React Forms Architecture

import { useState, type FormEvent } from "react";
import { z } from "zod";

const contactSchema = z.object({
  name: z.string().min(2, "Name must be at least 2 characters"),
  email: z.string().email("Enter a valid email address"),
  message: z.string().min(10, "Message must be at least 10 characters"),
});

type ContactFormData = z.infer<typeof contactSchema>;

export function ContactForm() {
  const [values, setValues] = useState<ContactFormData>({
    name: "",
    email: "",
    message: "",
  });
  const [errors, setErrors] = useState<Partial<Record<keyof ContactFormData, string>>>({});
  const [submitting, setSubmitting] = useState(false);

  function handleChange(field: keyof ContactFormData, value: string) {
    setValues((prev) => ({ ...prev, [field]: value }));
    // Clear field error on change
    if (errors[field]) setErrors((prev) => ({ ...prev, [field]: undefined }));
  }

  async function handleSubmit(e: FormEvent) {
    e.preventDefault();
    const result = contactSchema.safeParse(values);

    if (!result.success) {
      const fieldErrors: typeof errors = {};
      for (const issue of result.error.issues) {
        const field = issue.path[0] as keyof ContactFormData;
        fieldErrors[field] = issue.message;
      }
      setErrors(fieldErrors);
      return;
    }

    setSubmitting(true);
    try {
      await submitContact(result.data);
    } finally {
      setSubmitting(false);
    }
  }

  return (
    <form onSubmit={handleSubmit} noValidate>
      <div>
        <label htmlFor="name">Name</label>
        <input
          id="name"
          value={values.name}
          onChange={(e) => handleChange("name", e.target.value)}
          aria-describedby={errors.name ? "name-error" : undefined}
          aria-invalid={!!errors.name}
        />
        {errors.name && <p id="name-error" role="alert">{errors.name}</p>}
      </div>
      {/* email and message fields follow the same pattern */}
      <button type="submit" disabled={submitting}>
        {submitting ? "Sending..." : "Send message"}
      </button>
    </form>
  );
}

"use server";

import { z } from "zod";
import { redirect } from "next/navigation";

const schema = z.object({
  name: z.string().min(2),
  email: z.string().email(),
  message: z.string().min(10),
});

export async function submitContactAction(
  _prevState: unknown,
  formData: FormData
) {
  const result = schema.safeParse({
    name: formData.get("name"),
    email: formData.get("email"),
    message: formData.get("message"),
  });

  if (!result.success) {
    return {
      errors: result.error.flatten().fieldErrors,
      values: Object.fromEntries(formData),
    };
  }

  await saveContact(result.data);
  redirect("/contact/success");
}

"use client";

import { useActionState } from "react";
import { submitContactAction } from "@/app/contact/actions";

export function ContactFormAction() {
  const [state, action, pending] = useActionState(submitContactAction, null);

  return (
    <form action={action} noValidate>
      <div>
        <label htmlFor="name">Name</label>
        <input
          id="name"
          name="name"
          defaultValue={(state?.values?.name as string) ?? ""}
          aria-describedby={state?.errors?.name ? "name-error" : undefined}
          aria-invalid={!!state?.errors?.name}
        />
        {state?.errors?.name && (
          <p id="name-error" role="alert">{state.errors.name[0]}</p>
        )}
      </div>

      <div>
        <label htmlFor="email">Email</label>
        <input
          id="email"
          name="email"
          type="email"
          defaultValue={(state?.values?.email as string) ?? ""}
          aria-describedby={state?.errors?.email ? "email-error" : undefined}
          aria-invalid={!!state?.errors?.email}
        />
        {state?.errors?.email && (
          <p id="email-error" role="alert">{state.errors.email[0]}</p>
        )}
      </div>

      <button type="submit" disabled={pending}>
        {pending ? "Sending..." : "Send message"}
      </button>
    </form>
  );
}

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

const registrationSchema = z.object({
  firstName: z.string().min(1, "First name is required"),
  lastName: z.string().min(1, "Last name is required"),
  email: z.string().email("Enter a valid email address"),
  password: z
    .string()
    .min(8, "Password must be at least 8 characters")
    .regex(/[A-Z]/, "Password must contain an uppercase letter")
    .regex(/[0-9]/, "Password must contain a number"),
  confirmPassword: z.string(),
}).refine((data) => data.password === data.confirmPassword, {
  message: "Passwords do not match",
  path: ["confirmPassword"],
});

type RegistrationFormData = z.infer<typeof registrationSchema>;

export function RegistrationForm() {
  const {
    register,
    handleSubmit,
    formState: { errors, isSubmitting, isSubmitSuccessful },
    setError,
    reset,
  } = useForm<RegistrationFormData>({
    resolver: zodResolver(registrationSchema),
    defaultValues: {
      firstName: "",
      lastName: "",
      email: "",
      password: "",
      confirmPassword: "",
    },
  });

  async function onSubmit(data: RegistrationFormData) {
    try {
      await registerUser(data);
      reset();
    } catch (err) {
      if (err instanceof ApiError && err.code === "EMAIL_TAKEN") {
        setError("email", { message: "This email is already registered" });
      } else {
        setError("root", { message: "Registration failed. Please try again." });
      }
    }
  }

  return (
    <form onSubmit={handleSubmit(onSubmit)} noValidate>
      {errors.root && (
        <div role="alert" aria-live="assertive">
          {errors.root.message}
        </div>
      )}

      <div>
        <label htmlFor="firstName">First name</label>
        <input
          id="firstName"
          {...register("firstName")}
          aria-describedby={errors.firstName ? "firstName-error" : undefined}
          aria-invalid={!!errors.firstName}
        />
        {errors.firstName && (
          <p id="firstName-error" role="alert">{errors.firstName.message}</p>
        )}
      </div>

