Typing Server Components

Q: Can a Server Component be an async function?

Yes. Server Components can be async and return Promise. This is unique to RSC -- client components cannot be async.

Q: What is useActionState and how does it replace useFormState?

useActionState (React 19) returns [state, formAction, isPending]. It replaces the older useFormState with full type inference and a built-in pending flag.

Q: Can I use React.FC for Server Components?

No. React.FC does not support async components. Use regular function declarations: async function Page() { ... }.

Q: What does formData.get() return and how do I narrow it?

It returns FormDataEntryValue | null, which is string | File | null. Always check typeof value === "string" before using it as a string.

Recipe

Type React Server Components (RSC), async components, and Server Actions in Next.js. Understand how TypeScript works with the server/client boundary.

Working Example

// app/users/page.tsx - Async Server Component
type User = {
  id: string;
  name: string;
  email: string;
};
 
async function getUsers(): Promise<User[]> {
  const res = await fetch("https://api.example.com/users", {
    cache: "force-cache",
  });
  if (!res.ok) throw new Error("Failed to fetch users");
  return res.json() as Promise<User[]>;
}
 
export default async function UsersPage() {
  const users = await getUsers();
 
  return (
    <main>
      <h1>Users</h1>
      <ul>
        {users.map((user) => (
          <li key={user.id}>{user.name}</li>
        ))}
      </ul>
    </main>
  );
}

// Server Action with typed params
"use server";
 
type CreateUserInput = {
  name: string;
  email: string;
};
 
type ActionResult = {
  success: boolean;
  message: string;
};
 
export async function createUser(input: CreateUserInput): Promise<ActionResult> {
  // Validate and save to database
  if (!input.name || !input.email) {
    return { success: false, message: "Name and email are required" };
  }
 
  // ... database operation
  return { success: true, message: "User created" };
}

// Client component consuming the Server Action
"use client";
 
import { useActionState } from "react";
import { createUser } from "./actions";
 
export function CreateUserForm() {
  const [state, formAction, isPending] = useActionState(
    async (_prevState: ActionResult | null, formData: FormData) => {
      const result = await createUser({
        name: formData.get("name") as string,
        email: formData.get("email") as string,
      });
      return result;
    },
    null
  );
 
  return (
    <form action={formAction}>
      <input name="name" required />
      <input name="email" type="email" required />
      <button type="submit" disabled={isPending}>
        {isPending ? "Creating..." : "Create User"}
      </button>
      {state?.message && <p>{state.message}</p>}
    </form>
  );
}

Deep Dive

How It Works

Server Components can be async functions. TypeScript allows async function Page() that returns Promise<JSX.Element>. This is unique to RSC -- client components cannot be async.
Server Actions are async functions marked with "use server". They run on the server but can be called from client components. TypeScript enforces the parameter and return types across the boundary.
The "use client" and "use server" directives create serialization boundaries. Only serializable types (strings, numbers, plain objects, arrays, Date, FormData, etc.) can cross these boundaries.
useActionState (React 19) replaces the older useFormState. It returns [state, formAction, isPending] with full type inference from the action function.

Variations

Typed page params (Next.js 15):

// app/users/[id]/page.tsx
type PageProps = {
  params: Promise<{ id: string }>;
  searchParams: Promise<{ [key: string]: string | string[] | undefined }>;
};
 
export default async function UserPage({ params, searchParams }: PageProps) {
  const { id } = await params;
  const { tab } = await searchParams;
 
  const user = await getUser(id);
 
  return <div>{user.name}</div>;
}

Server Action with FormData:

"use server";
 
export async function submitForm(formData: FormData): Promise<ActionResult> {
  const name = formData.get("name");
  const email = formData.get("email");
 
  if (typeof name !== "string" || typeof email !== "string") {
    return { success: false, message: "Invalid form data" };
  }
 
  // ... process
  return { success: true, message: "Submitted" };
}

Typed layout component:

// app/layout.tsx
type RootLayoutProps = {
  children: React.ReactNode;
};
 
export default function RootLayout({ children }: RootLayoutProps) {
  return (
    <html lang="en">
      <body>{children}</body>
    </html>
  );
}

TypeScript Notes

In Next.js 15, params and searchParams are Promise types that must be awaited. This is a breaking change from Next.js 14.
Server Actions can only accept and return serializable values. Functions, class instances, and Symbols cannot cross the boundary.
The React.FC type does not support async components. Use regular function declarations for Server Components.

Gotchas

Importing a Server Action into a client component works, but passing a client function to a Server Component does not. Functions are not serializable.
formData.get() returns FormDataEntryValue | null (which is string | File | null). Always validate and narrow the type.
Forgetting "use server" on an action file means the function runs on the client, which will fail if it uses server-only APIs like database access.
Server Components cannot use hooks (useState, useEffect, etc.). TypeScript will not catch this -- it is a runtime error.

