Common Errors in Next.js and How to Fix Them

Common Errors in Next.js and How to Fix Them

Next.js is a popular React framework that simplifies building and deploying server-rendered applications. While it provides powerful tools, developers often encounter errors, especially when integrating newer features or third-party tools. This article will cover some of the most common errors in Next.js, why they happen, and how to resolve them.

1. Unhandled Rejection Due to Misconfigured Data Fetching

A frequent error in Next.js occurs during data fetching. Specifically, using client-only code in server-side rendering (SSR) methods like getServerSideProps can throw unhandled rejection errors.

Solution: Make sure your data-fetching logic does not include client-only code. The following example shows a correct implementation of getServerSideProps:

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

export default function ExamplePage({ data }) {
  return <div>{data.title}</div>;
}

In this setup, data is fetched on the server side without using any client-specific code (e.g., window or document), which prevents unhandled rejections.

2. 404 Error on Dynamic Routes

Dynamic routing is one of the most powerful features of Next.js, but it can lead to 404 errors if routes are misconfigured. This often occurs when developers fail to match the file path to the intended URL structure.

Solution: Ensure that the filename follows the [param].js convention, as shown below:

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

export default function BlogPost() {
  const router = useRouter();
  const { id } = router.query;

  return <h1>Blog Post: {id}</h1>;
}

Here, the URL /blog/123 will render the component and display Blog Post: 123. This error is also common when forgetting to include a route definition for the catch-all route, using [...param].js to capture multiple dynamic segments.

3. CSS Module or Styled-Component Conflicts

Next.js supports CSS Modules and styled-components, but conflicts arise when both are improperly configured. This often results in styling issues where CSS classes don’t apply correctly.

Solution: Choose a single styling approach or correctly configure both. When using styled-components, add a custom Babel configuration for SSR compatibility:

// .babelrc
{
  "presets": ["next/babel"],
  "plugins": [["styled-components", { "ssr": true }]]
}

This configuration ensures styled-components render correctly on the server, fixing issues with FOUC (Flash of Unstyled Content).

4. Next Router Errors

If you’re migrating from React Router or using the Next.js router incorrectly, you may encounter errors. The Next.js router, useRouter, is designed for server-rendered applications and is different from React Router.

Solution: Always use useRouter for Next.js projects. Here’s an example:

// pages/index.js
import { useRouter } from 'next/router';

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

  const navigate = () => {
    router.push('/about');
  };

  return <button onClick={navigate}>Go to About Page</button>;
}

export default HomePage;

Using router.push for navigation will eliminate most router-related errors and improve compatibility with Next.js’s features.

5. API Route Misconfigurations

Next.js API routes allow developers to build backend functionality directly within the app. Errors often arise when using improper HTTP methods or not correctly setting response headers.

Solution: Here’s a correct example of an API route with a POST method:

// pages/api/submit.js
export default function handler(req, res) {
  if (req.method === 'POST') {
    // Handle POST request
    res.status(200).json({ message: 'Success' });
  } else {
    res.setHeader('Allow', ['POST']);
    res.status(405).end(`Method ${req.method} Not Allowed`);
  }
}

This checks the request method and prevents method errors, which can lead to 405 responses.

Conclusion

Next.js errors often stem from misconfigured routes, data-fetching mistakes, or CSS issues. Following best practices, like correctly defining dynamic routes and using useRouter for navigation, will help prevent these common pitfalls. With these solutions, you’ll be able to build more robust Next.js applications.