      {/* Additional fields follow the same pattern */}

      <button type="submit" disabled={isSubmitting}>
        {isSubmitting ? "Creating account..." : "Create account"}
      </button>

      {isSubmitSuccessful && <p role="status">Account created successfully.</p>}
    </form>
  );
}

import { useForm, FormProvider } from "react-hook-form";
import { zodResolver } from "@hookform/resolvers/zod";
import { AddressFields } from "./AddressFields";
import { PaymentFields } from "./PaymentFields";
import { checkoutSchema, type CheckoutFormData } from "@/lib/schemas/checkout";

export function CheckoutForm() {
  const methods = useForm<CheckoutFormData>({
    resolver: zodResolver(checkoutSchema),
    defaultValues: { shipping: {}, billing: {}, payment: {} },
  });

  return (
    <FormProvider {...methods}>
      <form onSubmit={methods.handleSubmit(onSubmit)} noValidate>
        <fieldset>
          <legend>Shipping address</legend>
          <AddressFields prefix="shipping" />
        </fieldset>
        <fieldset>
          <legend>Payment details</legend>
          <PaymentFields />
        </fieldset>
        <button type="submit" disabled={methods.formState.isSubmitting}>
          Place order
        </button>
      </form>
    </FormProvider>
  );
}

// components/AddressFields.tsx — consumes context, no prop drilling
import { useFormContext } from "react-hook-form";
import type { CheckoutFormData } from "@/lib/schemas/checkout";

interface AddressFieldsProps {
  prefix: "shipping" | "billing";
}

export function AddressFields({ prefix }: AddressFieldsProps) {
  const { register, formState: { errors } } = useFormContext<CheckoutFormData>();
  const fieldErrors = errors[prefix];

  return (
    <>
      <div>
        <label htmlFor={`${prefix}.line1`}>Street address</label>
        <input
          id={`${prefix}.line1`}
          {...register(`${prefix}.line1`)}
          aria-invalid={!!fieldErrors?.line1}
          aria-describedby={fieldErrors?.line1 ? `${prefix}-line1-error` : undefined}
        />
        {fieldErrors?.line1 && (
          <p id={`${prefix}-line1-error`} role="alert">
            {fieldErrors.line1.message}
          </p>
        )}
      </div>
    </>
  );
}

import { Controller, useFormContext } from "react-hook-form";
import Select from "react-select";

interface Option {
  value: string;
  label: string;
}

interface SelectFieldProps {
  name: string;
  label: string;
  options: Option[];
  required?: boolean;
}

// Controller bridges RHF's ref-based system with third-party controlled components
// that manage their own internal state (react-select, date pickers, etc.)
export function SelectField({ name, label, options, required }: SelectFieldProps) {
  const { control, formState: { errors } } = useFormContext();
  const error = errors[name];

  return (
    <div>
      <label htmlFor={name}>
        {label}
        {required && <span aria-hidden="true"> *</span>}
      </label>
      <Controller
        name={name}
        control={control}
        render={({ field }) => (
          <Select
            inputId={name}
            options={options}
            value={options.find((o) => o.value === field.value) ?? null}
            onChange={(opt) => field.onChange(opt?.value ?? "")}
            onBlur={field.onBlur}
            aria-invalid={!!error}
            aria-describedby={error ? `${name}-error` : undefined}
          />
        )}
      />
      {error && (
        <p id={`${name}-error`} role="alert">
          {error.message as string}
        </p>
      )}
    </div>
  );
}

import { z } from "zod";

// Base schema — shared between create and edit forms
const profileBaseSchema = z.object({
  displayName: z
    .string()
    .min(2, "Display name must be at least 2 characters")
    .max(50, "Display name cannot exceed 50 characters"),
  bio: z.string().max(280, "Bio cannot exceed 280 characters").optional(),
  website: z
    .string()
    .url("Enter a valid URL including https://")
    .optional()
    .or(z.literal("")),
  timezone: z.enum(
    ["UTC", "America/New_York", "America/Los_Angeles", "Europe/London", "Australia/Melbourne"],
    { errorMap: () => ({ message: "Select a valid timezone" }) }
  ),
  notifications: z.object({
    email: z.boolean(),
    push: z.boolean(),
    digest: z.enum(["daily", "weekly", "never"]),
  }),
});

// Create schema adds password fields
export const createProfileSchema = profileBaseSchema.extend({
  email: z.string().email("Enter a valid email address"),
  password: z
    .string()
    .min(8, "Password must be at least 8 characters")
    .regex(/[^a-zA-Z0-9]/, "Password must contain a special character"),
  confirmPassword: z.string(),
}).refine(
  (data) => data.password === data.confirmPassword,
  { message: "Passwords do not match", path: ["confirmPassword"] }
);

// Edit schema omits password — same validation, different shape
export const editProfileSchema = profileBaseSchema;

// Type inference — no manual interface declarations needed
export type CreateProfileInput = z.infer<typeof createProfileSchema>;
export type EditProfileInput = z.infer<typeof editProfileSchema>;

import { z } from "zod";

