Next.js Server Actions

Server Actions

Priority: P1 (HIGH)

Handle form submissions and mutations without creating API endpoints.

Implementation

• Directive: Add 'use server' at the top of an async function.
• Usage: Pass to action prop of <form> or invoke from event handlers.

// actions.ts
'use server';
export async function createPost(formData: FormData) {
  const title = formData.get('title');
  await db.post.create({ title });
  revalidatePath('/posts'); // Refresh UI
}

Client Invocation

• Form: <form action={createPost}> (Progressive enhancements work without JS).
• Event Handler: onClick={() => createPost(data)}.
• Pending State: Use useFormStatus hook (must be inside a component rendered within the form).

P1: Operational Standard

1. Secure & Validate

Always validate inputs and authorization within the action.

'use server';
export async function updateProfile(prevState: any, formData: FormData) {
  const session = await auth();
  if (!session) throw new Error('Unauthorized');

  const validatedFields = ProfileSchema.safeParse(
    Object.fromEntries(formData.entries()),
  );
  if (!validatedFields.success)
    return { errors: validatedFields.error.flatten().fieldErrors };

  // mutation...
  revalidatePath('/profile');
  return { success: true };
}

2. Pending States

Use useActionState (React 19/Next.js 15+) for state handling and useFormStatus for button loading states.

Constraints

• Closures: Avoid defining actions inside components to prevent hidden closure encryption overhead and serialization bugs.
• Redirection: Use redirect() for success navigation; it throws an error that Next.js catches to handle the redirect.