form-patterns

1. Basic Form with Validation

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

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

type LoginFormData = z.infer<typeof loginSchema>;

export function LoginForm() {
  const {
    register,
    handleSubmit,
    formState: { errors, isSubmitting },
  } = useForm<LoginFormData>({
    resolver: zodResolver(loginSchema),
  });

  const onSubmit = async (data: LoginFormData) => {
    await login(data);
  };

  return (
    <form onSubmit={handleSubmit(onSubmit)} className="space-y-4">
      <div>
        <label htmlFor="email" className="block text-sm font-medium">
          Email
        </label>
        <input
          {...register("email")}
          type="email"
          id="email"
          className="mt-1 block w-full rounded-md border px-3 py-2"
        />
        {errors.email && (
          <p className="mt-1 text-sm text-red-600">{errors.email.message}</p>
        )}
      </div>

      <div>
        <label htmlFor="password" className="block text-sm font-medium">
          Password
        </label>
        <input
          {...register("password")}
          type="password"
          id="password"
          className="mt-1 block w-full rounded-md border px-3 py-2"
        />
        {errors.password && (
          <p className="mt-1 text-sm text-red-600">{errors.password.message}</p>
        )}
      </div>

      <button
        type="submit"
        disabled={isSubmitting}
        className="w-full rounded-md bg-primary px-4 py-2 text-white disabled:opacity-50"
      >
        {isSubmitting ? "Signing in..." : "Sign in"}
      </button>
    </form>
  );
}

2. Complex Schema Validation

const userSchema = z.object({
  name: z.string().min(2, "Name is too short"),
  email: z.string().email(),
  age: z.number().min(18, "Must be 18 or older").max(120),

  // Optional with default
  newsletter: z.boolean().default(false),

  // Enum
  role: z.enum(["user", "admin", "moderator"]),

  // Nested object
  address: z.object({
    street: z.string().min(1),
    city: z.string().min(1),
    zip: z.string().regex(/^\d{5}$/, "Invalid ZIP code"),
  }),

  // Array
  tags: z.array(z.string()).min(1, "Add at least one tag"),

  // Conditional validation
  password: z.string().min(8),
  confirmPassword: z.string(),
}).refine((data) => data.password === data.confirmPassword, {
  message: "Passwords don't match",
  path: ["confirmPassword"],
});

3. Reusable Form Field Component

import { useFormContext, FieldPath, FieldValues } from "react-hook-form";

interface FormFieldProps<T extends FieldValues> {
  name: FieldPath<T>;
  label: string;
  type?: "text" | "email" | "password" | "number";
  placeholder?: string;
}

export function FormField<T extends FieldValues>({
  name,
  label,
  type = "text",
  placeholder,
}: FormFieldProps<T>) {
  const {
    register,
    formState: { errors },
  } = useFormContext<T>();

  const error = errors[name];

  return (
    <div className="space-y-1">
      <label htmlFor={name} className="block text-sm font-medium">
        {label}
      </label>
      <input
        {...register(name)}
        type={type}
        id={name}
        placeholder={placeholder}
        className={cn(
          "block w-full rounded-md border px-3 py-2",
          error && "border-red-500 focus:ring-red-500"
        )}
      />
      {error && (
        <p className="text-sm text-red-600">
          {error.message as string}
        </p>
      )}
    </div>
  );
}

// Usage with FormProvider
function MyForm() {
  const methods = useForm<FormData>({ resolver: zodResolver(schema) });

  return (
    <FormProvider {...methods}>
      <form onSubmit={methods.handleSubmit(onSubmit)}>
        <FormField name="email" label="Email" type="email" />
        <FormField name="password" label="Password" type="password" />
      </form>
    </FormProvider>
  );
}

4. Dynamic Form Fields (Array)

import { useFieldArray, useForm } from "react-hook-form";

const schema = z.object({
  users: z.array(z.object({
    name: z.string().min(1),
    email: z.string().email(),
  })).min(1),
});

type FormData = z.infer<typeof schema>;

function DynamicForm() {
  const { control, register, handleSubmit } = useForm<FormData>({
    resolver: zodResolver(schema),
    defaultValues: {
      users: [{ name: "", email: "" }],
    },
  });

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

  return (
    <form onSubmit={handleSubmit(onSubmit)}>
      {fields.map((field, index) => (
        <div key={field.id} className="flex gap-4">
          <input {...register(`users.${index}.name`)} placeholder="Name" />
          <input {...register(`users.${index}.email`)} placeholder="Email" />
          <button type="button" onClick={() => remove(index)}>
            Remove
          </button>
        </div>
      ))}

      <button type="button" onClick={() => append({ name: "", email: "" })}>
        Add User
      </button>

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

5. Form with File Upload

const schema = z.object({
  name: z.string().min(1),
  avatar: z
    .instanceof(FileList)
    .refine((files) => files.length > 0, "Avatar is required")
    .refine(
      (files) => files[0]?.size <= 5 * 1024 * 1024,
      "File must be less than 5MB"
    )
    .refine(
      (files) => ["image/jpeg", "image/png"].includes(files[0]?.type),
      "Only JPEG or PNG allowed"
    ),
});

function FileUploadForm() {
  const { register, handleSubmit, watch, formState: { errors } } = useForm({
    resolver: zodResolver(schema),
  });

  const avatar = watch("avatar");
  const preview = avatar?.[0] ? URL.createObjectURL(avatar[0]) : null;

  return (
    <form onSubmit={handleSubmit(onSubmit)}>
      <input {...register("name")} />

      <div>
        <input
          {...register("avatar")}
          type="file"
          accept="image/jpeg,image/png"
        />
        {preview && <img src={preview} alt="Preview" className="w-20 h-20" />}
        {errors.avatar && <p>{errors.avatar.message}</p>}
      </div>

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

6. Server Actions (Next.js)

// actions.ts
"use server";

import { z } from "zod";

const schema = z.object({
  email: z.string().email(),
});

export async function subscribeAction(formData: FormData) {
  const result = schema.safeParse({
    email: formData.get("email"),
  });

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

  await subscribeToNewsletter(result.data.email);
  return { success: true };
}

// Component
"use client";

import { useFormState, useFormStatus } from "react-dom";
import { subscribeAction } from "./actions";

function SubmitButton() {
  const { pending } = useFormStatus();
  return (
    <button type="submit" disabled={pending}>
      {pending ? "Subscribing..." : "Subscribe"}
    </button>
  );
}

export function NewsletterForm() {
  const [state, formAction] = useFormState(subscribeAction, null);

  return (
    <form action={formAction}>
      <input name="email" type="email" required />
      {state?.error?.email && <p>{state.error.email}</p>}
      {state?.success && <p>Subscribed!</p>}
      <SubmitButton />
    </form>
  );
}

Best Practices

1. Always use Zod for schema validation
2. Show errors inline next to the field
3. Disable submit while submitting
4. Use FormProvider for deep nesting
5. Debounce async validation (username availability)
6. Reset form after successful submission