// Different validation rules based on payment method selection
const creditCardSchema = z.object({
  method: z.literal("credit_card"),
  cardNumber: z
    .string()
    .regex(/^d{16}$/, "Card number must be 16 digits"),
  expiryMonth: z.string().regex(/^(0[1-9]|1[0-2])$/, "Invalid month"),
  expiryYear: z.string().regex(/^d{4}$/, "Invalid year"),
  cvv: z.string().regex(/^d{3,4}$/, "CVV must be 3 or 4 digits"),
});

const bankTransferSchema = z.object({
  method: z.literal("bank_transfer"),
  bsb: z.string().regex(/^d{6}$/, "BSB must be 6 digits"),
  accountNumber: z.string().regex(/^d{6,10}$/, "Enter a valid account number"),
});

const paypalSchema = z.object({
  method: z.literal("paypal"),
  paypalEmail: z.string().email("Enter the email address for your PayPal account"),
});

// Discriminated union: Zod validates the correct branch based on 'method'
export const paymentSchema = z.discriminatedUnion("method", [
  creditCardSchema,
  bankTransferSchema,
  paypalSchema,
]);

export type PaymentFormData = z.infer<typeof paymentSchema>;

import { z } from "zod";

export const dateRangeSchema = z
  .object({
    startDate: z.string().regex(/^d{4}-d{2}-d{2}$/, "Enter a valid date"),
    endDate: z.string().regex(/^d{4}-d{2}-d{2}$/, "Enter a valid date"),
    maxDays: z.number().optional(),
  })
  .refine(
    (data) => new Date(data.endDate) >= new Date(data.startDate),
    { message: "End date must be on or after the start date", path: ["endDate"] }
  )
  .refine(
    (data) => {
      if (!data.maxDays) return true;
      const diff =
        (new Date(data.endDate).getTime() - new Date(data.startDate).getTime()) /
        (1000 * 60 * 60 * 24);
      return diff <= data.maxDays;
    },
    { message: "Date range exceeds the maximum allowed period", path: ["endDate"] }
  )
  .transform(({ maxDays: _maxDays, ...rest }) => rest); // strip internal field before submission

import { useFormContext, useWatch } from "react-hook-form";
import type { ProfileFormData } from "@/lib/schemas/profile";

// useWatch creates a subscription to a specific field value.
// Only this component re-renders when accountType changes —
// not the entire form.
export function AccountTypeFields() {
  const { register } = useFormContext<ProfileFormData>();
  const accountType = useWatch<ProfileFormData, "accountType">({ name: "accountType" });

  return (
    <>
      <div>
        <label htmlFor="accountType">Account type</label>
        <select id="accountType" {...register("accountType")}>
          <option value="personal">Personal</option>
          <option value="business">Business</option>
        </select>
      </div>

      {accountType === "business" && (
        <>
          <div>
            <label htmlFor="companyName">Company name</label>
            <input id="companyName" {...register("companyName")} />
          </div>
          <div>
            <label htmlFor="abn">ABN</label>
            <input id="abn" {...register("abn")} />
          </div>
        </>
      )}
    </>
  );
}

import { useForm } from "react-hook-form";
import { zodResolver } from "@hookform/resolvers/zod";
import { useEffect } from "react";
import { useQuery } from "@tanstack/react-query";
import { editProfileSchema, type EditProfileInput } from "@/lib/schemas/profile";

export function EditProfileForm({ userId }: { userId: string }) {
  const { data: profile } = useQuery({
    queryKey: ["profile", userId],
    queryFn: () => fetchProfile(userId),
  });

  const { register, handleSubmit, reset, formState: { errors, isDirty } } =
    useForm<EditProfileInput>({
      resolver: zodResolver(editProfileSchema),
    });

  // Populate form when server data arrives (or when profile changes externally)
  useEffect(() => {
    if (profile) {
      reset({
        displayName: profile.displayName,
        bio: profile.bio ?? "",
        website: profile.website ?? "",
        timezone: profile.timezone,
        notifications: profile.notifications,
      });
    }
  }, [profile, reset]);

  // Only submit changed fields — PATCH semantics
  async function onSubmit(data: EditProfileInput) {
    await updateProfile(userId, data);
    // Reset to new values after save so isDirty resets to false
    reset(data);
  }

  return (
    <form onSubmit={handleSubmit(onSubmit)} noValidate>
      {/* fields */}
      <button type="submit" disabled={!isDirty}>
        Save changes
      </button>
      {isDirty && <p>You have unsaved changes.</p>}
    </form>
  );
}

import { useFieldArray, useFormContext } from "react-hook-form";
import type { InvoiceFormData } from "@/lib/schemas/invoice";

export function InvoiceLineItems() {
  const { register, control, formState: { errors } } = useFormContext<InvoiceFormData>();

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

  return (
    <fieldset>
      <legend>Line items</legend>

      {fields.map((field, index) => (
        <div key={field.id} role="group" aria-label={`Line item ${index + 1}`}>
          <div>
            <label htmlFor={`lineItems.${index}.description`}>Description</label>
            <input
              id={`lineItems.${index}.description`}
              {...register(`lineItems.${index}.description`)}
              aria-invalid={!!errors.lineItems?.[index]?.description}
            />
            {errors.lineItems?.[index]?.description && (
              <p role="alert">{errors.lineItems[index].description?.message}</p>
            )}
          </div>

          <div>
            <label htmlFor={`lineItems.${index}.quantity`}>Qty</label>
            <input
              id={`lineItems.${index}.quantity`}
              type="number"
              min="1"
              {...register(`lineItems.${index}.quantity`, { valueAsNumber: true })}
            />
          </div>

          <div>
            <label htmlFor={`lineItems.${index}.unitPrice`}>Unit price</label>
            <input
              id={`lineItems.${index}.unitPrice`}
              type="number"
              min="0"
              step="0.01"
              {...register(`lineItems.${index}.unitPrice`, { valueAsNumber: true })}
            />
          </div>

