checks)
const usernameSchema = z.string().refine(async (username) => {
  // Check if username is available via API
  const response = await fetch(`/api/check-username?username=${username}`)
  const { available } = await response.json()
  return available
}, {
  message: 'Username is already taken',
})

Zod Transforms (Data Manipulation)

// Transform string to number
const ageSchema = z.string().transform((val) => parseInt(val, 10))

// Transform to uppercase
const uppercaseSchema = z.string().transform((val) => val.toUpperCase())

// Transform date string to Date object
const dateSchema = z.string().transform((val) => new Date(val))

// Trim whitespace
const trimmedSchema = z.string().transform((val) => val.trim())

// Complex transform
const userInputSchema = z.object({
  email: z.string().email().transform((val) => val.toLowerCase()),
  tags: z.string().transform((val) => val.split(',').map(tag => tag.trim())),
})

// Chain transform and refine
const positiveNumberSchema = z.string()
  .transform((val) => parseFloat(val))
  .refine((val) => !isNaN(val), { message: 'Must be a number' })
  .refine((val) => val > 0, { message: 'Must be positive' })

zodResolver Integration

import { zodResolver } from '@hookform/resolvers/zod'

const form = useForm<FormData>({
  resolver: zodResolver(schema),
})

What zodResolver Does:

Takes your Zod schema

Converts it to a format React Hook Form understands

Provides validation function that runs on form submission

Maps Zod errors to React Hook Form error format

Preserves type safety with TypeScript inference

zodResolver Options:

import { zodResolver } from '@hookform/resolvers/zod'

// With options
const form = useForm({
  resolver: zodResolver(schema, {
    async: false,     // Use async validation
    raw: false,       // Return raw Zod error
  }),
})

Form Registration Patterns

Pattern 1: Simple Input Registration

function BasicForm() {
  const { register, handleSubmit } = useForm<FormData>({
    resolver: zodResolver(schema),
  })

  return (
    <form onSubmit={handleSubmit(onSubmit)}>
      {/* Spread register result to input */}
      <input {...register('email')} />
      <input {...register('password')} />

      {/* With custom props */}
      <input
        {...register('username')}
        placeholder="Enter username"
        className="input"
      />
    </form>
  )
}

What register() Returns:

{
  onChange: (e) => void,
  onBlur: (e) => void,
  ref: (instance) => void,
  name: string,
}

Pattern 2: Controller (for Custom Components)

Use Controller when the input doesn't expose ref (like custom components, React Select, date pickers, etc.):

import { Controller } from 'react-hook-form'

function FormWithCustomInput() {
  const { control, handleSubmit } = useForm<FormData>({
    resolver: zodResolver(schema),
  })

  return (
    <form onSubmit={handleSubmit(onSubmit)}>
      <Controller
        name="category"
        control={control}
        render={({ field }) => (
          <CustomSelect
            {...field}                      // value, onChange, onBlur, ref
            options={categoryOptions}
          />
        )}
      />

      {/* With more control */}
      <Controller
        name="dateOfBirth"
        control={control}
        render={({ field, fieldState }) => (
          <div>
            <DatePicker
              selected={field.value}
              onChange={field.onChange}
              onBlur={field.onBlur}
            />
            {fieldState.error && (
              <span>{fieldState.error.message}</span>
            )}
          </div>
        )}
      />
    </form>
  )
}

When to Use Controller:

✅ Third-party UI libraries (React Select, Material-UI, Ant Design, etc.)

