Next.js getStaticPaths: fallback, blocking, revalidate Explained

• Introduction
• Step-by-Step Guide
• Code Example
• Additional Notes
• Summary
• Conclusion
• References

In Next.js, when you have pages with dynamic routes, you need a way to tell the framework which pages to pre-render at build time. This is where getStaticPaths comes in. The getStaticPaths function is crucial for optimizing dynamic routes in Next.js applications. It allows you to predefine the routes that will be statically generated at build time, resulting in faster page loads and improved SEO. The fallback option within getStaticPaths provides flexibility in handling scenarios where you might not be able to pre-render all possible routes at build time. Let's explore how getStaticPaths and its fallback option work, along with practical examples to illustrate their usage.

Let's break down how getStaticPaths and its fallback option work in Next.js.

Understanding getStaticPaths

In Next.js, you build websites with pre-rendered pages for optimal performance and SEO. When you have pages with dynamic routes (e.g., /products/[id].js), getStaticPaths comes into play. Its job is to tell Next.js in advance which dynamic routes to generate at build time.

Example: Product Pages

Imagine you have an e-commerce site with product pages. Here's how you'd use getStaticPaths:

// pages/products/[id].js
export async function getStaticPaths() {
  // Fetch a list of product IDs from your database or API
  const products = await fetch('https://api.example.com/products').then(res => res.json());

  // Map product IDs to paths
  const paths = products.map(product => ({
    params: { id: product.id.toString() } 
  }));

  return { paths, fallback: false }; 
}

export async function getStaticProps({ params }) {
  // Fetch data for a single product based on the 'id' param
  const product = await fetch(`https://api.example.com/products/${params.id}`).then(res => res.json());

  return { props: { product } };
}

// ... rest of your component to display product details

Explanation

1. getStaticPaths Function: This function is required when using getStaticProps with dynamic routes.
2. Fetching Data: Inside getStaticPaths, you fetch data (e.g., product IDs) that determine your dynamic routes.
3. Creating paths: You transform the fetched data into an array of objects. Each object represents a route with its params.
4. Returning paths and fallback: You return an object with the paths array and a fallback option.

The fallback Option

The fallback option controls how Next.js handles requests to paths that weren't pre-rendered at build time:

• fallback: false (Default):

  • Behavior: Next.js generates the specified paths at build time. Any requests to paths not listed in paths will result in a 404 error.
  • Use Case: When you know all possible routes in advance (e.g., a limited set of product pages).

• fallback: true:

  • Behavior: Next.js statically generates the paths specified in getStaticPaths.
    • For paths that are not generated at build time, Next.js will serve a fallback page on the first request.
    • In the background, Next.js will statically generate the page. Once complete, the page will be cached and served for all subsequent requests.
  • Use Case: Ideal when you have a large number of pages, and you want to avoid generating them all at build time. It provides a good balance between performance and SEO.

• fallback: 'blocking':

  • Behavior: Similar to fallback: true, but instead of showing a fallback, Next.js will wait until the page is fully generated on the server before responding. This results in a slightly slower first load but ensures users always see the most up-to-date content.
  • Use Case: Useful when you need to ensure that users always see the latest data, even on the first request.

Important Considerations

• Caching and Revalidation: By default, statically generated pages are cached. To update content, you can use Incremental Static Regeneration (ISR) by adding a revalidate property to getStaticProps.
• App Router: If you're using the new Next.js App Router, the concepts are similar, but the implementation might differ slightly. Refer to the App Router documentation for specifics.

Let me know if you'd like a deeper dive into a specific fallback option or have any other Next.js questions!

This Next.js code defines a dynamic product page component. It fetches product IDs from an API to pre-render pages at build time. The fallback option determines the behavior for pages not pre-rendered. It fetches individual product data based on the URL parameter and displays a loading state while generating the page if fallback is enabled. The code includes comments for revalidation and instructions for testing different fallback behaviors.

// pages/products/[id].js
import { useRouter } from 'next/router';

export async function getStaticPaths() {
  // Example: Fetching product IDs from an API
  const res = await fetch('https://api.example.com/products');
  const products = await res.json();

  const paths = products.map((product) => ({
    params: { id: product.id.toString() },
  }));

  return {
    paths,
    // Choose a 'fallback' option:
    // fallback: false, // 404 for non-pre-rendered paths
    fallback: true, // Fallback page, then static generation
    // fallback: 'blocking', // Wait for static generation
  };
}

export async function getStaticProps({ params }) {
  const res = await fetch(`https://api.example.com/products/${params.id}`);
  const product = await res.json();

  return {
    props: { product },
    // If using fallback: true or 'blocking', consider revalidate
    // revalidate: 60, // Regenerate page every 60 seconds
  };
}

export default function Product({ product }) {
  const router = useRouter();

  // Handle the fallback state (if applicable)
  if (router.isFallback) {
    return <div>Loading...</div>;
  }

  return (
    <div>
      <h1>{product.name}</h1>
      <p>{product.description}</p>
    </div>
  );
}

Explanation:

1. getStaticPaths:

   • Fetches product IDs from an API.
   • Maps IDs to paths for pre-rendering.
   • Sets fallback to demonstrate different behaviors.

2. getStaticProps:

   • Fetches product data based on the id parameter.
   • Includes revalidate (commented out) for ISR if needed.