          <button
            type="button"
            onClick={() => remove(index)}
            aria-label={`Remove line item ${index + 1}`}
          >
            Remove
          </button>
        </div>
      ))}

      <button
        type="button"
        onClick={() => append({ description: "", quantity: 1, unitPrice: 0 })}
      >
        Add line item
      </button>

      {errors.lineItems?.root && (
        <p role="alert">{errors.lineItems.root.message}</p>
      )}
    </fieldset>
  );
}

// Typed field descriptor — every field type has a strongly-typed config shape

type TextField = {
  type: "text" | "email" | "tel" | "url";
  name: string;
  label: string;
  placeholder?: string;
  required?: boolean;
  maxLength?: number;
};

type SelectField = {
  type: "select";
  name: string;
  label: string;
  options: Array<{ value: string; label: string }>;
  required?: boolean;
};

type CheckboxField = {
  type: "checkbox";
  name: string;
  label: string;
};

type TextareaField = {
  type: "textarea";
  name: string;
  label: string;
  rows?: number;
  maxLength?: number;
};

export type FieldDescriptor = TextField | SelectField | CheckboxField | TextareaField;

export interface FormDescriptor {
  fields: FieldDescriptor[];
  submitLabel?: string;
}

import { useForm } from "react-hook-form";
import type { FormDescriptor, FieldDescriptor } from "@/lib/form-renderer/types";

function renderField(field: FieldDescriptor, register: ReturnType<typeof useForm>["register"]) {
  const id = `field-${field.name}`;

  switch (field.type) {
    case "text":
    case "email":
    case "tel":
    case "url":
      return (
        <div key={field.name}>
          <label htmlFor={id}>{field.label}</label>
          <input
            id={id}
            type={field.type}
            placeholder={field.placeholder}
            {...register(field.name, { required: field.required })}
          />
        </div>
      );
    case "select":
      return (
        <div key={field.name}>
          <label htmlFor={id}>{field.label}</label>
          <select id={id} {...register(field.name, { required: field.required })}>
            <option value="">Select an option</option>
            {field.options.map((opt) => (
              <option key={opt.value} value={opt.value}>{opt.label}</option>
            ))}
          </select>
        </div>
      );
    case "checkbox":
      return (
        <div key={field.name}>
          <input id={id} type="checkbox" {...register(field.name)} />
          <label htmlFor={id}>{field.label}</label>
        </div>
      );
    case "textarea":
      return (
        <div key={field.name}>
          <label htmlFor={id}>{field.label}</label>
          <textarea id={id} rows={field.rows ?? 4} {...register(field.name)} />
        </div>
      );
  }
}

export function DynamicFormRenderer({ descriptor, onSubmit }: {
  descriptor: FormDescriptor;
  onSubmit: (data: Record<string, unknown>) => Promise<void>;
}) {
  const { register, handleSubmit } = useForm();

  return (
    <form onSubmit={handleSubmit(onSubmit)} noValidate>
      {descriptor.fields.map((field) => renderField(field, register))}
      <button type="submit">{descriptor.submitLabel ?? "Submit"}</button>
    </form>
  );
}

import { create } from "zustand";
import { persist } from "zustand/middleware";
import type { Step1Data, Step2Data, Step3Data } from "@/lib/schemas/onboarding";

type OnboardingData = Partial<Step1Data & Step2Data & Step3Data>;

interface OnboardingStore {
  currentStep: number;
  totalSteps: number;
  data: OnboardingData;
  completedSteps: Set<number>;

  setStepData: (step: number, data: Partial<OnboardingData>) => void;
  goToStep: (step: number) => void;
  nextStep: () => void;
  prevStep: () => void;
  reset: () => void;
}

export const useOnboardingStore = create<OnboardingStore>()(
  persist(
    (set, get) => ({
      currentStep: 1,
      totalSteps: 3,
      data: {},
      completedSteps: new Set(),

      setStepData: (step, stepData) =>
        set((state) => ({
          data: { ...state.data, ...stepData },
          completedSteps: new Set([...state.completedSteps, step]),
        })),

      goToStep: (step) => set({ currentStep: step }),

      nextStep: () =>
        set((state) => ({
          currentStep: Math.min(state.currentStep + 1, state.totalSteps),
        })),

      prevStep: () =>
        set((state) => ({
          currentStep: Math.max(state.currentStep - 1, 1),
        })),

      reset: () =>
        set({ currentStep: 1, data: {}, completedSteps: new Set() }),
    }),
    { name: "onboarding-draft" }  // persists to localStorage as a draft
  )
);

import { useForm } from "react-hook-form";
import { zodResolver } from "@hookform/resolvers/zod";
import { step1Schema, type Step1Data } from "@/lib/schemas/onboarding";
import { useOnboardingStore } from "@/stores/onboardingStore";

export function Step1PersonalInfo() {
  const { data, setStepData, nextStep } = useOnboardingStore();

  const { register, handleSubmit, formState: { errors } } = useForm<Step1Data>({
    resolver: zodResolver(step1Schema),
    defaultValues: {
      firstName: data.firstName ?? "",
      lastName: data.lastName ?? "",
      dateOfBirth: data.dateOfBirth ?? "",
    },
  });

  function onValid(stepData: Step1Data) {
    setStepData(1, stepData);
    nextStep();
  }

  return (
    <form onSubmit={handleSubmit(onValid)} noValidate>
      <div>
        <label htmlFor="firstName">First name</label>
        <input id="firstName" {...register("firstName")} aria-invalid={!!errors.firstName} />
        {errors.firstName && <p role="alert">{errors.firstName.message}</p>}
      </div>