✅ Custom components that don't expose ref

✅ Components that don't use onChange (like checkboxes with custom handlers)

✅ Need fine-grained control over field behavior

When NOT to Use Controller:

❌ Standard HTML inputs (use register instead - it's simpler and faster)

❌ When performance is critical (Controller adds minimal overhead)

Pattern 3: useController (Reusable Controlled Inputs)

import { useController } from 'react-hook-form'

// Reusable custom input component
function CustomInput({ name, control, label }) {
  const {
    field,
    fieldState: { error },
  } = useController({
    name,
    control,
    defaultValue: '',
  })

  return (
    <div>
      <label>{label}</label>
      <input {...field} />
      {error && <span>{error.message}</span>}
    </div>
  )
}

// Usage
function MyForm() {
  const { control, handleSubmit } = useForm({
    resolver: zodResolver(schema),
  })

  return (
    <form onSubmit={handleSubmit(onSubmit)}>
      <CustomInput name="email" control={control} label="Email" />
      <CustomInput name="username" control={control} label="Username" />
    </form>
  )
}

Error Handling

Displaying Errors

function FormWithErrors() {
  const { register, handleSubmit, formState: { errors } } = useForm<FormData>({
    resolver: zodResolver(schema),
  })

  return (
    <form onSubmit={handleSubmit(onSubmit)}>
      <div>
        <input {...register('email')} aria-invalid={errors.email ? 'true' : 'false'} />

        {/* Simple error display */}
        {errors.email && <span>{errors.email.message}</span>}

        {/* Accessible error display */}
        {errors.email && (
          <span role="alert" className="error">
            {errors.email.message}
          </span>
        )}

        {/* Error with icon */}
        {errors.email && (
          <div role="alert" className="error">
            <ErrorIcon />
            <span>{errors.email.message}</span>
          </div>
        )}
      </div>
    </form>
  )
}

Error Object Structure

// errors object structure
{
  email: {
    type: 'invalid_string',
    message: 'Invalid email address',
  },
  password: {
    type: 'too_small',
    message: 'Password must be at least 8 characters',
  },
  // Nested errors
  address: {
    street: {
      type: 'invalid_type',
      message: 'Expected string, received undefined',
    },
  },
}

Form-Level Validation Errors

const schema = z.object({
  password: z.string().min(8),
  confirmPassword: z.string(),
}).refine((data) => data.password === data.confirmPassword, {
  message: "Passwords don't match",
  path: ['confirmPassword'], // Attach error to confirmPassword field
})

// Without path - creates root error
.refine((data) => someCondition, {
  message: 'Form validation failed',
})

// Access root errors
const { formState: { errors } } = useForm()
errors.root?.message // Root-level error

Server Errors Integration

function FormWithServerErrors() {
  const { register, handleSubmit, setError, formState: { errors } } = useForm({
    resolver: zodResolver(schema),
  })

  const onSubmit = async (data) => {
    try {
      const response = await fetch('/api/submit', {
        method: 'POST',
        body: JSON.stringify(data),
      })

      if (!response.ok) {
        const { errors: serverErrors } = await response.json()

        // Map server errors to form fields
        Object.entries(serverErrors).forEach(([field, message]) => {
          setError(field, {
            type: 'server',
            message,
          })
        })

        return
      }

      // Success!
    } catch (error) {
      // Generic error
      setError('root', {
        type: 'server',
        message: 'An error occurred. Please try again.',
      })
    }
  }

  return (
    <form onSubmit={handleSubmit(onSubmit)}>
      {errors.root && <div role="alert">{errors.root.message}</div>}
      {/* ... */}
    </form>
  )
}

Advanced Patterns

Dynamic Form Fields (useFieldArray)

import { useForm, useFieldArray } from 'react-hook-form'
import { zodResolver } from '@hookform/resolvers/zod'
import { z } from 'zod'

const contactSchema = z.object({
  contacts: z.array(
    z.object({
      name: z.string().min(1, 'Name is required'),
      email: z.string().email('Invalid email'),
    })
  ).min(1, 'At least one contact is required'),
})

type ContactFormData = z.infer<typeof contactSchema>

function ContactListForm() {
  const { register, control, handleSubmit, formState: { errors } } = useForm<ContactFormData>({
    resolver: zodResolver(contactSchema),
    defaultValues: {
      contacts: [{ name: '', email: '' }],
    },
  })

  const { fields, append, remove } = useFieldArray({
    control,
    name: 'contacts',
  })

  return (
    <form onSubmit={handleSubmit(onSubmit)}>
      {fields.map((field, index) => (
        <div key={field.id}> {/* IMPORTANT: Use field.id, not index */}
          <input
            {...register(`contacts.${index}.name` as const)}
            placeholder="Name"
          />
          {errors.contacts?.[index]?.name && (
            <span>{errors.contacts[index].name.message}</span>
          )}