3. Product Component:

   • Uses router.isFallback to display a loading state while the page is statically generated (if fallback: true or 'blocking').
   • Renders product details once available.

To Test:

1. Run npm run build to build your Next.js project.
2. Start your development server: npm run dev or npm start.
3. Try accessing different product URLs:
   • URLs matching paths in getStaticPaths will load directly.
   • URLs not in paths will demonstrate the chosen fallback behavior.

Remember to replace the example API endpoints and data with your own. This code provides a practical starting point for understanding and implementing getStaticPaths and fallback in your Next.js applications.

General:

• Purpose: The primary goal of getStaticPaths is to improve performance and SEO by pre-rendering pages at build time. This is especially beneficial for pages with dynamic routes that would otherwise require server-side rendering on every request.
• Trade-offs: While pre-rendering offers performance benefits, it comes with a trade-off. You need to balance pre-rendering all possible paths (potentially time-consuming for large datasets) with providing a good user experience for paths not pre-rendered. The fallback option helps you manage this trade-off.

getStaticPaths:

• Required When: This function is mandatory when you're using getStaticProps to fetch data for a page with dynamic routes.
• Data Source: The data source for generating paths can be anything: your database, a headless CMS, a local JSON file, or an external API.
• Error Handling: If getStaticPaths throws an error, your build process will fail. Implement robust error handling to prevent this.

fallback Options:

• fallback: false:
  • Best for: Small, well-defined datasets where you know all possible routes at build time.
  • SEO Impact: Good for SEO as all pages are pre-rendered and indexed.
  • User Experience: Can lead to a poor user experience if a requested path is not found (results in a 404).
• fallback: true:
  • Best for: Large datasets where pre-rendering all paths is impractical or time-consuming.
  • SEO Impact: Good for SEO as most pages are pre-rendered. Search engines might initially see the fallback page but will eventually index the fully generated page.
  • User Experience: Provides a better user experience than fallback: false as users see a loading state while the page is being generated.
• fallback: 'blocking':
  • Best for: Situations where you need to ensure users always see the most up-to-date data, even on the first request, and a slight delay is acceptable.
  • SEO Impact: Similar to fallback: true, but search engines might treat the initial request as if the page doesn't exist until it's fully generated.
  • User Experience: The user experience might be slightly slower on the first load compared to fallback: true as the server waits for the page to be generated before responding.

Additional Tips:

• Incremental Static Regeneration (ISR): Use ISR with revalidate in getStaticProps to update static pages after they've been generated. This helps keep your content fresh without rebuilding the entire site.
• Caching: Leverage caching mechanisms (like a CDN) to further improve the performance of your statically generated pages.
• App Router Considerations: The concepts of getStaticPaths and fallback apply similarly to the Next.js App Router, but the implementation might differ. Refer to the App Router documentation for specific guidance.

This table summarizes the key aspects of getStaticPaths and its fallback option in Next.js:

This table provides a concise overview of getStaticPaths and fallback. For more detailed information, refer to the Next.js documentation.

getStaticPaths is a critical function in Next.js for pre-rendering dynamic routes at build time, leading to improved performance and SEO. By defining the paths to be statically generated and utilizing the fallback option to handle routes not pre-rendered, you can fine-tune your application's behavior and user experience. Understanding the different fallback options (false, true, and 'blocking') empowers you to make informed decisions based on your project's specific requirements, whether you need to prioritize fast initial page loads, handle large datasets efficiently, or ensure the most up-to-date content is displayed to users. By mastering getStaticPaths and its fallback option, you can leverage the full power of Next.js to build high-performance, SEO-friendly web applications.

• Functions: getStaticPaths | Next.js | API reference for getStaticPaths. Learn how to fetch data and generate static pages with getStaticPaths.
• wordpress - How to clear/delete cache in NextJs? - Stack Overflow | Jul 19, 2021 ... What is the difference between fallback false vs true vs blocking of getStaticPaths with and without revalidate in Next.js SSR/ISR? 6 · Next.
• Data Fetching: Incremental Static Regeneration (ISR) | Next.js | Learn how to create or update static pages at runtime with Incremental Static Regeneration.
• next.js - Should I use fallback: true or fallback: false in this scenario ... | Nov 22, 2022 ... The Next.js docs state. Web crawlers, such as Google, won't be served a fallback and instead the path will behave as in fallback: 'blocking'.
• Upgrading: App Router Migration | Next.js | Learn how to upgrade your existing Next.js application from the Pages Router to the App Router.
• Next.js ISR not working - Support - Netlify Support Forums | team: iamsterdam website: https://iamsterdam-redis-cache.netlify.app I want to move to use Nextjs’ Incremental Static Regeneration instead of SSR for most of my pages (loaded from a CMS), but I cannot get it to work. I have the following file structure set up: the file that’s using getStaticProps combined with getStaticPaths is located at pages/static-pages/[...slug]/index.js. getStaticProps correctly returns a revalidate property (set to 300 so it should be cached 5 minutes) and getStaticPat...
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• [Bug]: Fallback false is ignored · Issue #1179 · netlify/next-runtime ... | Summary Netlify triggers a new ODB function instead of returning the expected 404 when using fallback: false. See: https://github.com/netlify/netlify-plugin-nextjs/blob/6fd7bcc99aacf447559de46f60de...
• Data Fetching: getStaticPaths | Next.js | Fetch data and generate static pages with getStaticPaths. Learn more about this API for data fetching in Next.js.