[Next.js] Resolving Undefined Value Issue in Next.js SSR with Middleware Prefetching

Introduction

In this article, we will discuss a problem encountered while using Next.js for server-side rendering (SSR) where the user's language setting was returned as undefined. This issue was specific to Next.js, particularly related to middleware prefetching behavior. We will explain the cause of the problem, provide a detailed solution, and share relevant code examples to help you resolve similar issues in your Next.js projects.

Understanding the Problem

What Caused the Undefined Value?

The issue was caused by how Next.js handles link prefetching when middleware is used.

Starting from Next.js version 13, the framework began prefetching not just JavaScript files but also JSON data via the _data route for pages.

When middleware was added to the project, this prefetching sometimes resulted in empty JSON responses ({}). These empty responses led to critical data, such as the user's language preference, being undefined during page transitions.

Why Does This Happen?

When Next.js prefetches a page, it attempts to cache the response for faster navigation.

However, with middleware in place, these prefetch requests could cache invalid or empty data. If this cached empty response is used during an actual page load, it causes issues like missing or undefined values in the page's props.

Solution: How to Fix the Undefined Language Issue in Next.js

Step 1: Configure Middleware Prefetching

The first step to resolving this issue is to adjust the prefetching behavior in Next.js.

By setting the middlewarePrefetch option to 'strict' in your next.config.js file, you can prevent Next.js from caching and using invalid responses.

example code:

// next.config.js
module.exports = {
  experimental: {
    middlewarePrefetch: 'strict',
  },
};

This configuration ensures that only valid and complete responses are cached during prefetching, reducing the chances of encountering undefined values in your props.

 

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Step 2: Upgrade to the Latest Version of Next.js

Upgrading your Next.js version to 14.1.1-canary.37 or later is the most effective way to fix this issue permanently.

This version includes fixes that ensure correct handling of 404 responses and other edge cases during prefetching.

To upgrade, use the following commands:

For npm:

npm install next@canary

For Yarn:

yarn add next@canary

This upgrade addresses the root cause of the problem, ensuring that your application handles prefetching correctly even without the strict middleware setting.

Example Code: Implementing the Fix

Let’s look at an example where this issue might occur.

Suppose you have a page that uses getServerSideProps to fetch user-specific data, such as their language preference:

export async function getServerSideProps(context) {
  const userLanguage = context.req.headers['accept-language'] || 'en';
  return {
    props: {
      language: userLanguage,
    },
  };
}

const HomePage = ({ language }) => {
  return (
    <div>
      <h1>Welcome!</h1>
      <p>Your language preference is: {language}</p>
    </div>
  );
};

export default HomePage;

Without the strict prefetch setting, the language prop might incorrectly return undefined during some page navigations.

By applying the middlewarePrefetch: 'strict' setting and upgrading Next.js, you can ensure that the correct language is always displayed.

Conclusion

By following the steps outlined above, you can effectively resolve issues with undefined data in your Next.js SSR applications. Adjusting the middleware prefetch configuration and upgrading to the latest Next.js version are crucial steps in maintaining a reliable and robust application.

For more in-depth information and discussions, refer to the following resources:

• Next.js GitHub Discussion on Prefetching Empty Data
• Next.js Pull Request #60968
• Next.js Issue #52515

Implementing these solutions will not only fix the immediate problem but also improve the overall stability and performance of your Next.js application.