          <input
            {...register(`contacts.${index}.email` as const)}
            placeholder="Email"
          />
          {errors.contacts?.[index]?.email && (
            <span>{errors.contacts[index].email.message}</span>
          )}

          <button type="button" onClick={() => remove(index)}>
            Remove
          </button>
        </div>
      ))}

      <button
        type="button"
        onClick={() => append({ name: '', email: '' })}
      >
        Add Contact
      </button>

      <button type="submit">Submit</button>
    </form>
  )
}

useFieldArray API:

fields - Array of field items with unique IDs

append(value) - Add new item to end

prepend(value) - Add new item to beginning

insert(index, value) - Insert item at index

remove(index) - Remove item at index

update(index, value) - Update item at index

replace(values) - Replace entire array

Async Validation with Debouncing

import { useForm } from 'react-hook-form'
import { zodResolver } from '@hookform/resolvers/zod'
import { z } from 'zod'
import { useDebouncedCallback } from 'use-debounce' // npm install use-debounce

const usernameSchema = z.string().min(3).refine(async (username) => {
  const response = await fetch(`/api/check-username?username=${username}`)
  const { available } = await response.json()
  return available
}, {
  message: 'Username is already taken',
})

function AsyncValidationForm() {
  const { register, handleSubmit, trigger, formState: { errors, isValidating } } = useForm({
    resolver: zodResolver(z.object({ username: usernameSchema })),
    mode: 'onChange', // Validate on every change
  })

  // Debounce validation to avoid too many API calls
  const debouncedValidation = useDebouncedCallback(() => {
    trigger('username')
  }, 500) // Wait 500ms after user stops typing

  return (
    <form onSubmit={handleSubmit(onSubmit)}>
      <input
        {...register('username')}
        onChange={(e) => {
          register('username').onChange(e)
          debouncedValidation()
        }}
      />
      {isValidating && <span>Checking availability...</span>}
      {errors.username && <span>{errors.username.message}</span>}
    </form>
  )
}

Multi-Step Form (Wizard)

import { useState } from 'react'
import { useForm } from 'react-hook-form'
import { zodResolver } from '@hookform/resolvers/zod'
import { z } from 'zod'

// Step schemas
const step1Schema = z.object({
  name: z.string().min(1, 'Name is required'),
  email: z.string().email('Invalid email'),
})

const step2Schema = z.object({
  address: z.string().min(1, 'Address is required'),
  city: z.string().min(1, 'City is required'),
})

const step3Schema = z.object({
  cardNumber: z.string().regex(/^\d{16}$/, 'Invalid card number'),
  cvv: z.string().regex(/^\d{3,4}$/, 'Invalid CVV'),
})

// Combined schema for final validation
const fullSchema = step1Schema.merge(step2Schema).merge(step3Schema)

type FormData = z.infer<typeof fullSchema>

function MultiStepForm() {
  const [step, setStep] = useState(1)

  const { register, handleSubmit, trigger, formState: { errors } } = useForm<FormData>({
    resolver: zodResolver(fullSchema),
    mode: 'onChange',
  })

  const nextStep = async () => {
    let fieldsToValidate: (keyof FormData)[] = []

    if (step === 1) {
      fieldsToValidate = ['name', 'email']
    } else if (step === 2) {
      fieldsToValidate = ['address', 'city']
    }