      <div>
        <label htmlFor="lastName">Last name</label>
        <input id="lastName" {...register("lastName")} aria-invalid={!!errors.lastName} />
        {errors.lastName && <p role="alert">{errors.lastName.message}</p>}
      </div>

      <button type="submit">Continue</button>
    </form>
  );
}

import { useOnboardingStore } from "@/stores/onboardingStore";
import { Step1PersonalInfo } from "./Step1PersonalInfo";
import { Step2AccountDetails } from "./Step2AccountDetails";
import { Step3Preferences } from "./Step3Preferences";

const steps = [Step1PersonalInfo, Step2AccountDetails, Step3Preferences];

export function OnboardingWizard() {
  const { currentStep, totalSteps, completedSteps } = useOnboardingStore();
  const StepComponent = steps[currentStep - 1];

  return (
    <div>
      {/* Progress indicator */}
      <nav aria-label="Onboarding progress">
        <ol>
          {Array.from({ length: totalSteps }, (_, i) => i + 1).map((step) => (
            <li
              key={step}
              aria-current={step === currentStep ? "step" : undefined}
            >
              <span
                aria-label={
                  completedSteps.has(step)
                    ? `Step ${step}: completed`
                    : step === currentStep
                      ? `Step ${step}: current`
                      : `Step ${step}: upcoming`
                }
              >
                {step}
              </span>
            </li>
          ))}
        </ol>
      </nav>

      {/* Current step */}
      <div aria-live="polite" aria-atomic="true">
        <StepComponent />
      </div>
    </div>
  );
}

interface FormFieldProps {
  id: string;
  label: string;
  error?: string;
  hint?: string;
  required?: boolean;
  children: (props: {
    id: string;
    "aria-describedby"?: string;
    "aria-invalid"?: true;
    "aria-required"?: true;
  }) => React.ReactNode;
}

// Reusable wrapper that handles all ARIA associations automatically
export function FormField({ id, label, error, hint, required, children }: FormFieldProps) {
  const hintId = hint ? `${id}-hint` : undefined;
  const errorId = error ? `${id}-error` : undefined;
  const describedBy = [hintId, errorId].filter(Boolean).join(" ") || undefined;

  return (
    <div>
      <label htmlFor={id}>
        {label}
        {required && (
          <>
            <span aria-hidden="true"> *</span>
            <span className="sr-only"> (required)</span>
          </>
        )}
      </label>

      {hint && (
        <p id={hintId} className="field-hint">
          {hint}
        </p>
      )}

      {children({
        id,
        "aria-describedby": describedBy,
        "aria-invalid": error ? true : undefined,
        "aria-required": required ? true : undefined,
      })}

      {error && (
        <p id={errorId} role="alert" className="field-error">
          {error}
        </p>
      )}
    </div>
  );
}

import { useEffect, useRef } from "react";
import type { FieldErrors } from "react-hook-form";

// Moves focus to the first field with an error after a failed submit attempt.
// Essential for keyboard and screen reader users who need to know where errors are.
export function useFormFocusOnError<T extends Record<string, unknown>>(
  errors: FieldErrors<T>,
  submitCount: number
) {
  const prevSubmitCount = useRef(submitCount);

  useEffect(() => {
    if (submitCount === prevSubmitCount.current) return;
    prevSubmitCount.current = submitCount;

    const errorKeys = Object.keys(errors);
    if (errorKeys.length === 0) return;

    // Find the first errored input in DOM order
    const firstErrorField = document.querySelector<HTMLElement>(
      `[aria-invalid="true"], [name="${errorKeys[0]}"]`
    );

    firstErrorField?.focus();
  }, [errors, submitCount]);
}

// Screen readers need live region announcements for async state changes
// that happen outside the user's current focus area.

interface FormStatusAnnouncerProps {
  isSubmitting: boolean;
  isSuccess: boolean;
  successMessage: string;
  errorMessage?: string;
}

export function FormStatusAnnouncer({
  isSubmitting,
  isSuccess,
  successMessage,
  errorMessage,
}: FormStatusAnnouncerProps) {
  return (
    <>
      {/* Polite: announced after the user finishes their current action */}
      <div
        role="status"
        aria-live="polite"
        aria-atomic="true"
        className="sr-only"
      >
        {isSubmitting && "Submitting form, please wait."}
        {isSuccess && successMessage}
      </div>

      {/* Assertive: interrupts immediately — only for critical errors */}
      <div
        role="alert"
        aria-live="assertive"
        aria-atomic="true"
        className="sr-only"
      >
        {errorMessage}
      </div>
    </>
  );
}

import { useFormContext } from "react-hook-form";
import { useState, useEffect, useRef } from "react";

export function UsernameField() {
  const { register, formState: { errors }, setError, clearErrors, watch } =
    useFormContext<{ username: string }>();

  const username = watch("username");
  const [checking, setChecking] = useState(false);
  const abortRef = useRef<AbortController | null>(null);

  useEffect(() => {
    if (!username || username.length < 3) {
      clearErrors("username");
      return;
    }

