React Hook Form

Q: What is the difference between register and Controller?

register works with native HTML inputs by attaching ref, onChange, onBlur, and name Controller wraps controlled components (custom selects, date pickers) that need value and onChange props Use register for native inputs; use Controller for third-party or custom components

Q: What does isDirty track and how do you reset it?

isDirty is true when any field value differs from its defaultValues Call reset(data) after a successful save to update the baseline and set isDirty to false Omitting fields from defaultValues leads to incorrect dirty tracking

Q: How do you load default values from an async data source?

Pass an async function to defaultValues and RHF resolves it automatically

Q: What does useFieldArray provide and when should you use it?

It manages arrays of objects with append, remove, insert, move, swap, replace, and update Each item gets a stable field.id for use as a React key Use it for dynamic lists like ingredients, line items, or addresses

Q: Gotcha: Why should you never overwrite the ref returned by register?

register returns a ref that RHF uses to track the input element Overwriting it disconnects RHF from the DOM element Fix: use Controller if you need a custom ref, or merge refs with a callback ref

Q: Gotcha: Why does valueAsNumber return NaN for empty inputs?

When the input is empty, Number("") returns NaN valueAsNumber: true uses this conversion, resulting in NaN in the form data Fix: combine with setValueAs or use Zod's z.coerce.number() via a resolver instead

Q: How do you trigger validation manually for a single field in TypeScript?

trigger accepts typed field names from the form's generic parameter

Q: How do you pass form methods to child components with proper TypeScript types?

Use UseFormReturn as the prop type to get full type safety

Q: What validation modes are available and how do they differ?

"onSubmit" (default): validates only when the form is submitted "onBlur": validates when a field loses focus "onChange": validates on every keystroke reValidateMode controls re-validation behavior after the first error

Performant, flexible forms with minimal re-renders - setup, register, Controller, and core patterns.

Recipe

Quick-reference recipe card - copy-paste ready.

"use client";
 
import { useForm, Controller } from "react-hook-form";
 
interface LoginForm {
  email: string;
  password: string;
  rememberMe: boolean;
}
 
function LoginForm() {
  const {
    register,
    handleSubmit,
    control,
    formState: { errors, isSubmitting },
  } = useForm<LoginForm>({
    defaultValues: { email: "", password: "", rememberMe: false },
  });
 
  async function onSubmit(data: LoginForm) {
    await fetch("/api/login", { method: "POST", body: JSON.stringify(data) });
  }
 
  return (
    <form onSubmit={handleSubmit(onSubmit)}>
      {/* register - for native inputs */}
      <input {...register("email", { required: "Email is required" })} />
      {errors.email && <span>{errors.email.message}</span>}
 
      <input type="password" {...register("password", { minLength: 8 })} />
 
      {/* Controller - for controlled components */}
      <Controller
        name="rememberMe"
        control={control}
        render={({ field }) => (
          <label>
            <input type="checkbox" checked={field.value} onChange={field.onChange} />
            Remember me
          </label>
        )}
      />
 
      <button type="submit" disabled={isSubmitting}>Log in</button>
    </form>
  );
}

When to reach for this: When you need a form with validation, error handling, and good performance - especially when the form has more than a couple of fields.

Working Example

"use client";
 
import { useForm, useFieldArray } from "react-hook-form";
 
interface Ingredient {
  name: string;
  amount: string;
}
 
interface RecipeFormData {
  title: string;
  description: string;
  servings: number;
  ingredients: Ingredient[];
}
 
export function RecipeEditor() {
  const {
    register,
    handleSubmit,
    control,
    formState: { errors, isSubmitting, isDirty },
    reset,
    watch,
  } = useForm<RecipeFormData>({
    defaultValues: {
      title: "",
      description: "",
      servings: 4,
      ingredients: [{ name: "", amount: "" }],
    },
  });
 
  const { fields, append, remove } = useFieldArray({
    control,
    name: "ingredients",
  });
 
  const watchTitle = watch("title");
 
  function onSubmit(data: RecipeFormData) {
    console.log("Recipe:", data);
    reset(data); // clears isDirty
  }
 
  return (
    <form onSubmit={handleSubmit(onSubmit)} className="max-w-lg space-y-4">
      <h2 className="text-lg font-bold">
        {watchTitle || "New Recipe"}
      </h2>
 
      <div>
        <input
          {...register("title", { required: "Title is required" })}
          placeholder="Recipe title"
          className="w-full rounded border p-2"
        />
        {errors.title && <p className="text-sm text-red-600">{errors.title.message}</p>}
      </div>
 
