Router cache on client in NextJS - Performance & Optimization

Performance: Router cache on client

MEDIUM IMPACT

This affects page load speed and interaction responsiveness by reducing network requests and speeding up navigation.

Navigating between pages in a Next.js app

NextJS

import { useRouter } from 'next/router';
import { useState, useEffect } from 'react';

const cache = new Map();

function Page() {
  const router = useRouter();
  const [pageData, setPageData] = useState(null);

  async function goToAbout() {
    if (cache.has('/about')) {
      setPageData(cache.get('/about'));
    } else {
      const res = await fetch('/about-data.json');
      const data = await res.json();
      cache.set('/about', data);
      setPageData(data);
    }
    router.push('/about');
  }

  return <button onClick={goToAbout}>About</button>;
}

Caches fetched data on the client to avoid repeated network requests, speeding up navigation and reducing reflows.

📈 Performance Gainreduces network requests by 100%, cuts rendering delay to under 100ms

Navigating between pages in a Next.js app

NextJS

import { useRouter } from 'next/router';

function Page() {
  const router = useRouter();

  function goToAbout() {
    router.push('/about'); // no caching, fetches fresh data every time
  }

  return <button onClick={goToAbout}>About</button>;
}

Each navigation triggers a full fetch and render, causing slower page transitions and higher network load.

📉 Performance Costblocks rendering for 200-500ms per navigation, triggers multiple reflows

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
No client cache	High (new nodes on each page)	Multiple per navigation	High due to full repaint	[X] Bad
With client cache	Low (reuse nodes/data)	Single or none	Low due to partial repaint	[OK] Good

Rendering Pipeline

When navigating, the router checks the client cache before fetching data. If cached, it skips network fetch and layout recalculation is minimized.

→Network Fetch

→Layout

→Paint

→Composite

⚠️ BottleneckNetwork Fetch and Layout

Core Web Vital Affected

LCP, INP

This affects page load speed and interaction responsiveness by reducing network requests and speeding up navigation.

Optimization Tips

1Cache route data on the client to avoid repeated network requests.

2Reuse DOM nodes and data to minimize reflows and repaints.

3Measure navigation performance using browser DevTools to verify caching effectiveness.

Performance Quiz - 3 Questions

What is the main performance benefit of caching routes on the client in Next.js?

A Reduces network requests and speeds up page transitions

B Increases bundle size significantly

C Improves SEO by preloading all pages

D Prevents any JavaScript execution on navigation

DevTools: Performance

How to check: Record a navigation in the Performance panel. Look for network requests and layout events during page transitions.

What to look for: Fewer network requests and shorter layout times indicate effective router caching.

Practice

1. What is the main purpose of router cache on the client in Next.js?

easy

A. To disable client-side navigation completely

B. To speed up navigation by storing visited pages locally

C. To increase server load by fetching pages repeatedly

D. To prevent users from accessing certain pages

Solution

1. Step 1: Understand router cache concept

   Router cache stores pages visited by the user on the client side to avoid reloading them from the server.

2. Step 2: Identify the effect on navigation speed

   By storing pages locally, navigation becomes faster and smoother because the browser doesn't need to fetch the page again.

3. Final Answer:

   To speed up navigation by storing visited pages locally -> Option B

4. Quick Check:

   Router cache = faster navigation [OK]

Hint: Router cache means storing pages locally for faster navigation [OK]

Common Mistakes:

• Thinking router cache increases server load
• Confusing cache with disabling navigation
• Assuming cache blocks page access

2. Which Next.js router method is used to prefetch a page and cache it on the client?

easy

A. router.prefetch()

B. router.load()

C. router.cache()

D. router.fetch()

Solution

1. Step 1: Recall Next.js router methods

   Next.js provides a method called router.prefetch() to load pages in advance.

2. Step 2: Match method to caching behavior

   router.prefetch() loads the page and stores it in the client cache for faster navigation.

3. Final Answer:

   router.prefetch() -> Option A

4. Quick Check:

   Prefetch method caches pages [OK]

Hint: Prefetch means load ahead to cache page [OK]

Common Mistakes:

• Using non-existent methods like router.load()
• Confusing fetch with prefetch
• Assuming cache() is a router method

3. Given this code snippet in a Next.js component:

import { useRouter } from 'next/router';

export default function Home() {
  const router = useRouter();
  router.prefetch('/about');
  return <button onClick={() => router.push('/about')>Go to About</button>;
}

What happens when the user clicks the button?

medium

A. An error occurs because prefetch cannot be called inside the component

B. The /about page reloads from the server every time

C. The button does nothing because prefetch disables navigation

D. The /about page loads instantly from client cache

Solution

1. Step 1: Understand router.prefetch usage

   The code calls router.prefetch('/about') inside the component, which loads and caches the /about page on the client.

2. Step 2: Analyze button click behavior

   When clicking the button, router.push('/about') navigates to the cached page, making the load instant without server fetch.

3. Final Answer:

   The /about page loads instantly from client cache -> Option D

4. Quick Check:

   Prefetch + push = instant load [OK]

Hint: Prefetch before push means instant page load [OK]

Common Mistakes:

• Thinking prefetch disables navigation
• Assuming page reloads from server always
• Believing prefetch causes errors inside components

4. Identify the error in this Next.js code that tries to prefetch a page:

import { useRouter } from 'next/router';

export default function Page() {
  const router = useRouter();

  function handleClick() {
    router.prefetch('/contact')
  }

  return <button onClick={handleClick}>Load Contact</button>;
}

medium

A. router.prefetch must be called outside event handlers

B. Missing semicolon after router.prefetch call causes syntax error

C. No error; code correctly prefetches on button click

D. useRouter cannot be used inside functional components

Solution

1. Step 1: Check router.prefetch usage

   Calling router.prefetch('/contact') inside an event handler is valid and triggers prefetch when the button is clicked.

2. Step 2: Verify syntax and hooks usage

   The code has correct syntax and useRouter is properly used inside the functional component.

3. Final Answer:

   No error; code correctly prefetches on button click -> Option C

4. Quick Check:

   Prefetch in event handler is valid [OK]

Hint: Prefetch can run inside event handlers safely [OK]

Common Mistakes:

• Thinking prefetch must run only on mount
• Assuming missing semicolon breaks code
• Believing useRouter is invalid in functional components

5. You want to improve user experience by preloading multiple pages on client side in Next.js. Which approach correctly prefetches pages for /about and /contact when the homepage loads?

hard

A. Call router.prefetch('/about') and router.prefetch('/contact') inside a useEffect hook with empty dependency array

B. Call router.prefetch('/about') and router.prefetch('/contact') directly in the component body

C. Call router.push('/about') and router.push('/contact') inside useEffect

D. Use getServerSideProps to prefetch pages on the server

Solution

1. Step 1: Understand prefetch timing

   Prefetching pages on client should happen after component mounts to avoid blocking rendering, so useEffect with empty dependencies is ideal.

2. Step 2: Avoid direct calls in component body or pushing pages

   Calling router.prefetch() directly in the component body runs on every render, which is inefficient. Using router.push() navigates instead of prefetching. getServerSideProps runs on server, not client cache.

3. Final Answer:

   Call router.prefetch('/about') and router.prefetch('/contact') inside a useEffect hook with empty dependency array -> Option A

4. Quick Check:

   Prefetch in useEffect = best practice [OK]

Hint: Prefetch pages inside useEffect to run once on mount [OK]

Common Mistakes:

• Prefetching directly in component body causing repeated calls
• Using router.push instead of prefetch
• Trying to prefetch on server side with getServerSideProps