    // Cancel any previous in-flight check
    abortRef.current?.abort();
    abortRef.current = new AbortController();

    const timeout = setTimeout(async () => {
      setChecking(true);
      try {
        const res = await fetch(`/api/users/check-username?username=${username}`, {
          signal: abortRef.current!.signal,
        });
        const { available } = await res.json() as { available: boolean };

        if (!available) {
          setError("username", { type: "manual", message: "This username is already taken" });
        } else {
          clearErrors("username");
        }
      } catch (err) {
        if ((err as Error).name !== "AbortError") {
          // Silently fail async checks — do not block submit over a network error
        }
      } finally {
        setChecking(false);
      }
    }, 400); // 400ms debounce

    return () => {
      clearTimeout(timeout);
      abortRef.current?.abort();
    };
  }, [username, setError, clearErrors]);

  return (
    <div>
      <label htmlFor="username">
        Username
        {checking && <span aria-live="polite"> Checking availability...</span>}
      </label>
      <input
        id="username"
        {...register("username")}
        aria-invalid={!!errors.username}
        aria-describedby={errors.username ? "username-error" : undefined}
      />
      {errors.username && (
        <p id="username-error" role="alert">{errors.username.message}</p>
      )}
      {!errors.username && username && username.length >= 3 && !checking && (
        <p role="status" aria-live="polite">Username is available.</p>
      )}
    </div>
  );
}

// WRONG: Watch at the form root — re-renders the entire form on every change
export function ProductForm() {
  const { register, watch } = useForm<ProductFormData>();
  const category = watch("category"); // subscribes the whole component to all changes

  return (
    <form>
      <input {...register("name")} />
      {/* Every keystroke in 'name' re-renders this entire component */}
      {category === "digital" && <DigitalOptions />}
    </form>
  );
}

// CORRECT: Isolate the subscription to the component that needs it
export function ProductForm() {
  const { register } = useForm<ProductFormData>();

  return (
    <form>
      <input {...register("name")} />
      <CategoryDependentFields />  {/* only this re-renders on category change */}
    </form>
  );
}

function CategoryDependentFields() {
  const category = useWatch<ProductFormData, "category">({ name: "category" });
  return category === "digital" ? <DigitalOptions /> : null;
}

// Validates on blur initially (less disruptive during typing),
// then re-validates on change once an error has appeared.
// This is the best UX/performance balance for most production forms.
const { register, handleSubmit, formState } = useForm<FormData>({
  resolver: zodResolver(schema),
  mode: "onBlur",
  reValidateMode: "onChange",
});

import { lazy, Suspense, useState } from "react";

// For forms with many sections (e.g. a 40-field employee profile),
// lazy-load sections that are not immediately visible to reduce initial render cost
const EmploymentHistorySection = lazy(() => import("./sections/EmploymentHistorySection"));
const EducationSection = lazy(() => import("./sections/EducationSection"));
const ReferenceSection = lazy(() => import("./sections/ReferenceSection"));

export function EmployeeProfileForm() {
  const [openSection, setOpenSection] = useState<string | null>("personal");

  return (
    <form>
      <PersonalDetailsSection />

      <details open={openSection === "employment"} onToggle={() => setOpenSection("employment")}>
        <summary>Employment history</summary>
        <Suspense fallback={<SectionSkeleton />}>
          {openSection === "employment" && <EmploymentHistorySection />}
        </Suspense>
      </details>

      {/* Additional sections follow the same pattern */}
    </form>
  );
}

"use server";

import { z } from "zod";
import { revalidatePath } from "next/cache";
import { editProfileSchema } from "@/lib/schemas/profile";
import { getServerSession } from "@/lib/auth";

export interface ProfileActionState {
  success: boolean;
  errors?: Partial<Record<string, string[]>>;
  message?: string;
}

export async function updateProfileAction(
  _prevState: ProfileActionState,
  formData: FormData
): Promise<ProfileActionState> {
  const session = await getServerSession();
  if (!session) return { success: false, message: "Unauthorised" };

  const raw = {
    displayName: formData.get("displayName"),
    bio: formData.get("bio"),
    website: formData.get("website"),
    timezone: formData.get("timezone"),
  };

  const result = editProfileSchema.safeParse(raw);

  if (!result.success) {
    return {
      success: false,
      errors: result.error.flatten().fieldErrors,
    };
  }

  try {
    await db.profile.update({
      where: { userId: session.userId },
      data: result.data,
    });
    revalidatePath("/settings/profile");
    return { success: true, message: "Profile updated successfully." };
  } catch (err) {
    return { success: false, message: "Failed to save profile. Please try again." };
  }
}

import { useEffect } from "react";
import type { UseFormSetError, FieldValues, Path } from "react-hook-form";

// After a failed server-side mutation, map the error response back to RHF fields.
// Works with both REST API error responses and Server Action state.

interface ServerErrors {
  fieldErrors?: Partial<Record<string, string[]>>;
  message?: string;
}

export function useServerFormErrors<T extends FieldValues>(
  serverErrors: ServerErrors | null,
  setError: UseFormSetError<T>
) {
  useEffect(() => {
    if (!serverErrors) return;

    if (serverErrors.fieldErrors) {
      for (const [field, messages] of Object.entries(serverErrors.fieldErrors)) {
        if (messages && messages.length > 0) {
          setError(field as Path<T>, {
            type: "server",
            message: messages[0],
          });
        }
      }
    }

    if (serverErrors.message && !serverErrors.fieldErrors) {
      setError("root" as Path<T>, {
        type: "server",
        message: serverErrors.message,
      });
    }
  }, [serverErrors, setError]);
}