      <textarea
        {...register("description")}
        placeholder="Description"
        className="w-full rounded border p-2"
        rows={3}
      />
 
      <input
        type="number"
        {...register("servings", { valueAsNumber: true, min: 1 })}
        className="w-32 rounded border p-2"
      />
 
      <fieldset className="space-y-2">
        <legend className="font-semibold">Ingredients</legend>
        {fields.map((field, index) => (
          <div key={field.id} className="flex gap-2">
            <input
              {...register(`ingredients.${index}.name`, { required: true })}
              placeholder="Ingredient"
              className="flex-1 rounded border p-2"
            />
            <input
              {...register(`ingredients.${index}.amount`)}
              placeholder="Amount"
              className="w-24 rounded border p-2"
            />
            <button type="button" onClick={() => remove(index)} className="text-red-500">
              Remove
            </button>
          </div>
        ))}
        <button
          type="button"
          onClick={() => append({ name: "", amount: "" })}
          className="text-sm text-blue-600"
        >
          + Add ingredient
        </button>
      </fieldset>
 
      <div className="flex gap-3">
        <button
          type="submit"
          disabled={isSubmitting}
          className="rounded bg-blue-600 px-4 py-2 text-white disabled:opacity-50"
        >
          Save
        </button>
        {isDirty && <span className="self-center text-sm text-amber-600">Unsaved changes</span>}
      </div>
    </form>
  );
}

What this demonstrates:

useForm with typed defaultValues
register for native inputs with validation rules
useFieldArray for dynamic lists
watch for real-time field observation
isDirty tracking and reset after save

Deep Dive

How It Works

RHF uses uncontrolled inputs by default - register attaches ref, onChange, onBlur, and name to the input
Form state lives in a ref, not React state - only the parts you subscribe to cause re-renders
handleSubmit runs validation first, then calls your onSubmit only if valid
formState properties (errors, isDirty, isValid, etc.) are lazily evaluated via Proxy - only accessed properties trigger re-renders
Controller bridges controlled components (custom selects, date pickers) into RHF

Variations

Validation modes:

const { register } = useForm({
  mode: "onBlur",         // validate on blur (default: "onSubmit")
  reValidateMode: "onChange", // re-validate on change after first error
});

Set values programmatically:

const { setValue, getValues, trigger } = useForm();
 
// Set a single field
setValue("email", "new@example.com", { shouldValidate: true });
 
// Get all values
const all = getValues();
 
// Trigger validation manually
await trigger("email");       // single field
await trigger();              // all fields

Form-level default values from async data:

const { reset } = useForm<ProfileForm>({
  defaultValues: async () => {
    const res = await fetch("/api/profile");
    return res.json();
  },
});

TypeScript Notes

// Strongly typed register - catches typos at compile time
register("emial"); // TS error: "emial" is not a key of LoginForm
 
// Typed errors
errors.email?.message; // string | undefined
 
// UseFormReturn type for passing form methods as props
import type { UseFormReturn } from "react-hook-form";
 
function FormSection({ form }: { form: UseFormReturn<LoginForm> }) {
  return <input {...form.register("email")} />;
}
 
// FieldPath for generic field components
import type { FieldPath, FieldValues } from "react-hook-form";
 
function TextInput<T extends FieldValues>({
  name,
  control,
}: {
  name: FieldPath<T>;
  control: Control<T>;
}) {
  return <Controller name={name} control={control} render={({ field }) => <input {...field} />} />;
}

Gotchas

Default values must be complete - RHF uses defaultValues to determine the initial isDirty state. Omitting fields leads to incorrect dirty tracking. Fix: Always provide every field in defaultValues.
register returns a ref - Do not overwrite the ref returned by register. Fix: Use Controller if you need a custom ref, or merge refs with a callback ref.
Re-renders from formState - Destructuring formState at the top level subscribes to all properties. Fix: Only destructure the properties you need: const { errors } = formState.
valueAsNumber returns NaN for empty inputs - If the input is empty, valueAsNumber: true gives NaN. Fix: Combine with setValueAs or use Zod coercion via a resolver.
useFieldArray keys - Always use field.id as the key, not the array index. Using index causes state bugs when reordering or removing items.