    // Validate current step fields
    const isValid = await trigger(fieldsToValidate)

    if (isValid) {
      setStep(step + 1)
    }
  }

  const prevStep = () => setStep(step - 1)

  const onSubmit = (data: FormData) => {
    console.log('Final data:', data)
  }

  return (
    <form onSubmit={handleSubmit(onSubmit)}>
      {/* Progress indicator */}
      <div className="progress">
        Step {step} of 3
      </div>

      {/* Step 1 */}
      {step === 1 && (
        <div>
          <h2>Personal Information</h2>
          <input {...register('name')} placeholder="Name" />
          {errors.name && <span>{errors.name.message}</span>}

          <input {...register('email')} placeholder="Email" />
          {errors.email && <span>{errors.email.message}</span>}
        </div>
      )}

      {/* Step 2 */}
      {step === 2 && (
        <div>
          <h2>Address</h2>
          <input {...register('address')} placeholder="Address" />
          {errors.address && <span>{errors.address.message}</span>}

          <input {...register('city')} placeholder="City" />
          {errors.city && <span>{errors.city.message}</span>}
        </div>
      )}

      {/* Step 3 */}
      {step === 3 && (
        <div>
          <h2>Payment</h2>
          <input {...register('cardNumber')} placeholder="Card Number" />
          {errors.cardNumber && <span>{errors.cardNumber.message}</span>}

          <input {...register('cvv')} placeholder="CVV" />
          {errors.cvv && <span>{errors.cvv.message}</span>}
        </div>
      )}

      {/* Navigation */}
      <div>
        {step > 1 && (
          <button type="button" onClick={prevStep}>
            Previous
          </button>
        )}
        {step < 3 ? (
          <button type="button" onClick={nextStep}>
            Next
          </button>
        ) : (
          <button type="submit">Submit</button>
        )}
      </div>
    </form>
  )
}

Conditional Validation

import { z } from 'zod'

// Schema with conditional validation
const formSchema = z.discriminatedUnion('accountType', [
  z.object({
    accountType: z.literal('personal'),
    name: z.string().min(1),
  }),
  z.object({
    accountType: z.literal('business'),
    companyName: z.string().min(1),
    taxId: z.string().regex(/^\d{9}$/),
  }),
])

// Alternative: Using refine
const conditionalSchema = z.object({
  hasDiscount: z.boolean(),
  discountCode: z.string().optional(),
}).refine((data) => {
  // If hasDiscount is true, discountCode is required
  if (data.hasDiscount && !data.discountCode) {
    return false
  }
  return true
}, {
  message: 'Discount code is required when discount is enabled',
  path: ['discountCode'],
})

shadcn/ui Integration

Using Form Component (Legacy)

import { zodResolver } from '@hookform/resolvers/zod'
import { useForm } from 'react-hook-form'
import { z } from 'zod'
import {
  Form,
  FormControl,
  FormDescription,
  FormField,
  FormItem,
  FormLabel,
  FormMessage,
} from '@/components/ui/form'
import { Input } from '@/components/ui/input'

const formSchema = z.object({
  username: z.string().min(2, 'Username must be at least 2 characters'),
  email: z.string().email('Invalid email address'),
})

function ProfileForm() {
  const form = useForm<z.infer<typeof formSchema>>({
    resolver: zodResolver(formSchema),
    defaultValues: {
      username: '',
      email: '',
    },
  })

  return (
    <Form {...form}>
      <form onSubmit={form.handleSubmit(onSubmit)} className="space-y-8">
        <FormField
          control={form.control}
          name="username"
          render={({ field }) => (
            <FormItem>
              <FormLabel>Username</FormLabel>
              <FormControl>
                <Input placeholder="shadcn" {...field} />
              </FormControl>
              <FormDescription>
                This is your public display name.
              </FormDescription>
              <FormMessage />
            </FormItem>
          )}
        />

        <FormField
          control={form.control}
          name="email"
          render={({ field }) => (
            <FormItem>
              <FormLabel>Email</FormLabel>
              <FormControl>
                <Input type="email" placeholder="email@example.com" {...field} />
              </FormControl>
              <FormMessage />
            </FormItem>
          )}
        />

        <button type="submit">Submit</button>
      </form>
    </Form>
  )
}

Note: shadcn/ui states "We are not actively developing the Form component anymore." They recommend using the Field component for new implementations.

