tools-only Server Components and Server Actions
React 19 Server Components run on server, can be async, enable zero-bundle data fetching. Server Actions handle mutations with progressive enhancement.
Async Server Component
Server Components can be async functions — await data fetching directly.
// app/users/[id]/page.tsx
type PageProps = {
params: { id: string };
searchParams?: { tab?: string; edit?: string };
};
export default async function UserPage({ params, searchParams }: PageProps) {
// Runs on server - no client bundle
const user = await fetchUser(params.id);
const posts = await fetchUserPosts(params.id);
return (
<div>
<header>
<h1>{user.name}</h1>
<p>{user.email}</p>
</header>
<UserTabs user={user} posts={posts} activeTab={searchParams?.tab} />
{searchParams?.edit === 'true' && (
<UserEditForm user={user} />
)}
</div>
);
}
async function fetchUser(id: string): Promise<User> {
const res = await fetch(`https://api.example.com/users/${id}`, {
cache: 'no-store', // Or 'force-cache', 'revalidate'
});
if (!res.ok) throw new Error('Failed to fetch user');
return res.json();
}
Parallel Data Fetching
Fetch multiple resources in parallel with Promise.all.
type DashboardProps = {
params: { userId: string };
};
export default async function Dashboard({ params }: DashboardProps) {
// Parallel fetching
const [user, stats, activity] = await Promise.all([
fetchUser(params.userId),
fetchUserStats(params.userId),
fetchRecentActivity(params.userId),
]);
return (
<div>
<UserHeader user={user} />
<StatsGrid stats={stats} />
<ActivityFeed items={activity} />
</div>
);
}
Sequential vs Waterfall Fetching
// ❌ Waterfall - slow
async function SlowPage() {
const user = await fetchUser('123');
const posts = await fetchUserPosts(user.id); // Waits for user
const comments = await fetchPostComments(posts[0].id); // Waits for posts
return <div>...</div>;
}
// ✅ Parallel - fast
async function FastPage() {
const userPromise = fetchUser('123');
const postsPromise = fetchUserPosts('123');
const [user, posts] = await Promise.all([userPromise, postsPromise]);
// If comments depend on posts, fetch after
const comments = await fetchPostComments(posts[0].id);
return <div>...</div>;
}
Server Actions - Form Mutations
Server Actions marked with 'use server' — run on server, callable from client.
// actions/user.ts
'use server';
import { revalidatePath, revalidateTag } from 'next/cache';
import { redirect } from 'next/navigation';
import { z } from 'zod';
const updateUserSchema = z.object({
name: z.string().min(2, 'Name must be at least 2 characters'),
email: z.string().email('Invalid email address'),
bio: z.string().max(500, 'Bio must be less than 500 characters').optional(),
});
type FormState = {
success?: boolean;
errors?: Record<string, string[]>;
message?: string;
};
export async function updateUser(
userId: string,
prevState: FormState,
formData: FormData
): Promise<FormState> {
// Validate
const parsed = updateUserSchema.safeParse({
name: formData.get('name'),
email: formData.get('email'),
bio: formData.get('bio'),
});
if (!parsed.success) {
return {
success: false,
errors: parsed.error.flatten().fieldErrors,
};
}
try {
// Mutate database
await db.user.update({
where: { id: userId },
data: parsed.data,
});
// Revalidate cached data
revalidatePath(`/users/${userId}`);
revalidateTag(`user-${userId}`);
return { success: true, message: 'Profile updated successfully' };
} catch (error) {
return {
success: false,
message: 'Failed to update profile. Please try again.',
};
}
}
export async function deleteUser(userId: string) {
await db.user.delete({ where: { id: userId } });
revalidatePath('/users');
redirect('/users'); // Navigate after mutation
}
Client Component Using Server Action
// components/UserForm.tsx
'use client';
import { useActionState } from 'react';
import { updateUser } from '@/actions/user';
type FormState = {
success?: boolean;
errors?: Record<string, string[]>;
message?: string;
};
export function UserEditForm({ userId, initialData }: Props) {
const [state, formAction, isPending] = useActionState<FormState, FormData>(
(prevState, formData) => updateUser(userId, prevState, formData),
{}
);
return (
<form action={formAction}>
<div>
<label htmlFor="name">Name</label>
<input
id="name"
name="name"
defaultValue={initialData.name}
required
aria-invalid={!!state.errors?.name}
/>
{state.errors?.name?.map((error) => (
<p key={error} className="error">
{error}
</p>
))}
</div>
<div>
<label htmlFor="email">Email</label>
<input
id="email"
name="email"
type="email"
defaultValue={initialData.email}
required
aria-invalid={!!state.errors?.email}
/>
{state.errors?.email?.map((error) => (
<p key={error} className="error">
{error}
</p>
))}
</div>
<div>
<label htmlFor="bio">Bio</label>
<textarea
id="bio"
name="bio"
defaultValue={initialData.bio}
aria-invalid={!!state.errors?.bio}
/>
{state.errors?.bio?.map((error) => (
<p key={error} className="error">
{error}
</p>
))}
</div>
{state.message && (
<div className={state.success ? 'success' : 'error'}>{state.message}</div>
)}
<button type="submit" disabled={isPending}>
{isPending ? 'Saving...' : 'Save Changes'}
</button>
</form>
);
}
Programmatic Server Action
Call Server Actions directly from client code, not just forms.
'use client';
import { deleteUser } from '@/actions/user';
import { useTransition } from 'react';
export function DeleteButton({ userId }: { userId: string }) {
const [isPending, startTransition] = useTransition();
const handleDelete = () => {
if (!confirm('Are you sure you want to delete this user?')) return;
startTransition(async () => {
try {
await deleteUser(userId);
// deleteUser calls redirect(), navigation happens automatically
} catch (error) {
console.error('Failed to delete user:', error);
}
});
};
return (
<button disabled={isPending}>
{isPending ? 'Deleting...' : 'Delete User'}
</button>
);
}
use() Hook - Unwrapping Promises
Pass promises from Server to Client components, unwrap with use().
