React Hook Form Writer

This skill helps you write new forms and refactor existing forms to use react-hook-form following project best practices.

When to Use

• Creating new form components from scratch
• Converting existing forms to react-hook-form
• Adding validation to forms
• Implementing complex form patterns (nested forms, field arrays, multi-step)

Core Principles

1. Always Use Zod for Validation

Define schemas with Zod and integrate via zodResolver:

import { z } from "zod";
import { zodResolver } from "@hookform/resolvers/zod";

const formSchema = z.object({
  name: z.string().min(1, "Name is required"),
  email: z.string().email("Invalid email address"),
  age: z.number().min(18, "Must be at least 18"),
});

type FormValues = z.infer<typeof formSchema>;

const form = useForm<FormValues>({
  resolver: zodResolver(formSchema),
  defaultValues: {
    name: "",
    email: "",
    age: 18,
  },
});

2. Prefer useController Over Controller

Use useController hook for better composability in custom field components:

// Good: useController
function TextField({ name, control, label }: TextFieldProps) {
  const { field, fieldState } = useController({ name, control });

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

// Avoid: Controller component (less composable)
<Controller
  name="name"
  control={control}
  render={({ field }) => <input {...field} />}
/>

3. Uncontrolled by Default

Leverage react-hook-form's uncontrolled approach for native inputs:

// Good: Uncontrolled with register
<input {...register("name")} />

// Only use Controller/useController for third-party controlled components
// (e.g., shadcn Select, custom date pickers, rich text editors)

4. Use field.onChange for User Interactions, setValue for Programmatic Updates

When working with useController, use field.onChange for user interactions:

// Good: field.onChange for user interactions
const { field } = useController({ name: "status", control });

<Select onValueChange={field.onChange} value={field.value}>
  {options.map((opt) => (
    <SelectItem key={opt.value} value={opt.value}>
      {opt.label}
    </SelectItem>
  ))}
</Select>

// Bad: setValue for user interactions (breaks controller lifecycle)
<Select onValueChange={(v) => setValue("status", v)} value={watch("status")}>

Use setValue (from useFormContext) for programmatic updates in effects:

// Good: setValue for programmatic initialization
const { setValue } = useFormContext();
const { field } = useController({ name: "status" });

useEffect(() => {
  if (externalData) {
    setValue("status", externalData.defaultStatus); // ✓ programmatic
  }
}, [externalData, setValue]);

const handleUserSelect = (value: string) => {
  field.onChange(value); // ✓ user interaction
};

CRITICAL: Never use field.onChange inside useEffect dependencies

useController returns new field objects on every render. Including them in useEffect dependencies while also calling field.onChange() inside the effect causes infinite loops:

// BAD: Infinite loop - field objects change every render
const { field } = useController({ name: "status" });

useEffect(() => {
  field.onChange(defaultValue); // Triggers re-render
}, [field, defaultValue]); // field changes → effect runs → onChange → re-render → repeat

// GOOD: Use setValue (stable) for programmatic updates in effects
const { setValue } = useFormContext();
const { field } = useController({ name: "status" });

useEffect(() => {
  setValue("status", defaultValue); // setValue is stable
}, [defaultValue, setValue]);

// User interactions still use field.onChange
const handleSelect = (value: string) => {
  field.onChange(value);
};

Summary:

• field.onChange → user interaction handlers (onClick, onSelect, etc.)
• setValue → programmatic updates in useEffect or callbacks based on external data

5. Always Provide Default Values

Always provide defaultValues in useForm for all fields:

// Good: All fields have defaults
const form = useForm<FormValues>({
  resolver: zodResolver(formSchema),
  defaultValues: {
    name: "",
    email: "",
    items: [],
    settings: {
      notifications: true,
      theme: "light",
    },
  },
});

// Bad: Missing defaultValues causes controlled/uncontrolled warnings
const form = useForm<FormValues>({
  resolver: zodResolver(formSchema),
});

6. Watch Specific Fields Only

Never use watch() without parameters:

// Good: Watch specific fields
const selectedType = watch("type");
const [name, email] = watch(["name", "email"]);

// Bad: Watches everything, causes unnecessary re-renders
const allValues = watch();

7. Use Dot Notation for Nested Fields

const schema = z.object({
  user: z.object({
    profile: z.object({
      firstName: z.string(),
      lastName: z.string(),
    }),
  }),
});

