Next.js: Server Actions и мутации — пример кода

// actions.ts — a mutation as a plain server function
'use server';
import { revalidateTag } from 'next/cache';
import { auth } from '@/lib/auth';
import { z } from 'zod';

const Schema = z.object({ title: z.string().min(1).max(200) });

export async function createTodo(_prev: unknown, formData: FormData) {
  const session = await auth();                 // ← authZ is YOURS. Do it first.
  if (!session) return { error: 'Unauthorized' };

  const parsed = Schema.safeParse({ title: formData.get('title') });
  if (!parsed.success) return { error: 'Invalid input' };  // never trust the client

  await db.todo.create({ data: { ...parsed.data, userId: session.userId } });
  revalidateTag('todos');                       // purge the cache the read uses
  return { ok: true };
}

// The form: progressive enhancement + pending + result state, no useEffect.
'use client';
import { useActionState } from 'react';       // React 19 (was useFormState)
import { useFormStatus } from 'react-dom';
import { createTodo } from './actions';

function Submit() {
  const { pending } = useFormStatus();          // must be INSIDE the 
  return {pending ? 'Saving…' : 'Add'};
}

export function TodoForm() {
  const [state, action] = useActionState(createTodo, null);
  return (
    
      
      
      {state?.error && {state.error}}
    
  );
}