Using Field Component (Recommended)

Check shadcn/ui documentation for the latest Field component API as it's the actively maintained approach.

Performance Optimization

Form Mode Strategies

// Best performance - validate only on submit
const form = useForm({
  mode: 'onSubmit',
  resolver: zodResolver(schema),
})

// Good balance - validate on blur
const form = useForm({
  mode: 'onBlur',
  resolver: zodResolver(schema),
})

// Live feedback - validate on every change
const form = useForm({
  mode: 'onChange',
  resolver: zodResolver(schema),
})

// Maximum validation - all events
const form = useForm({
  mode: 'all',
  resolver: zodResolver(schema),
})

Controlled vs Uncontrolled Inputs

// Uncontrolled (better performance) - use register
<input {...register('email')} />

// Controlled (more control) - use Controller
<Controller
  name="email"
  control={control}
  render={({ field }) => <Input {...field} />}
/>

Recommendation: Use register for standard inputs, Controller only when necessary (third-party components, custom behavior).

Isolation with Controller

// BAD: Entire form re-renders when any field changes
function BadForm() {
  const { watch } = useForm()
  const values = watch() // Watches ALL fields

  return <div>{JSON.stringify(values)}</div>
}

// GOOD: Only re-render when specific field changes
function GoodForm() {
  const { watch } = useForm()
  const email = watch('email') // Watches only email field

  return <div>{email}</div>
}

shouldUnregister Flag

const form = useForm({
  resolver: zodResolver(schema),
  shouldUnregister: true, // Remove field data when unmounted
})

When to use:

✅ Multi-step forms where steps have different fields

✅ Conditional fields that should not persist

✅ Want to clear data when component unmounts

When NOT to use:

❌ Want to preserve form data when toggling visibility

❌ Navigating between form sections (tabs, accordions)

Accessibility Best Practices

ARIA Attributes

function AccessibleForm() {
  const { register, handleSubmit, formState: { errors } } = useForm({
    resolver: zodResolver(schema),
  })

  return (
    <form onSubmit={handleSubmit(onSubmit)}>
      <div>
        <label htmlFor="email">Email</label>
        <input
          id="email"
          {...register('email')}
          aria-invalid={errors.email ? 'true' : 'false'}
          aria-describedby={errors.email ? 'email-error' : undefined}
        />
        {errors.email && (
          <span id="email-error" role="alert">
            {errors.email.message}
          </span>
        )}
      </div>
    </form>
  )
}

Error Announcements

import { useEffect } from 'react'

function FormWithAnnouncements() {
  const { formState: { errors, isSubmitted } } = useForm()

  // Announce errors to screen readers
  useEffect(() => {
    if (isSubmitted && Object.keys(errors).length > 0) {
      const errorCount = Object.keys(errors).length
      const announcement = `Form submission failed with ${errorCount} error${errorCount > 1 ? 's' : ''}`

      // Create live region for announcement
      const liveRegion = document.createElement('div')
      liveRegion.setAttribute('role', 'alert')
      liveRegion.setAttribute('aria-live', 'assertive')
      liveRegion.textContent = announcement
      document.body.appendChild(liveRegion)

      setTimeout(() => {
        document.body.removeChild(liveRegion)
      }, 1000)
    }
  }, [errors, isSubmitted])

  return (
    <form>
      {/* ... */}
    </form>
  )
}

Focus Management

import { useRef, useEffect } from 'react'

function FormWithFocus() {
  const { handleSubmit, formState: { errors } } = useForm()
  const firstErrorRef = useRef<HTMLInputElement>(null)