Alternatives

Alternative	Use When	Don't Use When
Native FormData	Simple server action forms with minimal client logic	You need field-level validation and dynamic fields
Formik	You are in a legacy codebase already using Formik	Starting a new project (RHF has better performance)
React 19 `useActionState`	Server-first forms without client-side JS	You need instant field validation and complex UX
Tanstack Form	You want framework-agnostic form logic	You need the widest ecosystem of resolvers and UI libraries

FAQs

What is the difference between register and Controller?

register works with native HTML inputs by attaching ref, onChange, onBlur, and name
Controller wraps controlled components (custom selects, date pickers) that need value and onChange props
Use register for native inputs; use Controller for third-party or custom components

How does react-hook-form achieve minimal re-renders?

Form state lives in a ref, not React state, so updates do not trigger re-renders by default
formState properties are lazily evaluated via Proxy -- only accessed properties trigger re-renders
Only destructure what you need: const { errors } = formState instead of the whole object

What does isDirty track and how do you reset it?

isDirty is true when any field value differs from its defaultValues
Call reset(data) after a successful save to update the baseline and set isDirty to false
Omitting fields from defaultValues leads to incorrect dirty tracking

How do you load default values from an async data source?

const { reset } = useForm<ProfileForm>({
  defaultValues: async () => {
    const res = await fetch("/api/profile");
    return res.json();
  },
});

Pass an async function to defaultValues and RHF resolves it automatically

What does useFieldArray provide and when should you use it?

It manages arrays of objects with append, remove, insert, move, swap, replace, and update
Each item gets a stable field.id for use as a React key
Use it for dynamic lists like ingredients, line items, or addresses

Gotcha: Why should you never overwrite the ref returned by register?

register returns a ref that RHF uses to track the input element
Overwriting it disconnects RHF from the DOM element
Fix: use Controller if you need a custom ref, or merge refs with a callback ref

Gotcha: Why does valueAsNumber return NaN for empty inputs?

When the input is empty, Number("") returns NaN
valueAsNumber: true uses this conversion, resulting in NaN in the form data
Fix: combine with setValueAs or use Zod's z.coerce.number() via a resolver instead

How do you trigger validation manually for a single field in TypeScript?

const { trigger } = useForm<FormData>();
await trigger("email");  // validates just the email field
await trigger();          // validates all fields

trigger accepts typed field names from the form's generic parameter

How do you pass form methods to child components with proper TypeScript types?

import type { UseFormReturn } from "react-hook-form";
 
function FormSection({ form }: { form: UseFormReturn<LoginForm> }) {
  return <input {...form.register("email")} />;
}

Use UseFormReturn<T> as the prop type to get full type safety

What validation modes are available and how do they differ?

"onSubmit" (default): validates only when the form is submitted
"onBlur": validates when a field loses focus
"onChange": validates on every keystroke
reValidateMode controls re-validation behavior after the first error

How does watch differ from getValues?

watch("title") subscribes to changes and triggers re-renders when the value changes
getValues("title") reads the current value without subscribing to changes
Use watch for reactive UI updates; use getValues for one-time reads

RHF + Zod - Zod resolver integration
Form Patterns Basic - login, signup, contact
Form Patterns Complex - multi-step, dynamic fields
shadcn Form - shadcn Form component integration

Recipe

Quick-reference recipe card - copy-paste ready.

"use client";
 
import { useForm, Controller } from "react-hook-form";
 
interface LoginForm {
  email: string;
  password: string;
  rememberMe: boolean;
}
 
function LoginForm() {
  const {
    register,
    handleSubmit,
    control,
    formState: { errors, isSubmitting },
  } = useForm<LoginForm>({
    defaultValues: { email: "", password: "", rememberMe: false },
  });
 
  async function onSubmit(data: LoginForm) {
    await fetch("/api/login", { method: "POST", body: JSON.stringify(data) });
  }
 
  return (
    <form onSubmit={handleSubmit(onSubmit)}>
      {/* register - for native inputs */}
      <input {...register("email", { required: "Email is required" })} />
      {errors.email && <span>{errors.email.message}</span>}
 
      <input type="password" {...register("password", { minLength: 8 })} />
 
      {/* Controller - for controlled components */}
      <Controller
        name="rememberMe"
        control={control}
        render={({ field }) => (
          <label>
            <input type="checkbox" checked={field.value} onChange={field.onChange} />
            Remember me
          </label>
        )}
      />
 
      <button type="submit" disabled={isSubmitting}>Log in</button>
    </form>
  );
}

When to reach for this: When you need a form with validation, error handling, and good performance - especially when the form has more than a couple of fields.

