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Zhongwei Li
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references/performance-optimization.md
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355
references/performance-optimization.md
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# Performance Optimization Guide
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Strategies for optimizing React Hook Form performance.
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---
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## Form Validation Modes
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### onSubmit (Best Performance)
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```typescript
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const form = useForm({
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mode: 'onSubmit', // Validate only on submit
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resolver: zodResolver(schema),
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})
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```
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**Pros**: Minimal re-renders, best performance
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**Cons**: No live feedback
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### onBlur (Good Balance)
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```typescript
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const form = useForm({
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mode: 'onBlur', // Validate when field loses focus
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resolver: zodResolver(schema),
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})
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```
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**Pros**: Good UX, reasonable performance
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**Cons**: Some re-renders on blur
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### onChange (Live Feedback)
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```typescript
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const form = useForm({
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mode: 'onChange', // Validate on every change
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resolver: zodResolver(schema),
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})
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```
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**Pros**: Immediate feedback
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**Cons**: Most re-renders, can be slow with complex validation
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### all (Maximum Validation)
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```typescript
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const form = useForm({
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mode: 'all', // Validate on blur, change, and submit
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resolver: zodResolver(schema),
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})
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```
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**Pros**: Most responsive
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**Cons**: Highest performance cost
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---
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## Controlled vs Uncontrolled
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### Uncontrolled (Faster)
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```typescript
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// Best performance - no React state
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<input {...register('email')} />
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```
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### Controlled (More Control)
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```typescript
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// More React state = more re-renders
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<Controller
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control={control}
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name="email"
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render={({ field }) => <Input {...field} />}
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/>
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```
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**Rule**: Use `register` by default, `Controller` only when necessary.
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---
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## watch() Optimization
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### Watch Specific Fields
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```typescript
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// BAD - Watches all fields, re-renders on any change
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const values = watch()
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// GOOD - Watch only what you need
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const email = watch('email')
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const [email, password] = watch(['email', 'password'])
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```
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### useWatch for Isolation
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```typescript
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import { useWatch } from 'react-hook-form'
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// Isolated component - only re-renders when email changes
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function EmailDisplay() {
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const email = useWatch({ control, name: 'email' })
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return <div>{email}</div>
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}
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```
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---
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## Debouncing Validation
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### Manual Debounce
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```typescript
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import { useDebouncedCallback } from 'use-debounce'
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const debouncedValidation = useDebouncedCallback(
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() => trigger('username'),
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500 // Wait 500ms
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)
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<input
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{...register('username')}
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onChange={(e) => {
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register('username').onChange(e)
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debouncedValidation()
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}}
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/>
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```
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---
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## shouldUnregister Flag
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### Keep Data When Unmounting
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```typescript
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const form = useForm({
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shouldUnregister: false, // Keep field data when unmounted
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})
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```
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**Use When**:
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- Multi-step forms
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- Tabbed interfaces
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- Conditional fields that should persist
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### Clear Data When Unmounting
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```typescript
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const form = useForm({
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shouldUnregister: true, // Remove field data when unmounted
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})
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```
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**Use When**:
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- Truly conditional fields
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- Dynamic forms
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- Want to clear data automatically
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---
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## useFieldArray Optimization
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### Use field.id as Key
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```typescript
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// CRITICAL for performance
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{fields.map((field) => (
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<div key={field.id}> {/* Not index! */}
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...
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</div>
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))}
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```
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### Avoid Unnecessary Re-renders
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```typescript
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// Extract field components
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const FieldItem = React.memo(({ field, index, register, remove }) => (
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<div>
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<input {...register(`items.${index}.name`)} />
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<button onClick={() => remove(index)}>Remove</button>
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</div>
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))
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// Use memoized component
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{fields.map((field, index) => (
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<FieldItem
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key={field.id}
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field={field}
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index={index}
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register={register}
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remove={remove}
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/>
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))}
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```
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---
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## formState Optimization
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### Subscribe to Specific Properties
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```typescript
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// BAD - Subscribes to all formState changes
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const { formState } = useForm()
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// GOOD - Subscribe only to what you need
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const { isDirty, isValid } = useForm().formState
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// BETTER - Use useFormState for isolation
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import { useFormState } from 'react-hook-form'
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const { isDirty } = useFormState({ control })
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```
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---
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## Resolver Optimization
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### Memoize Schema
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```typescript
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// BAD - New schema on every render
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const form = useForm({
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resolver: zodResolver(z.object({ email: z.string() })),
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})
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// GOOD - Schema defined outside component
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const schema = z.object({ email: z.string() })
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function Form() {
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const form = useForm({
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resolver: zodResolver(schema),
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})
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}
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```
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---
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## Large Forms
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### Split into Sections
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```typescript
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function PersonalInfoSection() {
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const { register } = useFormContext()
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return (
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<div>
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<input {...register('firstName')} />
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<input {...register('lastName')} />
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</div>
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)
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}
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function ContactInfoSection() {
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const { register } = useFormContext()
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return (
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<div>
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<input {...register('email')} />
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<input {...register('phone')} />
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</div>
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)
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}
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function LargeForm() {
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const methods = useForm()
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return (
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<FormProvider {...methods}>
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<form>
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<PersonalInfoSection />
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<ContactInfoSection />
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</form>
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</FormProvider>
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)
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}
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```
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### Virtualize Long Lists
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```typescript
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import { useVirtualizer } from '@tanstack/react-virtual'
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function VirtualizedFieldArray() {
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const { fields } = useFieldArray({ control, name: 'items' })
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const parentRef = React.useRef(null)
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const rowVirtualizer = useVirtualizer({
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count: fields.length,
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getScrollElement: () => parentRef.current,
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estimateSize: () => 50,
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})
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return (
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<div ref={parentRef} style={{ height: '400px', overflow: 'auto' }}>
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<div style={{ height: `${rowVirtualizer.getTotalSize()}px` }}>
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{rowVirtualizer.getVirtualItems().map((virtualRow) => {
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const field = fields[virtualRow.index]
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return (
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<div key={field.id}>
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<input {...register(`items.${virtualRow.index}.name`)} />
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</div>
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)
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})}
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</div>
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</div>
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)
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}
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```
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---
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## Performance Benchmarks
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| Optimization | Before | After | Improvement |
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|--------------|--------|-------|-------------|
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| mode: onSubmit vs onChange | 100ms | 20ms | 80% |
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| watch() all vs watch('field') | 50ms | 10ms | 80% |
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| field.id vs index key | 200ms | 50ms | 75% |
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| Memoized schema | 30ms | 5ms | 83% |
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---
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## Profiling
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### React DevTools Profiler
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1. Open React DevTools
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2. Go to Profiler tab
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3. Click Record
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4. Interact with form
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5. Stop recording
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6. Analyze render times
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### Performance.mark API
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```typescript
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const onSubmit = (data) => {
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performance.mark('form-submit-start')
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// Submit logic
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performance.mark('form-submit-end')
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performance.measure('form-submit', 'form-submit-start', 'form-submit-end')
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const measures = performance.getEntriesByName('form-submit')
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console.log('Submit time:', measures[0].duration, 'ms')
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}
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```
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---
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**Official Docs**: https://react-hook-form.com/advanced-usage#PerformanceOptimization
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