Alternatives

Approach	Pros	Cons
Async Server Components	Direct data fetching, zero client JS	Cannot use hooks or browser APIs
Server Actions	Type-safe mutations, progressive enhancement	Only serializable params and returns
API Route Handlers	Full HTTP control, any client can call	Manual fetch + typing on client
tRPC	End-to-end type safety	Additional dependency and setup
Server-side `getServerSideProps` (Pages Router)	Familiar pattern	Pages Router only, being phased out

FAQs

Can a Server Component be an async function?

Yes. Server Components can be async and return Promise<JSX.Element>.
This is unique to RSC -- client components cannot be async.

What is the `"use server"` directive and where does it go?

It marks a file (or individual function) as a Server Action.
Place it at the top of the file or at the beginning of the function body.
Server Actions run on the server but can be called from client components.

What types can cross the server/client boundary?

Only serializable types: strings, numbers, booleans, plain objects, arrays, Date, FormData.
Functions, class instances, Symbols, and Maps cannot cross the boundary.

How do I type page params in Next.js 15?

type PageProps = {
  params: Promise<{ id: string }>;
  searchParams: Promise<{ [key: string]: string | string[] | undefined }>;
};
 
export default async function Page({ params, searchParams }: PageProps) {
  const { id } = await params;
}

What is `useActionState` and how does it replace `useFormState`?

useActionState (React 19) returns [state, formAction, isPending].
It replaces the older useFormState with full type inference and a built-in pending flag.

Gotcha: What happens if I forget the `"use server"` directive on an action file?

The function will run on the client instead of the server.
If it uses server-only APIs (database, secrets), it will fail at runtime.

Can I use `React.FC` for Server Components?

No. React.FC does not support async components.
Use regular function declarations: async function Page() { ... }.

Gotcha: Can Server Components use hooks like useState or useEffect?

No. Server Components cannot use any hooks.
TypeScript will not catch this -- it produces a runtime error, not a compile-time error.

How should I type a layout component?

type RootLayoutProps = {
  children: React.ReactNode;
};
 
export default function RootLayout({ children }: RootLayoutProps) {
  return <html><body>{children}</body></html>;
}

What does `formData.get()` return and how do I narrow it?

It returns FormDataEntryValue | null, which is string | File | null.
Always check typeof value === "string" before using it as a string.

Can I pass a client function to a Server Component?

No. Functions are not serializable and cannot cross the client-to-server boundary.
Importing a Server Action into a client component works, but the reverse does not.

How do I type a Server Action that accepts FormData?

"use server";
 
export async function submitForm(formData: FormData): Promise<ActionResult> {
  const name = formData.get("name");
  if (typeof name !== "string") {
    return { success: false, message: "Invalid" };
  }
  // process...
  return { success: true, message: "Done" };
}

Recipe

Type React Server Components (RSC), async components, and Server Actions in Next.js. Understand how TypeScript works with the server/client boundary.

Working Example

// app/users/page.tsx - Async Server Component
type User = {
  id: string;
  name: string;
  email: string;
};
 
async function getUsers(): Promise<User[]> {
  const res = await fetch("https://api.example.com/users", {
    cache: "force-cache",
  });
  if (!res.ok) throw new Error("Failed to fetch users");
  return res.json() as Promise<User[]>;
}
 
export default async function UsersPage() {
  const users = await getUsers();
 
  return (
    <main>
      <h1>Users</h1>
      <ul>
        {users.map((user) => (
          <li key={user.id}>{user.name}</li>
        ))}
      </ul>
    </main>
  );
}

// Server Action with typed params
"use server";
 
type CreateUserInput = {
  name: string;
  email: string;
};
 
type ActionResult = {
  success: boolean;
  message: string;
};
 
export async function createUser(input: CreateUserInput): Promise<ActionResult> {
  // Validate and save to database
  if (!input.name || !input.email) {
    return { success: false, message: "Name and email are required" };
  }
 
  // ... database operation
  return { success: true, message: "User created" };
}

// Client component consuming the Server Action
"use client";
 
import { useActionState } from "react";
import { createUser } from "./actions";
 
export function CreateUserForm() {
  const [state, formAction, isPending] = useActionState(
    async (_prevState: ActionResult | null, formData: FormData) => {
      const result = await createUser({
        name: formData.get("name") as string,
        email: formData.get("email") as string,
      });
      return result;
    },
    null
  );
 
  return (
    <form action={formAction}>
      <input name="name" required />
      <input name="email" type="email" required />
      <button type="submit" disabled={isPending}>
        {isPending ? "Creating..." : "Create User"}
      </button>
      {state?.message && <p>{state.message}</p>}
    </form>
  );
}

Deep Dive

How It Works

Server Components can be async functions. TypeScript allows async function Page() that returns Promise<JSX.Element>. This is unique to RSC -- client components cannot be async.
Server Actions are async functions marked with "use server". They run on the server but can be called from client components. TypeScript enforces the parameter and return types across the boundary.
The "use client" and "use server" directives create serialization boundaries. Only serializable types (strings, numbers, plain objects, arrays, Date, FormData, etc.) can cross these boundaries.
useActionState (React 19) replaces the older useFormState. It returns [state, formAction, isPending] with full type inference from the action function.