// Access nested fields with dot notation
<input {...register("user.profile.firstName")} />

8. Use useFieldArray for Dynamic Lists

const { fields, append, remove } = useFieldArray({
  control,
  name: "items",
});

return (
  <div>
    {fields.map((field, index) => (
      <div key={field.id}>
        <input {...register(`items.${index}.name`)} />
        <button type="button" onClick={() => remove(index)}>
          Remove
        </button>
      </div>
    ))}
    <button type="button" onClick={() => append({ name: "" })}>
      Add Item
    </button>
  </div>
);

9. Proper Form Submission

const onSubmit = async (data: FormValues) => {
  try {
    await submitToApi(data);
  } catch (error) {
    // Handle API errors, optionally set form errors
    form.setError("root", { message: "Submission failed" });
  }
};

<form onSubmit={form.handleSubmit(onSubmit)}>
  {/* fields */}
  {form.formState.errors.root && (
    <div className="error">{form.formState.errors.root.message}</div>
  )}
  <button type="submit" disabled={form.formState.isSubmitting}>
    Submit
  </button>
</form>

10. Reset Forms Correctly

// Good: Reset with new values
form.reset({
  name: "New Name",
  email: "new@email.com",
});

// Good: Reset to default values
form.reset();

// Bad: Manual field clearing
setValue("name", "");
setValue("email", "");

11. Sub-form Validation with trigger()

// Validate specific fields (useful for multi-step forms)
const isStepValid = await form.trigger(["name", "email"]);

if (isStepValid) {
  goToNextStep();
}

12. Error Display Pattern

// Access errors via formState.errors
const {
  formState: { errors },
} = form;

<div>
  <input {...register("email")} />
  {errors.email && (
    <span className="text-red-500">{errors.email.message}</span>
  )}
</div>

Complete Example

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

const schema = z.object({
  name: z.string().min(1, "Name is required"),
  email: z.string().email("Invalid email"),
  role: z.enum(["admin", "user", "guest"]),
  tags: z.array(z.object({ value: z.string().min(1) })),
});

type FormValues = z.infer<typeof schema>;

function MyForm() {
  const form = useForm<FormValues>({
    resolver: zodResolver(schema),
    defaultValues: {
      name: "",
      email: "",
      role: "user",
      tags: [],
    },
  });

  const { fields, append, remove } = useFieldArray({
    control: form.control,
    name: "tags",
  });

  const onSubmit = async (data: FormValues) => {
    console.log(data);
  };

  return (
    <form onSubmit={form.handleSubmit(onSubmit)}>
      <div>
        <label>Name</label>
        <input {...form.register("name")} />
        {form.formState.errors.name && (
          <span>{form.formState.errors.name.message}</span>
        )}
      </div>

      <div>
        <label>Email</label>
        <input {...form.register("email")} />
        {form.formState.errors.email && (
          <span>{form.formState.errors.email.message}</span>
        )}
      </div>

      <RoleSelect control={form.control} />

      <div>
        <label>Tags</label>
        {fields.map((field, index) => (
          <div key={field.id}>
            <input {...form.register(`tags.${index}.value`)} />
            <button type="button" onClick={() => remove(index)}>
              Remove
            </button>
          </div>
        ))}
        <button type="button" onClick={() => append({ value: "" })}>
          Add Tag
        </button>
      </div>

      <button type="submit" disabled={form.formState.isSubmitting}>
        Submit
      </button>
    </form>
  );
}

// Custom controlled component using useController
function RoleSelect({ control }: { control: Control<FormValues> }) {
  const { field, fieldState } = useController({
    name: "role",
    control,
  });

  return (
    <div>
      <label>Role</label>
      <select onChange={field.onChange} value={field.value} ref={field.ref}>
        <option value="admin">Admin</option>
        <option value="user">User</option>
        <option value="guest">Guest</option>
      </select>
      {fieldState.error && <span>{fieldState.error.message}</span>}
    </div>
  );
}

Refactoring Checklist

When refactoring existing forms to react-hook-form:

1. Define Zod schema matching existing validation
2. Set up useForm with zodResolver and defaultValues
3. Replace controlled inputs with register() where possible
4. Use useController for third-party controlled components
5. Replace manual state management with form state
6. Convert submit handlers to use handleSubmit
7. Update error display to use formState.errors
8. Replace manual arrays with useFieldArray
9. Remove unnecessary useState for form values