  // Focus first error field on validation failure
  useEffect(() => {
    if (Object.keys(errors).length > 0) {
      firstErrorRef.current?.focus()
    }
  }, [errors])

  return (
    <form onSubmit={handleSubmit(onSubmit)}>
      <input
        {...register('email')}
        ref={errors.email ? firstErrorRef : undefined}
      />
    </form>
  )
}

Critical Rules

Always Do

✅ Set defaultValues to prevent "uncontrolled to controlled" warnings

const form = useForm({
  defaultValues: { email: '', password: '' }, // ALWAYS set defaults
})

✅ Use zodResolver for Zod integration

const form = useForm({
  resolver: zodResolver(schema), // Required for Zod validation
})

✅ Type forms with z.infer

type FormData = z.infer<typeof schema> // Automatic type inference

✅ Validate on both client AND server

// Client
const form = useForm({ resolver: zodResolver(schema) })

// Server
const data = schema.parse(await req.json()) // SAME schema

✅ Use formState.errors for error display

{errors.email && <span role="alert">{errors.email.message}</span>}

✅ Add ARIA attributes for accessibility

<input
  {...register('email')}
  aria-invalid={errors.email ? 'true' : 'false'}
  aria-describedby="email-error"
/>

✅ Use field.id for useFieldArray keys

{fields.map((field) => <div key={field.id}>{/* ... */}</div>)}

✅ Debounce async validation

const debouncedValidation = useDebouncedCallback(() => trigger('username'), 500)

Never Do

❌ Skip server-side validation (security vulnerability!)

// BAD: Only client validation
const form = useForm({ resolver: zodResolver(schema) })
// API endpoint has no validation

// GOOD: Validate on both client and server
const form = useForm({ resolver: zodResolver(schema) })
// API: schema.parse(data) on server too

❌ Use Zod v4 without checking type inference

// Issue #13109: Zod v4 has type inference changes
// Test your types carefully when upgrading

❌ Forget to spread {...field} in Controller

// BAD
<Controller render={({ field }) => <Input value={field.value} />} />

// GOOD
<Controller render={({ field }) => <Input {...field} />} />

❌ Mutate form values directly

// BAD
const values = getValues()
values.email = 'new@email.com' // Direct mutation

// GOOD
setValue('email', 'new@email.com') // Use setValue

❌ Use inline validation without debouncing

// BAD: Validates on every keystroke
const form = useForm({ mode: 'onChange' })

// GOOD: Debounce async validation
const debouncedTrigger = useDebouncedCallback(() => trigger(), 500)

❌ Mix controlled and uncontrolled inputs

// BAD: Mixing patterns
<input {...register('email')} value={email} onChange={setEmail} />

// GOOD: Choose one pattern
<input {...register('email')} /> // Uncontrolled
// OR
<Controller render={({ field }) => <Input {...field} />} /> // Controlled

❌ Use index as key in useFieldArray

// BAD
{fields.map((field, index) => <div key={index}>{/* ... */}</div>)}

// GOOD
{fields.map((field) => <div key={field.id}>{/* ... */}</div>)}

❌ Forget defaultValues for all fields

// BAD: Missing defaults causes warnings
const form = useForm({
  resolver: zodResolver(schema),
})

// GOOD: Set defaults for all fields
const form = useForm({
  resolver: zodResolver(schema),
  defaultValues: { email: '', password: '', remember: false },
})

Known Issues Prevention

This skill prevents 12 documented issues:

Issue #1: Zod v4 Type Inference Errors

Error: Type inference doesn't work correctly with Zod v4
Source: GitHub Issue #13109 (Closed 2025-11-01)
Why It Happens: Zod v4 changed how types are inferred
Prevention: Use correct type patterns: type FormData = z.infer<typeof schema>
Note: Resolved in react-hook-form v7.66.x+. Upgrade to latest version to avoid this issue.

