Server Actions are async functions marked with 'use server' that run on the server and can be called from Client or Server Components — Next.js's answer to form mutations.
Common Interview Questions
Q: Define a Server Action
Answer: Place 'use server' at file or function level. Actions receive FormData or serializable arguments.
"use server";
export async function createPost(formData: FormData) {
const title = formData.get("title");
await db.post.create({ data: { title: String(title) } });
revalidatePath("/posts");
}
Q: Call from a form?
Answer: Pass action to form action prop for progressive enhancement — works without JavaScript.
<form action={createPost}>
<input name="title" />
<button type="submit">Create</button>
</form>
Q: Security considerations?
Answer: Always authenticate and authorize inside the action. Never trust client input. Actions are POST endpoints — validate CSRF protection (Next.js handles for same-origin).
Q: useFormState / useActionState?
Answer: Hook for action return values and pending state in Client Components — show validation errors without manual fetch boilerplate.
Key Points for Interviews
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Colocate mutations with UI
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Works with forms and programmatic calls
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Revalidate cache after mutations
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Validate on server always
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Alternative to REST for internal app mutations