Server state vs client state in NextJS

Introduction

Server state and client state help manage data in web apps. Server state is data from the server, client state is data stored in the browser.

When you fetch user info from a database on the server.

When you store if a user clicked a button on the page.

When you want to show live data from a server API.

When you keep track of form inputs before sending them.

When you cache server data to avoid extra loading.

Syntax

NextJS

/* Server state example: fetching data in Next.js server component */
export default async function Page() {
  const res = await fetch('https://api.example.com/data');
  const data = await res.json();
  return <div>{data.message}</div>;
}

/* Client state example: using useState in a React component */
import { useState } from 'react';

export default function Counter() {
  const [count, setCount] = useState(0);
  return (
    <button onClick={() => setCount(count + 1)}>
      Count: {count}
    </button>
  );
}

Server state is usually fetched asynchronously and can be shared across users.

Client state is local to the user and changes instantly on interaction.

Examples

This fetches user data on the server and shows the name.

NextJS

export default async function Page() {
  const res = await fetch('https://api.example.com/user');
  const user = await res.json();
  return <h1>Hello, {user.name}</h1>;
}

This toggles a button label on the client when clicked.

NextJS

import { useState } from 'react';

export default function Toggle() {
  const [on, setOn] = useState(false);
  return <button onClick={() => setOn(!on)}>{on ? 'ON' : 'OFF'}</button>;
}

Sample Program

This example shows client state with a counter button that updates instantly on click. It also shows server state by fetching a post title from an API and displaying it.

NextJS

import { useState } from 'react';

// Client state: count button
function Counter() {
  const [count, setCount] = useState(0);

  return (
    <div>
      <h2>Client State Example</h2>
      <button onClick={() => setCount(count + 1)} aria-label="Increment count">
        Count: {count}
      </button>
    </div>
  );
}

// Server state: fetch message
export async function getServerSideProps() {
  const res = await fetch('https://jsonplaceholder.typicode.com/posts/1');
  const post = await res.json();
  return { props: { title: post.title } };
}

export default function Page({ title }) {
  return (
    <div>
      <Counter />
      <h2>Server State Example: {title}</h2>
    </div>
  );
}

Important Notes

Server state updates when you fetch new data from the server or reload the page.

Client state updates immediately on user actions without needing to reload.

In Next.js, server state can be fetched in server components or data fetching methods like getServerSideProps.

Summary

Server state is data from the server, shared and fetched asynchronously.

Client state is local data in the browser, updated instantly by user actions.

Use server state for data from APIs, client state for UI interactions.

Practice

1. Which of the following best describes server state in Next.js?

easy

A. UI component states like button clicks or form inputs.

B. Temporary data stored only in the browser memory.

C. CSS styles applied to components dynamically.

D. Data fetched from APIs or databases and shared across users.

Solution

1. Step 1: Understand server state meaning

   Server state is data that comes from the server, such as API responses or database queries, and is shared among users.

2. Step 2: Compare with client state

   Client state is local to the browser and changes instantly with user actions, unlike server state.

3. Final Answer:

   Data fetched from APIs or databases and shared across users. -> Option D

4. Quick Check:

   Server state = shared API data [OK]

Hint: Server state = shared data from backend, not local UI [OK]

Common Mistakes:

• Confusing server state with client UI state
• Thinking server state is local to one user
• Mixing server state with CSS or styling

2. Which syntax correctly uses React's useState hook for client state in Next.js?

easy

A. const [count, setCount] = useState(0)

B. const [data, setData] = useState(fetch('/api/data'))

C. const data = await fetch('/api/data')

D. const count = useState(0)

Solution

1. Step 1: Identify correct useState syntax

   useState returns an array with current state and a setter function, so destructuring with [state, setState] is correct.

2. Step 2: Check each option

   const [count, setCount] = useState(0) correctly uses const [count, setCount] = useState(0). const [data, setData] = useState(fetch('/api/data')) wrongly calls fetch inside useState. const data = await fetch('/api/data') is an async fetch, not useState. const count = useState(0) misses destructuring.

3. Final Answer:

   const [count, setCount] = useState(0) -> Option A

4. Quick Check:

   useState returns [state, setter] array [OK]

Hint: useState always destructures [value, setter] array [OK]

Common Mistakes:

• Calling async fetch inside useState
• Not destructuring useState result
• Assigning useState directly to a variable

3. Given this Next.js component, what will be displayed initially?

import { useState, useEffect } from 'react';

export default function Example() {
  const [serverData, setServerData] = useState(null);

  useEffect(() => {
    fetch('/api/data')
      .then(res => res.json())
      .then(data => setServerData(data.message));
  }, []);

  return 
{serverData ?? 'Loading...'}
;
}

medium

A. undefined

B. Loading...

C. null

D. The message from /api/data immediately

Solution

1. Step 1: Check initial state value

   The state serverData starts as null, so the component shows 'Loading...' initially because of the nullish coalescing operator.

2. Step 2: Understand useEffect fetch timing

   The fetch runs after the first render, so the message from the API is not available immediately.

3. Final Answer:

   Loading... -> Option B

4. Quick Check:

   Initial state null shows fallback text [OK]

Hint: Initial null state shows fallback before fetch completes [OK]

Common Mistakes:

• Assuming fetch is synchronous and immediate
• Expecting undefined or null to render as text
• Ignoring useEffect timing

4. Identify the bug in this Next.js component that fetches server state and updates client state:

import { useState, useEffect } from 'react';

export default function DataFetcher() {
  const [data, setData] = useState();

  useEffect(async () => {
    const res = await fetch('/api/data');
    const json = await res.json();
    setData(json);
  }, []);

  return <div>{data.message}</div>;
}

medium

A. useEffect callback cannot be async directly

B. Missing initial state value for useState

C. setData should not be called inside useEffect

D. data.message will always be undefined

Solution

1. Step 1: Check useEffect callback syntax

   React's useEffect callback cannot be async directly; it should call an async function inside.

2. Step 2: Identify correct fix

   Wrap the async fetch logic inside a separate async function inside useEffect, then call it.

3. Final Answer:

   useEffect callback cannot be async directly -> Option A

4. Quick Check:

   Async function inside useEffect, not async callback [OK]

Hint: useEffect callback must be sync; call async function inside [OK]

Common Mistakes:

• Making useEffect callback async directly
• Not handling initial undefined state safely
• Assuming data.message exists before fetch

5. You want to build a Next.js page that shows a list of products fetched from the server and allows users to filter them locally without refetching. Which approach correctly separates server state and client state?

hard

A. Store products in client state only and fetch on every filter change.

B. Fetch products inside useEffect and store in server state for filtering.

C. Fetch products with getServerSideProps and store in client state with useState for filtering.

D. Fetch products with getStaticProps and filter on server on every user input.

Solution

1. Step 1: Understand server state fetching

   Using getServerSideProps fetches data on the server before rendering, providing server state.

2. Step 2: Manage client state for filtering

   Storing the fetched products in useState allows local filtering without refetching, which is client state.

3. Final Answer:

   Fetch products with getServerSideProps and store in client state with useState for filtering. -> Option C

4. Quick Check:

   Server fetch + client filter state = Fetch products with getServerSideProps and store in client state with useState for filtering. [OK]

Hint: Fetch server data once, filter locally with client state [OK]

Common Mistakes:

• Fetching data on every filter change
• Filtering on server for every user input
• Mixing server and client state responsibilities