Issue #2: Uncontrolled to Controlled Warning

Error: "A component is changing an uncontrolled input to be controlled"
Source: React documentation
Why It Happens: Not setting defaultValues causes undefined -> value transition
Prevention: Always set defaultValues for all fields

Issue #3: Nested Object Validation Errors

Error: Errors for nested fields don't display correctly
Source: Common React Hook Form issue
Why It Happens: Accessing nested errors incorrectly
Prevention: Use optional chaining: errors.address?.street?.message

Issue #4: Array Field Re-renders

Error: Form re-renders excessively with array fields
Source: Performance issue
Why It Happens: Not using field.id as key
Prevention: Use key={field.id} in useFieldArray map

Issue #5: Async Validation Race Conditions

Error: Multiple validation requests cause conflicting results
Source: Common async pattern issue
Why It Happens: No debouncing or request cancellation
Prevention: Debounce validation and cancel pending requests

Issue #6: Server Error Mapping

Error: Server validation errors don't map to form fields
Source: Integration issue
Why It Happens: Server error format doesn't match React Hook Form format
Prevention: Use setError() to map server errors to fields

Issue #7: Default Values Not Applied

Error: Form fields don't show default values
Source: Common mistake
Why It Happens: defaultValues set after form initialization
Prevention: Set defaultValues in useForm options, not useState

Issue #8: Controller Field Not Updating

Error: Custom component doesn't update when value changes
Source: Common Controller issue
Why It Happens: Not spreading {...field} in render function
Prevention: Always spread {...field} to custom component

Issue #9: useFieldArray Key Warnings

Error: React warning about duplicate keys in list
Source: React list rendering
Why It Happens: Using array index as key instead of field.id
Prevention: Use field.id: key={field.id}

Issue #10: Schema Refinement Error Paths

Error: Custom validation errors appear at wrong field
Source: Zod refinement behavior
Why It Happens: Not specifying path in refinement options
Prevention: Add path option: refine(..., { message: '...', path: ['fieldName'] })

Issue #11: Transform vs Preprocess Confusion

Error: Data transformation doesn't work as expected
Source: Zod API confusion
Why It Happens: Using wrong method for use case
Prevention: Use transform for output transformation, preprocess for input transformation

Issue #12: Multiple Resolver Conflicts

Error: Form validation doesn't work with multiple resolvers
Source: Configuration error
Why It Happens: Trying to use multiple validation libraries
Prevention: Use single resolver (zodResolver), combine schemas if needed

Templates

See the templates/ directory for working examples:

basic-form.tsx - Simple login/signup form

advanced-form.tsx - Nested objects, arrays, conditional fields

shadcn-form.tsx - shadcn/ui Form component integration

server-validation.ts - Server-side validation with same schema

async-validation.tsx - Async validation with debouncing

dynamic-fields.tsx - useFieldArray for adding/removing items

multi-step-form.tsx - Wizard with per-step validation

custom-error-display.tsx - Custom error formatting

package.json - Complete dependencies

References

See the references/ directory for deep-dive documentation:

zod-schemas-guide.md - Comprehensive Zod schema patterns

rhf-api-reference.md - Complete React Hook Form API

error-handling.md - Error messages, formatting, accessibility

accessibility.md - WCAG compliance, ARIA attributes

performance-optimization.md - Form modes, validation strategies

shadcn-integration.md - shadcn/ui Form vs Field components

top-errors.md - 12 common errors with solutions

links-to-official-docs.md - Organized documentation links

Official Documentation

React Hook Form: https://react-hook-form.com/

Zod: https://zod.dev/

@hookform/resolvers: https://github.com/react-hook-form/resolvers

shadcn/ui Form: https://ui.shadcn.com/docs/components/form

License: MIT
Last Verified: 2025-11-20
Maintainer: Jeremy Dawes (jeremy@jezweb.net)