// Server Component
async function UserPage({ params }: { params: { id: string } }) {
// Don't await - pass promise to client
const userPromise = fetchUser(params.id);
return (
<Suspense fallback={<UserSkeleton />}>
<UserProfile userPromise={userPromise} />
</Suspense>
);
}
// Client Component
'use client';
import { use } from 'react';
type Props = {
userPromise: Promise<User>;
};
export function UserProfile({ userPromise }: Props) {
// Suspends until resolved
const user = use(userPromise);
return (
<div>
<h1>{user.name}</h1>
<p>{user.email}</p>
</div>
);
}
use() with Context
use() also unwraps context — alternative to useContext.
'use client';
import { use } from 'react';
import { ThemeContext } from './ThemeProvider';
export function ThemedButton() {
const theme = use(ThemeContext); // Same as useContext(ThemeContext)
return <button className={theme.mode}>{theme.primaryColor}</button>;
}
Streaming with Suspense
Stream components as they resolve — faster initial page load.
// Server Component
export default async function Page() {
return (
<div>
<h1>Dashboard</h1>
{/* Renders immediately */}
<StaticContent />
{/* Streams when ready */}
<Suspense fallback={<Spinner />}>
<SlowComponent />
</Suspense>
{/* Independent stream */}
<Suspense fallback={<Skeleton />}>
<AnotherSlowComponent />
</Suspense>
</div>
);
}
async function SlowComponent() {
const data = await slowFetch(); // Takes 2s
return <div>{data}</div>;
}
async function AnotherSlowComponent() {
const data = await anotherSlowFetch(); // Takes 1s
return <div>{data}</div>;
}
Error Handling in Server Components
Use error.tsx for error boundaries.
// app/users/[id]/error.tsx
'use client';
export default function Error({
error,
reset,
}: {
error: Error & { digest?: string };
reset: () => void;
}) {
return (
<div>
<h2>Something went wrong!</h2>
<p>{error.message}</p>
<button again</button>
</div>
);
}
// Server Component throws error
export default async function UserPage({ params }: Props) {
const user = await fetchUser(params.id);
if (!user) {
throw new Error('User not found'); // Caught by error.tsx
}
return <div>{user.name}</div>;
}
Loading States with loading.tsx
// app/users/[id]/loading.tsx
export default function Loading() {
return <UserSkeleton />;
}
// Automatically wraps page in Suspense
// No need for manual Suspense boundary
Server-Only Code
Ensure code never runs on client.
// lib/server-only-utils.ts
import 'server-only'; // Throws if imported in client component
export async function getSecretKey() {
return process.env.SECRET_KEY; // Safe - never in client bundle
}
export async function hashPassword(password: string) {
const bcrypt = await import('bcrypt');
return bcrypt.hash(password, 10);
}
Client-Only Code
Ensure code never runs on server.
// lib/client-only-utils.ts
import 'client-only';
export function useLocalStorage(key: string) {
// localStorage only available in browser
const [value, setValue] = useState(() => localStorage.getItem(key));
useEffect(() => {
localStorage.setItem(key, value || '');
}, [key, value]);
return [value, setValue] as const;
}
Mixing Server and Client Components
// app/page.tsx (Server Component)
export default async function Page() {
const data = await fetchData();
return (
<div>
{/* Server Component - can be async */}
<ServerComponent data={data} />
{/* Client Component - interactive */}
<ClientComponent initialData={data} />
</div>
);
}
// ServerComponent.tsx (Server Component - default)
export function ServerComponent({ data }: { data: Data }) {
return <div>{data.title}</div>;
}
// ClientComponent.tsx (Client Component)
'use client';
export function ClientComponent({ initialData }: { initialData: Data }) {
const [data, setData] = useState(initialData);
return (
<button => setData({ ...data, count: data.count + 1 })}>
{data.count}
</button>
);
}
Server Component Patterns
// ✅ Server Component can:
// - Be async
// - Fetch data directly
// - Access backend resources (DB, filesystem)
// - Use server-only packages
// - Pass serializable props to client components
export default async function Page() {
const db = await connectDB(); // Direct DB access
const users = await db.user.findMany();
return <UserList users={users} />; // Pass serializable data
}
// ❌ Server Component cannot:
// - Use hooks (useState, useEffect, etc)
// - Use browser APIs (localStorage, window, etc)
// - Add event listeners (onClick, onChange, etc)
// - Use React Context
// ✅ Client Component can:
// - Use hooks
// - Use browser APIs
// - Add event listeners
// - Use React Context
// - Import Server Components as children
'use client';
export function Layout({ children }: { children: React.ReactNode }) {
const [theme, setTheme] = useState('light');
return (
<div className={theme}>
<button => setTheme('dark')}>Toggle</button>
{children} {/* Server Component can be child */}
</div>
);
}
// ❌ Client Component cannot:
// - Be async
// - Directly access backend resources
// - Import server-only packages
Progressive Enhancement with Server Actions
Forms work without JavaScript when using Server Actions.
// components/AddTodoForm.tsx
import { addTodo } from '@/actions/todos';
export function AddTodoForm() {
return (
<form action={addTodo}>
<input name="title" required />
<button type="submit">Add Todo</button>
{/* Works without JS - progressive enhancement */}
</form>
);
}
// actions/todos.ts
'use server';
export async function addTodo(formData: FormData) {
const title = formData.get('title');
if (typeof title !== 'string') return;
await db.todo.create({ data: { title } });
revalidatePath('/todos');
}