Working Example

"use client";
 
import { useForm, useFieldArray } from "react-hook-form";
 
interface Ingredient {
  name: string;
  amount: string;
}
 
interface RecipeFormData {
  title: string;
  description: string;
  servings: number;
  ingredients: Ingredient[];
}
 
export function RecipeEditor() {
  const {
    register,
    handleSubmit,
    control,
    formState: { errors, isSubmitting, isDirty },
    reset,
    watch,
  } = useForm<RecipeFormData>({
    defaultValues: {
      title: "",
      description: "",
      servings: 4,
      ingredients: [{ name: "", amount: "" }],
    },
  });
 
  const { fields, append, remove } = useFieldArray({
    control,
    name: "ingredients",
  });
 
  const watchTitle = watch("title");
 
  function onSubmit(data: RecipeFormData) {
    console.log("Recipe:", data);
    reset(data); // clears isDirty
  }
 
  return (
    <form onSubmit={handleSubmit(onSubmit)} className="max-w-lg space-y-4">
      <h2 className="text-lg font-bold">
        {watchTitle || "New Recipe"}
      </h2>
 
      <div>
        <input
          {...register("title", { required: "Title is required" })}
          placeholder="Recipe title"
          className="w-full rounded border p-2"
        />
        {errors.title && <p className="text-sm text-red-600">{errors.title.message}</p>}
      </div>
 
      <textarea
        {...register("description")}
        placeholder="Description"
        className="w-full rounded border p-2"
        rows={3}
      />
 
      <input
        type="number"
        {...register("servings", { valueAsNumber: true, min: 1 })}
        className="w-32 rounded border p-2"
      />
 
      <fieldset className="space-y-2">
        <legend className="font-semibold">Ingredients</legend>
        {fields.map((field, index) => (
          <div key={field.id} className="flex gap-2">
            <input
              {...register(`ingredients.${index}.name`, { required: true })}
              placeholder="Ingredient"
              className="flex-1 rounded border p-2"
            />
            <input
              {...register(`ingredients.${index}.amount`)}
              placeholder="Amount"
              className="w-24 rounded border p-2"
            />
            <button type="button" onClick={() => remove(index)} className="text-red-500">
              Remove
            </button>
          </div>
        ))}
        <button
          type="button"
          onClick={() => append({ name: "", amount: "" })}
          className="text-sm text-blue-600"
        >
          + Add ingredient
        </button>
      </fieldset>
 
      <div className="flex gap-3">
        <button
          type="submit"
          disabled={isSubmitting}
          className="rounded bg-blue-600 px-4 py-2 text-white disabled:opacity-50"
        >
          Save
        </button>
        {isDirty && <span className="self-center text-sm text-amber-600">Unsaved changes</span>}
      </div>
    </form>
  );
}

What this demonstrates:

useForm with typed defaultValues
register for native inputs with validation rules
useFieldArray for dynamic lists
watch for real-time field observation
isDirty tracking and reset after save

Deep Dive

How It Works

RHF uses uncontrolled inputs by default - register attaches ref, onChange, onBlur, and name to the input
Form state lives in a ref, not React state - only the parts you subscribe to cause re-renders
handleSubmit runs validation first, then calls your onSubmit only if valid
formState properties (errors, isDirty, isValid, etc.) are lazily evaluated via Proxy - only accessed properties trigger re-renders
Controller bridges controlled components (custom selects, date pickers) into RHF

Variations

Validation modes:

const { register } = useForm({
  mode: "onBlur",         // validate on blur (default: "onSubmit")
  reValidateMode: "onChange", // re-validate on change after first error
});

Set values programmatically:

const { setValue, getValues, trigger } = useForm();
 
// Set a single field
setValue("email", "new@example.com", { shouldValidate: true });
 
// Get all values
const all = getValues();
 
// Trigger validation manually
await trigger("email");       // single field
await trigger();              // all fields

Form-level default values from async data:

const { reset } = useForm<ProfileForm>({
  defaultValues: async () => {
    const res = await fetch("/api/profile");
    return res.json();
  },
});

TypeScript Notes

// Strongly typed register - catches typos at compile time
register("emial"); // TS error: "emial" is not a key of LoginForm
 
// Typed errors
errors.email?.message; // string | undefined
 
// UseFormReturn type for passing form methods as props
import type { UseFormReturn } from "react-hook-form";
 
function FormSection({ form }: { form: UseFormReturn<LoginForm> }) {
  return <input {...form.register("email")} />;
}
 