Variations

Typed page params (Next.js 15):

// app/users/[id]/page.tsx
type PageProps = {
  params: Promise<{ id: string }>;
  searchParams: Promise<{ [key: string]: string | string[] | undefined }>;
};
 
export default async function UserPage({ params, searchParams }: PageProps) {
  const { id } = await params;
  const { tab } = await searchParams;
 
  const user = await getUser(id);
 
  return <div>{user.name}</div>;
}

Server Action with FormData:

"use server";
 
export async function submitForm(formData: FormData): Promise<ActionResult> {
  const name = formData.get("name");
  const email = formData.get("email");
 
  if (typeof name !== "string" || typeof email !== "string") {
    return { success: false, message: "Invalid form data" };
  }
 
  // ... process
  return { success: true, message: "Submitted" };
}

Typed layout component:

// app/layout.tsx
type RootLayoutProps = {
  children: React.ReactNode;
};
 
export default function RootLayout({ children }: RootLayoutProps) {
  return (
    <html lang="en">
      <body>{children}</body>
    </html>
  );
}

TypeScript Notes

In Next.js 15, params and searchParams are Promise types that must be awaited. This is a breaking change from Next.js 14.
Server Actions can only accept and return serializable values. Functions, class instances, and Symbols cannot cross the boundary.
The React.FC type does not support async components. Use regular function declarations for Server Components.

Gotchas

Importing a Server Action into a client component works, but passing a client function to a Server Component does not. Functions are not serializable.
formData.get() returns FormDataEntryValue | null (which is string | File | null). Always validate and narrow the type.
Forgetting "use server" on an action file means the function runs on the client, which will fail if it uses server-only APIs like database access.
Server Components cannot use hooks (useState, useEffect, etc.). TypeScript will not catch this -- it is a runtime error.

Alternatives

Approach	Pros	Cons
Async Server Components	Direct data fetching, zero client JS	Cannot use hooks or browser APIs
Server Actions	Type-safe mutations, progressive enhancement	Only serializable params and returns
API Route Handlers	Full HTTP control, any client can call	Manual fetch + typing on client
tRPC	End-to-end type safety	Additional dependency and setup
Server-side `getServerSideProps` (Pages Router)	Familiar pattern	Pages Router only, being phased out

FAQs

Can a Server Component be an async function?

Yes. Server Components can be async and return Promise<JSX.Element>.
This is unique to RSC -- client components cannot be async.

What is the `"use server"` directive and where does it go?

It marks a file (or individual function) as a Server Action.
Place it at the top of the file or at the beginning of the function body.
Server Actions run on the server but can be called from client components.

What types can cross the server/client boundary?

Only serializable types: strings, numbers, booleans, plain objects, arrays, Date, FormData.
Functions, class instances, Symbols, and Maps cannot cross the boundary.

How do I type page params in Next.js 15?

type PageProps = {
  params: Promise<{ id: string }>;
  searchParams: Promise<{ [key: string]: string | string[] | undefined }>;
};
 
export default async function Page({ params, searchParams }: PageProps) {
  const { id } = await params;
}

What is `useActionState` and how does it replace `useFormState`?

useActionState (React 19) returns [state, formAction, isPending].
It replaces the older useFormState with full type inference and a built-in pending flag.

Gotcha: What happens if I forget the `"use server"` directive on an action file?

The function will run on the client instead of the server.
If it uses server-only APIs (database, secrets), it will fail at runtime.

Can I use `React.FC` for Server Components?

No. React.FC does not support async components.
Use regular function declarations: async function Page() { ... }.

Gotcha: Can Server Components use hooks like useState or useEffect?

No. Server Components cannot use any hooks.
TypeScript will not catch this -- it produces a runtime error, not a compile-time error.

How should I type a layout component?

type RootLayoutProps = {
  children: React.ReactNode;
};
 
export default function RootLayout({ children }: RootLayoutProps) {
  return <html><body>{children}</body></html>;
}

What does `formData.get()` return and how do I narrow it?

It returns FormDataEntryValue | null, which is string | File | null.
Always check typeof value === "string" before using it as a string.

Can I pass a client function to a Server Component?

No. Functions are not serializable and cannot cross the client-to-server boundary.
Importing a Server Action into a client component works, but the reverse does not.

How do I type a Server Action that accepts FormData?

"use server";
 
export async function submitForm(formData: FormData): Promise<ActionResult> {
  const name = formData.get("name");
  if (typeof name !== "string") {
    return { success: false, message: "Invalid" };
  }
  // process...
  return { success: true, message: "Done" };
}

Typing Server Components

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Typing Server Components