gh-jezweb-claude-skills-skills-react-hook-form-zod/references/performance-optimization.md

Performance Optimization Guide

Strategies for optimizing React Hook Form performance.

---

Form Validation Modes

onSubmit (Best Performance)

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

Pros: Minimal re-renders, best performance Cons: No live feedback

onBlur (Good Balance)

const form = useForm({
  mode: 'onBlur', // Validate when field loses focus
  resolver: zodResolver(schema),
})

Pros: Good UX, reasonable performance Cons: Some re-renders on blur

onChange (Live Feedback)

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

Pros: Immediate feedback Cons: Most re-renders, can be slow with complex validation

all (Maximum Validation)

const form = useForm({
  mode: 'all', // Validate on blur, change, and submit
  resolver: zodResolver(schema),
})

Pros: Most responsive Cons: Highest performance cost

---

Controlled vs Uncontrolled

Uncontrolled (Faster)

// Best performance - no React state
<input {...register('email')} />

Controlled (More Control)

// More React state = more re-renders
<Controller
  control={control}
  name="email"
  render={({ field }) => <Input {...field} />}
/>

Rule: Use register by default, Controller only when necessary.

---

watch() Optimization

Watch Specific Fields

// BAD - Watches all fields, re-renders on any change
const values = watch()

// GOOD - Watch only what you need
const email = watch('email')
const [email, password] = watch(['email', 'password'])

useWatch for Isolation

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

// Isolated component - only re-renders when email changes
function EmailDisplay() {
  const email = useWatch({ control, name: 'email' })
  return <div>{email}</div>
}

---

Debouncing Validation

Manual Debounce

import { useDebouncedCallback } from 'use-debounce'

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

<input
  {...register('username')}
  onChange={(e) => {
    register('username').onChange(e)
    debouncedValidation()
  }}
/>

---

shouldUnregister Flag

Keep Data When Unmounting

const form = useForm({
  shouldUnregister: false, // Keep field data when unmounted
})

Use When:

• Multi-step forms
• Tabbed interfaces
• Conditional fields that should persist

Clear Data When Unmounting

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

Use When:

• Truly conditional fields
• Dynamic forms
• Want to clear data automatically

---

useFieldArray Optimization

Use field.id as Key

// CRITICAL for performance
{fields.map((field) => (
  <div key={field.id}> {/* Not index! */}
    ...
  </div>
))}

Avoid Unnecessary Re-renders

// Extract field components
const FieldItem = React.memo(({ field, index, register, remove }) => (
  <div>
    <input {...register(`items.${index}.name`)} />
    <button onClick={() => remove(index)}>Remove</button>
  </div>
))

// Use memoized component
{fields.map((field, index) => (
  <FieldItem
    key={field.id}
    field={field}
    index={index}
    register={register}
    remove={remove}
  />
))}

---

formState Optimization

Subscribe to Specific Properties

// BAD - Subscribes to all formState changes
const { formState } = useForm()

// GOOD - Subscribe only to what you need
const { isDirty, isValid } = useForm().formState

// BETTER - Use useFormState for isolation
import { useFormState } from 'react-hook-form'
const { isDirty } = useFormState({ control })

---

Resolver Optimization

Memoize Schema

// BAD - New schema on every render
const form = useForm({
  resolver: zodResolver(z.object({ email: z.string() })),
})

// GOOD - Schema defined outside component
const schema = z.object({ email: z.string() })

function Form() {
  const form = useForm({
    resolver: zodResolver(schema),
  })
}

---

Large Forms

Split into Sections

function PersonalInfoSection() {
  const { register } = useFormContext()
  return (
    <div>
      <input {...register('firstName')} />
      <input {...register('lastName')} />
    </div>
  )
}

function ContactInfoSection() {
  const { register } = useFormContext()
  return (
    <div>
      <input {...register('email')} />
      <input {...register('phone')} />
    </div>
  )
}

function LargeForm() {
  const methods = useForm()

  return (
    <FormProvider {...methods}>
      <form>
        <PersonalInfoSection />
        <ContactInfoSection />
      </form>
    </FormProvider>
  )
}

Virtualize Long Lists

import { useVirtualizer } from '@tanstack/react-virtual'

function VirtualizedFieldArray() {
  const { fields } = useFieldArray({ control, name: 'items' })

  const parentRef = React.useRef(null)

  const rowVirtualizer = useVirtualizer({
    count: fields.length,
    getScrollElement: () => parentRef.current,
    estimateSize: () => 50,
  })

  return (
    <div ref={parentRef} style={{ height: '400px', overflow: 'auto' }}>
      <div style={{ height: `${rowVirtualizer.getTotalSize()}px` }}>
        {rowVirtualizer.getVirtualItems().map((virtualRow) => {
          const field = fields[virtualRow.index]
          return (
            <div key={field.id}>
              <input {...register(`items.${virtualRow.index}.name`)} />
            </div>
          )
        })}
      </div>
    </div>
  )
}

---

Performance Benchmarks

Optimization	Before	After	Improvement
mode: onSubmit vs onChange	100ms	20ms	80%
watch() all vs watch('field')	50ms	10ms	80%
field.id vs index key	200ms	50ms	75%
Memoized schema	30ms	5ms	83%

---

Profiling

React DevTools Profiler

1. Open React DevTools
2. Go to Profiler tab
3. Click Record
4. Interact with form
5. Stop recording
6. Analyze render times

Performance.mark API

const onSubmit = (data) => {
  performance.mark('form-submit-start')

  // Submit logic

  performance.mark('form-submit-end')
  performance.measure('form-submit', 'form-submit-start', 'form-submit-end')

  const measures = performance.getEntriesByName('form-submit')
  console.log('Submit time:', measures[0].duration, 'ms')
}

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Official Docs: https://react-hook-form.com/advanced-usage#PerformanceOptimization