// FieldPath for generic field components
import type { FieldPath, FieldValues } from "react-hook-form";
 
function TextInput<T extends FieldValues>({
  name,
  control,
}: {
  name: FieldPath<T>;
  control: Control<T>;
}) {
  return <Controller name={name} control={control} render={({ field }) => <input {...field} />} />;
}

Gotchas

Default values must be complete - RHF uses defaultValues to determine the initial isDirty state. Omitting fields leads to incorrect dirty tracking. Fix: Always provide every field in defaultValues.
register returns a ref - Do not overwrite the ref returned by register. Fix: Use Controller if you need a custom ref, or merge refs with a callback ref.
Re-renders from formState - Destructuring formState at the top level subscribes to all properties. Fix: Only destructure the properties you need: const { errors } = formState.
valueAsNumber returns NaN for empty inputs - If the input is empty, valueAsNumber: true gives NaN. Fix: Combine with setValueAs or use Zod coercion via a resolver.
useFieldArray keys - Always use field.id as the key, not the array index. Using index causes state bugs when reordering or removing items.

Alternatives

Alternative	Use When	Don't Use When
Native FormData	Simple server action forms with minimal client logic	You need field-level validation and dynamic fields
Formik	You are in a legacy codebase already using Formik	Starting a new project (RHF has better performance)
React 19 `useActionState`	Server-first forms without client-side JS	You need instant field validation and complex UX
Tanstack Form	You want framework-agnostic form logic	You need the widest ecosystem of resolvers and UI libraries

FAQs

What is the difference between register and Controller?

register works with native HTML inputs by attaching ref, onChange, onBlur, and name
Controller wraps controlled components (custom selects, date pickers) that need value and onChange props
Use register for native inputs; use Controller for third-party or custom components

How does react-hook-form achieve minimal re-renders?

Form state lives in a ref, not React state, so updates do not trigger re-renders by default
formState properties are lazily evaluated via Proxy -- only accessed properties trigger re-renders
Only destructure what you need: const { errors } = formState instead of the whole object

What does isDirty track and how do you reset it?

isDirty is true when any field value differs from its defaultValues
Call reset(data) after a successful save to update the baseline and set isDirty to false
Omitting fields from defaultValues leads to incorrect dirty tracking

How do you load default values from an async data source?

const { reset } = useForm<ProfileForm>({
  defaultValues: async () => {
    const res = await fetch("/api/profile");
    return res.json();
  },
});

Pass an async function to defaultValues and RHF resolves it automatically

What does useFieldArray provide and when should you use it?

It manages arrays of objects with append, remove, insert, move, swap, replace, and update
Each item gets a stable field.id for use as a React key
Use it for dynamic lists like ingredients, line items, or addresses

Gotcha: Why should you never overwrite the ref returned by register?

register returns a ref that RHF uses to track the input element
Overwriting it disconnects RHF from the DOM element
Fix: use Controller if you need a custom ref, or merge refs with a callback ref

Gotcha: Why does valueAsNumber return NaN for empty inputs?

When the input is empty, Number("") returns NaN
valueAsNumber: true uses this conversion, resulting in NaN in the form data
Fix: combine with setValueAs or use Zod's z.coerce.number() via a resolver instead

How do you trigger validation manually for a single field in TypeScript?

const { trigger } = useForm<FormData>();
await trigger("email");  // validates just the email field
await trigger();          // validates all fields

trigger accepts typed field names from the form's generic parameter

How do you pass form methods to child components with proper TypeScript types?

import type { UseFormReturn } from "react-hook-form";
 
function FormSection({ form }: { form: UseFormReturn<LoginForm> }) {
  return <input {...form.register("email")} />;
}

Use UseFormReturn<T> as the prop type to get full type safety

What validation modes are available and how do they differ?

"onSubmit" (default): validates only when the form is submitted
"onBlur": validates when a field loses focus
"onChange": validates on every keystroke
reValidateMode controls re-validation behavior after the first error

How does watch differ from getValues?

watch("title") subscribes to changes and triggers re-renders when the value changes
getValues("title") reads the current value without subscribing to changes
Use watch for reactive UI updates; use getValues for one-time reads

RHF + Zod - Zod resolver integration
Form Patterns Basic - login, signup, contact
Form Patterns Complex - multi-step, dynamic fields
shadcn Form - shadcn Form component integration

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