"use client";

import { useOptimistic, useTransition } from "react";
import { toggleTodoAction } from "@/app/todos/actions";

interface Todo {
  id: string;
  text: string;
  completed: boolean;
}

export function TodoItem({ todo }: { todo: Todo }) {
  const [isPending, startTransition] = useTransition();
  const [optimisticTodo, updateOptimistic] = useOptimistic(
    todo,
    (current, completed: boolean) => ({ ...current, completed })
  );

  function handleToggle() {
    startTransition(async () => {
      // Optimistic update: show new state immediately
      updateOptimistic(!optimisticTodo.completed);
      // Server action: perform the real change
      await toggleTodoAction(todo.id, !todo.completed);
      // On success: React reconciles with real server state
      // On failure: React reverts to the original todo value
    });
  }

  return (
    <li>
      <label>
        <input
          type="checkbox"
          checked={optimisticTodo.completed}
          onChange={handleToggle}
          disabled={isPending}
          aria-label={optimisticTodo.text}
        />
        <span style={{ textDecoration: optimisticTodo.completed ? "line-through" : "none" }}>
          {optimisticTodo.text}
        </span>
      </label>
    </li>
  );
}

import { useMutation, useQueryClient } from "@tanstack/react-query";
import type { EditProfileInput } from "@/lib/schemas/profile";

export function useUpdateProfile(userId: string) {
  const queryClient = useQueryClient();

  return useMutation({
    mutationFn: (data: EditProfileInput) => updateProfileApi(userId, data),

    onMutate: async (newData) => {
      // Cancel any outgoing refetches to avoid overwriting optimistic update
      await queryClient.cancelQueries({ queryKey: ["profile", userId] });

      // Snapshot the previous value for rollback
      const previousProfile = queryClient.getQueryData(["profile", userId]);

      // Optimistically update the cache
      queryClient.setQueryData(["profile", userId], (old: unknown) => ({
        ...(old as object),
        ...newData,
      }));

      return { previousProfile };
    },

    onError: (_err, _newData, context) => {
      // Roll back to snapshot on error
      if (context?.previousProfile) {
        queryClient.setQueryData(["profile", userId], context.previousProfile);
      }
    },

    onSettled: () => {
      // Always refetch after mutation to ensure server-side truth
      queryClient.invalidateQueries({ queryKey: ["profile", userId] });
    },
  });
}

import { useForm } from "react-hook-form";
import { zodResolver } from "@hookform/resolvers/zod";
import { useUpdateProfile } from "@/hooks/useUpdateProfile";
import { editProfileSchema, type EditProfileInput } from "@/lib/schemas/profile";

export function ProfileForm({ userId }: { userId: string }) {
  const { mutateAsync } = useUpdateProfile(userId);

  const {
    register,
    handleSubmit,
    setError,
    formState: { errors, isSubmitting },
  } = useForm<EditProfileInput>({
    resolver: zodResolver(editProfileSchema), // Tier 1: client validation
  });

  async function onSubmit(data: EditProfileInput) {
    try {
      await mutateAsync(data);
    } catch (err) {
      if (err instanceof ValidationError) {
        // Tier 2: server validation — map field errors back to RHF
        for (const [field, message] of Object.entries(err.fieldErrors)) {
          setError(field as keyof EditProfileInput, {
            type: "server",
            message: message as string,
          });
        }
      } else {
        // Tier 3: network / system error — not field-specific, shown at form level
        setError("root.serverError", {
          type: "server",
          message: "Unable to save your profile. Please check your connection and try again.",
        });
      }
    }
  }

  return (
    <form onSubmit={handleSubmit(onSubmit)} noValidate>
      {/* Tier 3 error: shown above the form */}
      {errors.root?.serverError && (
        <div role="alert" aria-live="assertive">
          {errors.root.serverError.message}
        </div>
      )}

      <div>
        <label htmlFor="displayName">Display name</label>
        <input
          id="displayName"
          {...register("displayName")}
          aria-invalid={!!errors.displayName}
          aria-describedby={errors.displayName ? "displayName-error" : undefined}
        />
        {errors.displayName && (
          // Tier 1 and Tier 2 errors: shown inline below the field
          <p id="displayName-error" role="alert">
            {errors.displayName.message}
          </p>
        )}
      </div>

      <button type="submit" disabled={isSubmitting}>
        {isSubmitting ? "Saving..." : "Save changes"}
      </button>
    </form>
  );
}

// Normalise different server error shapes into a consistent format
// for use with RHF's setError.

export interface NormalisedFormErrors {
  fieldErrors: Record<string, string>;
  formError?: string;
}

// REST API: { errors: [{ field: "email", message: "Already registered" }] }
export function normaliseRestErrors(
  body: { errors?: Array<{ field: string; message: string }>; message?: string }
): NormalisedFormErrors {
  const fieldErrors: Record<string, string> = {};

  for (const error of body.errors ?? []) {
    fieldErrors[error.field] = error.message;
  }

  return {
    fieldErrors,
    formError: Object.keys(fieldErrors).length === 0 ? body.message : undefined,
  };
}

// Zod flatten format: { fieldErrors: { email: ["Already registered"] } }
export function normaliseZodErrors(
  body: { fieldErrors?: Record<string, string[]> }
): NormalisedFormErrors {
  const fieldErrors: Record<string, string> = {};

  for (const [field, messages] of Object.entries(body.fieldErrors ?? {})) {
    if (messages.length > 0) fieldErrors[field] = messages[0];
  }

  return { fieldErrors };
}

// Apply to RHF
export function applyNormalisedErrors<T extends Record<string, unknown>>(
  errors: NormalisedFormErrors,
  setError: (field: string, error: { type: string; message: string }) => void
) {
  for (const [field, message] of Object.entries(errors.fieldErrors)) {
    setError(field, { type: "server", message });
  }
  if (errors.formError) {
    setError("root.serverError", { type: "server", message: errors.formError });
  }
}

// ANTI-PATTERN
// mode: "onChange" fires full Zod schema validation on every keystroke.
// On a large form, this means running every refine and transform on every key press.
// The result: input lag, unnecessary error flashing, and a poor user experience.
const { register } = useForm({ mode: "onChange" });

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

// ANTI-PATTERN
// Syncing every field change to a global store creates unnecessary global re-renders,
// couples the form to global state, and makes the form impossible to reuse.
const useFormStore = create((set) => ({
  name: "",
  email: "",
  setName: (name) => set({ name }),
  setEmail: (email) => set({ email }),
}));

function NameField() {
  const { name, setName } = useFormStore();
  return <input value={name} onChange={(e) => setName(e.target.value)} />;
}

// CORRECT: Form state lives in the form. Persist only when you need cross-session
// or cross-route access (multi-step forms, unsaved drafts).
// Use RHF for form state. Use Zustand only for the step coordinator in multi-step flows.

