Server state vs client state in NextJS - Performance Comparison

Performance: Server state vs client state

HIGH IMPACT

This concept affects page load speed and interaction responsiveness by determining where data is fetched and stored.

Fetching and managing data for a Next.js page

NextJS

export async function getServerSideProps() {
  const res = await fetch('https://api.example.com/data');
  const data = await res.json();
  return { props: { data } };
}

export default function Page({ data }) {
  return <div>{data.content}</div>;
}

Fetching data on the server delivers fully rendered HTML to the client, improving LCP and reducing loading states.

📈 Performance GainImproves LCP by delivering content immediately; reduces INP by avoiding client fetch delays.

Fetching and managing data for a Next.js page

NextJS

export default function Page() {
  const [data, setData] = React.useState(null);
  React.useEffect(() => {
    fetch('/api/data').then(res => res.json()).then(setData);
  }, []);
  if (!data) return <p>Loading...</p>;
  return <div>{data.content}</div>;
}

Fetching data on the client delays Largest Contentful Paint (LCP) because the page renders without data first, causing a loading state.

📉 Performance CostBlocks LCP until client fetch completes; increases INP due to delayed content availability.

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Client fetch on mount	Minimal initial DOM	Multiple reflows due to loading state changes	Multiple paints for loading and content	[X] Bad
Server-side data fetch	Full DOM with data on load	Single reflow on initial render	Single paint with content	[OK] Good
Client state for UI interactions	DOM updates on interaction	Reflows only on state change	Paints only on changed elements	[OK] Good
Server state for UI interactions	Network requests on each interaction	Reflows delayed by network	Paints delayed by data arrival	[X] Bad

Rendering Pipeline

Server state fetching happens before HTML is sent, affecting the Critical Rendering Path by delivering ready content. Client state updates happen after initial render, affecting interaction responsiveness and repaint.

→HTML Parsing

→Style Calculation

→Layout

→Paint

→Composite

⚠️ BottleneckFor server state: HTML generation and network latency; for client state: JavaScript execution and re-rendering.

Core Web Vital Affected

LCP, INP

This concept affects page load speed and interaction responsiveness by determining where data is fetched and stored.

Optimization Tips

1Fetch data on the server to improve initial page load speed (LCP).

2Keep interactive UI state on the client to improve responsiveness (INP).

3Avoid refetching server state on every user interaction to reduce delays.

Performance Quiz - 3 Questions

Which approach improves Largest Contentful Paint (LCP) in Next.js?

A Storing all UI state on the server

B Fetching data on the client after rendering

C Fetching data on the server before rendering

D Using client state for initial data fetch

DevTools: Performance

How to check: Record a page load and interaction in DevTools Performance panel. Look for long tasks during load and delays after user input.

What to look for: Check LCP timing for server vs client data fetch. Look at interaction latency and scripting time for client state responsiveness.

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