ISR (Incremental Static Regeneration) in NextJS

Introduction

ISR lets your Next.js site update static pages after you build it, without rebuilding the whole site. This keeps pages fast and fresh.

You want fast pages that update with new data every few seconds or minutes.

Your site has many pages but only some change often.

You want to avoid rebuilding the entire site for small content updates.

You want to serve static pages but still show recent info like blog posts or products.

Syntax

NextJS

export async function getStaticProps() {
  // fetch data here
  const data = {}; // example placeholder
  return {
    props: { data },
    revalidate: 10, // seconds
  };
}

The revalidate key tells Next.js how often to update the page.

When a user visits after revalidate seconds, Next.js regenerates the page in the background.

Examples

This example fetches posts and updates the page every 60 seconds.

NextJS

export async function getStaticProps() {
  const data = await fetch('https://api.example.com/posts').then(res => res.json());
  return {
    props: { posts: data },
    revalidate: 60, // update every 60 seconds
  };
}

This example returns a simple message and regenerates the page every 5 seconds.

NextJS

export async function getStaticProps() {
  return {
    props: { message: 'Hello!' },
    revalidate: 5, // update every 5 seconds
  };
}

Sample Program

This Next.js page shows the current time when the page was generated. It updates every 10 seconds using ISR.

NextJS

import React from 'react';

export default function TimePage({ time }) {
  return (
    <main>
      <h1>Current Time</h1>
      <p>The page was generated at: {time}</p>
    </main>
  );
}

export async function getStaticProps() {
  const time = new Date().toLocaleTimeString();
  return {
    props: { time },
    revalidate: 10, // regenerate page every 10 seconds
  };
}

Important Notes

ISR helps balance speed and freshness by updating pages in the background.

If revalidate is not set, the page is only generated once at build time.

ISR works only with getStaticProps, not with server-side rendering.

Summary

ISR updates static pages after build without full rebuilds.

Use revalidate in getStaticProps to set update frequency.

It keeps pages fast and content fresh automatically.

Practice

1. What is the main purpose of Incremental Static Regeneration (ISR) in Next.js?

easy

A. To disable static generation completely

B. To generate pages only on the client side

C. To update static pages after build without rebuilding the entire site

D. To force server-side rendering on every request

Solution

1. Step 1: Understand ISR concept

   ISR allows static pages to be updated after the initial build without rebuilding the whole site.

2. Step 2: Compare options

   Options B, C, and D describe client-side rendering, disabling static generation, or forcing server-side rendering, which are not ISR features.

3. Final Answer:

   To update static pages after build without rebuilding the entire site -> Option C

4. Quick Check:

   ISR updates static pages incrementally = A [OK]

Hint: ISR updates static pages without full rebuilds [OK]

Common Mistakes:

• Confusing ISR with client-side rendering
• Thinking ISR disables static generation
• Believing ISR forces server-side rendering

2. Which Next.js function should you use to enable ISR by specifying a revalidation time?

easy

A. getServerSideProps

B. getStaticProps

C. getInitialProps

D. useEffect

Solution

1. Step 1: Identify ISR enabling function

   ISR is enabled by returning a revalidate property inside getStaticProps.

2. Step 2: Check other options

   getServerSideProps is for server-side rendering, getInitialProps is legacy, and useEffect is a React hook for client-side effects.

3. Final Answer:

   getStaticProps -> Option B

4. Quick Check:

   ISR uses getStaticProps with revalidate = D [OK]

Hint: Use getStaticProps with revalidate for ISR [OK]

Common Mistakes:

• Using getServerSideProps instead of getStaticProps
• Confusing client-side hooks with data fetching
• Using deprecated getInitialProps for ISR

3. Given this code snippet, what will be the behavior of the page?

export async function getStaticProps() {
  return {
    props: { time: Date.now() },
    revalidate: 10,
  };
}

medium

A. The page throws a syntax error

B. The page never updates after build

C. The page updates on every request

D. The page updates every 10 seconds with new time

Solution

1. Step 1: Analyze revalidate property

   The revalidate: 10 means Next.js will regenerate the page at most every 10 seconds.

2. Step 2: Understand page update behavior

   The page will serve the static content initially, then update the static page in the background every 10 seconds.

3. Final Answer:

   The page updates every 10 seconds with new time -> Option D

4. Quick Check:

   revalidate 10 means update every 10 seconds = A [OK]

Hint: revalidate sets update interval in seconds [OK]

Common Mistakes:

• Thinking page never updates after build
• Confusing ISR with server-side rendering
• Assuming syntax error due to revalidate

4. Identify the error in this ISR setup:

export async function getStaticProps() {
  return {
    props: { data: 'Hello' },
    revalidate: '60',
  };
}

medium

A. revalidate must be a number, not a string

B. props must be a function, not an object

C. getStaticProps cannot be async

D. Missing fallback property

Solution

1. Step 1: Check revalidate type

   The revalidate property must be a number representing seconds, not a string.

2. Step 2: Verify other parts

   props is correctly an object, getStaticProps can be async, and fallback is unrelated here.

3. Final Answer:

   revalidate must be a number, not a string -> Option A

4. Quick Check:

   revalidate type must be number = C [OK]

Hint: revalidate must be numeric seconds, not string [OK]

Common Mistakes:

• Passing revalidate as string instead of number
• Thinking getStaticProps can't be async
• Confusing fallback with revalidate

5. You want a blog page to update its static content every 5 minutes but also show a fallback loading page on first visit to new posts. Which ISR setup is correct?

hard

A. Use getStaticProps with revalidate: 300 and fallback: 'blocking' in getStaticPaths

B. Use getServerSideProps with revalidate: 300

C. Use getStaticProps with revalidate: '300' and no fallback

D. Use getStaticPaths with fallback: false and no revalidate

Solution

1. Step 1: Understand ISR with fallback

   To update static pages every 5 minutes (300 seconds) and show fallback loading for new pages, use revalidate: 300 in getStaticProps and fallback: 'blocking' in getStaticPaths.

2. Step 2: Check other options

   Use getServerSideProps with revalidate: 300 uses server-side rendering, not ISR. Use getStaticProps with revalidate: '300' and no fallback has revalidate as string and no fallback. Use getStaticPaths with fallback: false and no revalidate disables fallback and revalidate, so no ISR updates or loading fallback.

3. Final Answer:

   Use getStaticProps with revalidate: 300 and fallback: 'blocking' in getStaticPaths -> Option A

4. Quick Check:

   ISR with fallback blocking and revalidate 300 = B [OK]

Hint: Combine revalidate with fallback: 'blocking' for ISR + loading [OK]

Common Mistakes:

• Using server-side rendering instead of ISR
• Passing revalidate as string
• Setting fallback to false disables loading fallback