// ANTI-PATTERN: Error is visually present but not accessible to screen readers.
// The input has no aria-invalid, the error has no id, and no aria-describedby links them.
{errors.email && (
  <p style={{ color: "red" }}>{errors.email.message}</p>
)}

// CORRECT: Full accessible association
<input
  id="email"
  {...register("email")}
  aria-invalid={!!errors.email}                          // tells screen reader field is invalid
  aria-describedby={errors.email ? "email-error" : undefined}  // links to error message
/>
{errors.email && (
  <p id="email-error" role="alert">                      // role="alert" announces immediately
    {errors.email.message}
  </p>
)}

// ANTI-PATTERN
// A single 300-line component handles layout, validation, submission,
// error display, and all conditional rendering. Untestable. Unmaintainable.
// Impossible for two engineers to work on without conflicts.
export function GiantCheckoutForm() {
  const [step, setStep] = useState(1);
  const [values, setValues] = useState({});
  const [errors, setErrors] = useState({});
  // ... 250 more lines of interleaved logic and JSX
}

// CORRECT: Compose from focused, single-responsibility pieces
export function CheckoutForm() {
  return (
    <FormProvider {...methods}>
      <form onSubmit={methods.handleSubmit(onSubmit)}>
        {step === 1 && <ShippingStep />}   // owns its own fields and validation display
        {step === 2 && <PaymentStep />}    // independently testable
        {step === 3 && <ReviewStep />}     // reads accumulated state, submits
      </form>
    </FormProvider>
  );
}

import { useForm } from "@tanstack/react-form";
import { zodValidator } from "@tanstack/zod-form-adapter";
import { z } from "zod";

const registrationSchema = z.object({
  email: z.string().email("Enter a valid email address"),
  password: z.string().min(8, "Password must be at least 8 characters"),
});

export function RegistrationForm() {
  const form = useForm({
    defaultValues: { email: "", password: "" },
    onSubmit: async ({ value }) => {
      await registerUser(value);
    },
    validatorAdapter: zodValidator(),
  });

  return (
    <form
      onSubmit={(e) => {
        e.preventDefault();
        form.handleSubmit();
      }}
      noValidate
    >
      <form.Field
        name="email"
        validators={{ onChange: registrationSchema.shape.email }}
      >
        {(field) => (
          <div>
            <label htmlFor="email">Email</label>
            <input
              id="email"
              type="email"
              value={field.state.value}
              onBlur={field.handleBlur}
              onChange={(e) => field.handleChange(e.target.value)}
              aria-invalid={!!field.state.meta.errors.length}
              aria-describedby={field.state.meta.errors.length ? "email-error" : undefined}
            />
            {field.state.meta.errors.length > 0 && (
              <p id="email-error" role="alert">{field.state.meta.errors[0]}</p>
            )}
          </div>
        )}
      </form.Field>

      <form.Subscribe selector={(state) => state.isSubmitting}>
        {(isSubmitting) => (
          <button type="submit" disabled={isSubmitting}>
            {isSubmitting ? "Creating account..." : "Create account"}
          </button>
        )}
      </form.Subscribe>
    </form>
  );
}

import { useForm } from "react-hook-form";
import { zodResolver } from "@hookform/resolvers/zod";
import { useMutation } from "@tanstack/react-query";
import { editProfileSchema, type EditProfileInput } from "@/lib/schemas/profile";
import { applyNormalisedErrors, normaliseRestErrors } from "@/lib/form-errors";

export function ProfileForm({ userId }: { userId: string }) {
  const {
    register,
    handleSubmit,
    setError,
    formState: { errors, isDirty, isSubmitting },
  } = useForm<EditProfileInput>({
    resolver: zodResolver(editProfileSchema),
  });

  const { mutateAsync } = useMutation({
    mutationFn: (data: EditProfileInput) =>
      fetch(`/api/users/${userId}/profile`, {
        method: "PATCH",
        headers: { "Content-Type": "application/json" },
        body: JSON.stringify(data),
      }).then(async (res) => {
        if (!res.ok) throw await res.json();
        return res.json();
      }),
  });

  async function onSubmit(data: EditProfileInput) {
    try {
      await mutateAsync(data);
    } catch (err) {
      const normalised = normaliseRestErrors(err as { errors?: Array<{ field: string; message: string }>; message?: string });
      applyNormalisedErrors(normalised, setError);
    }
  }

  return (
    <form onSubmit={handleSubmit(onSubmit)} noValidate>
      {errors.root?.serverError && (
        <div role="alert">{errors.root.serverError.message}</div>
      )}
      <div>
        <label htmlFor="displayName">Display name</label>
        <input
          id="displayName"
          {...register("displayName")}
          aria-invalid={!!errors.displayName}
        />
        {errors.displayName && <p role="alert">{errors.displayName.message}</p>}
      </div>
      <button type="submit" disabled={isSubmitting || !isDirty}>
        {isSubmitting ? "Saving..." : "Save changes"}
      </button>
    </form>
  );
}