Resolving Next.js Error: Only Absolute URLs Are Supported

The “only absolute URLs are supported” error in Next.js typically occurs when developers use relative URLs in contexts like data fetching, API routes, or redirects. This error arises because Next.js requires the full URL, including the protocol and domain, to properly locate resources. Understanding and resolving this error is crucial for developers to ensure their applications function correctly and securely.

Understanding the Error

The error message “only absolute URLs are supported” in Next.js typically occurs when you use a relative URL in a context where an absolute URL is required, such as in API requests or redirects.

Absolute URLs

An absolute URL provides the complete address of a resource on the internet. It includes:

Protocol: Specifies the communication protocol (e.g., http, https).
Domain Name: The web address (e.g., example.com).
Path: The specific location of a resource on the server (e.g., /path/to/resource).

For example, https://example.com/path/to/resource is an absolute URL. It can be used from any location to access the specified resource.

Relative URLs

A relative URL specifies a resource’s location relative to the current page’s URL. It does not include the protocol or domain name, only the path.

For example, if the current page is https://example.com/folder/page.html, a relative URL like ../image.jpg points to https://example.com/image.jpg.

Key Differences

Scope: Absolute URLs are independent of the current page’s location, while relative URLs depend on the current page’s URL.
Usage: Absolute URLs are necessary for cross-domain requests and certain contexts in web development, such as API calls in Next.js. Relative URLs are useful for linking resources within the same domain.

In Next.js, using absolute URLs ensures that the application can correctly locate and fetch resources, regardless of the current page’s location.

Common Scenarios

Here are common scenarios where developers might encounter the “Next.js error: only absolute URLs are supported”:

API Calls:

// Incorrect
const res = await fetch('/api/data');

// Correct
const res = await fetch('https://example.com/api/data');

Redirects:

// Incorrect
res.redirect('/home');

// Correct
res.redirect('https://example.com/home');

Data Fetching:

// Incorrect
const data = await fetch('/products');

// Correct
const data = await fetch('https://example.com/products');

Middleware:

// Incorrect
NextResponse.redirect('/login');

// Correct
NextResponse.redirect('https://example.com/login');

Image Optimization:

// Incorrect
<Image src="/images/pic.jpg" alt="Picture" />

// Correct
<Image src="https://example.com/images/pic.jpg" alt="Picture" />

Static Site Generation (SSG):

// Incorrect
const res = await fetch('/api/posts');

// Correct
const res = await fetch('https://example.com/api/posts');

Server-Side Rendering (SSR):

// Incorrect
const res = await fetch('/api/user');

// Correct
const res = await fetch('https://example.com/api/user');

These examples illustrate the importance of using absolute URLs in various Next.js contexts to avoid this common error.

Troubleshooting Steps

Sure, here are the steps to troubleshoot and resolve the “Next.js error: only absolute URLs are supported”:

Identify the Error Source:
- Check your code for any fetch or API calls. The error typically occurs when using relative URLs (e.g., /api/data) instead of absolute URLs (e.g., https://example.com/api/data).
Locate the Problematic Code:
- Look for fetch calls, axios requests, or any other HTTP requests in your Next.js project. These are often found in API routes, getServerSideProps, getStaticProps, or client-side components.
Update to Absolute URLs:
- Replace relative URLs with absolute URLs. Ensure the URL includes the protocol (http or https) and the domain name.
```
// Incorrect
const res = await fetch('/api/data');

// Correct
const res = await fetch('https://example.com/api/data');
```
Use Environment Variables:
- To avoid hardcoding URLs, use environment variables. Create a .env.local file in your project root and define your base URL.
```
NEXT_PUBLIC_API_BASE_URL=https://example.com
```
- Update your fetch calls to use this environment variable.
```
const res = await fetch(`${process.env.NEXT_PUBLIC_API_BASE_URL}/api/data`);
```

Check Middleware and Redirects:

If you are using middleware or redirects, ensure they also use absolute URLs. For example, in middleware.ts:

import { NextResponse } from 'next/server';

export function middleware(req) {
  const url = req.nextUrl.clone();
  url.pathname = '/new-path';
  return NextResponse.redirect(url);
}

Test Your Changes:
- After making these changes, test your application to ensure the error is resolved. Check both server-side and client-side code paths.
Review Documentation:
- Refer to the Next.js documentation for more details on handling URLs in middleware and other advanced scenarios.

By following these steps, you should be able to identify and fix the “only absolute URLs are supported” error in your Next.js application.

Best Practices

To avoid the “only absolute URLs are supported” error in Next.js, follow these best practices:

Use Absolute URLs in Fetch Requests: Always include the full URL with the protocol (e.g., https://example.com/api/data). Avoid relative URLs like /api/data.
Environment Variables: Store your base URL in environment variables. This helps manage different environments (development, staging, production) easily.
```
const baseUrl = process.env.NEXT_PUBLIC_API_BASE_URL;
const response = await fetch(`${baseUrl}/api/data`);
```

Utility Functions: Create a utility function to construct absolute URLs. This ensures consistency across your project.

const getAbsoluteUrl = (path) => `${process.env.NEXT_PUBLIC_API_BASE_URL}${path}`;
const response = await fetch(getAbsoluteUrl('/api/data'));

Next.js Configuration: Use the next.config.js file to define your base URL and other configurations.

module.exports = {
  env: {
    NEXT_PUBLIC_API_BASE_URL: 'https://example.com',
  },
};

SSR and CSR Considerations: Ensure that your URLs are absolute in both server-side rendering (SSR) and client-side rendering (CSR) contexts. Use the same environment variables and utility functions to maintain consistency.
Custom Server Setup: If using a custom server, ensure that all routes and redirects use absolute URLs.

By consistently applying these practices, you can avoid the “only absolute URLs are supported” error and maintain a robust Next.js project.

To Resolve the ‘Only Absolute URLs Are Supported’ Error in Next.js

To resolve the “only absolute URLs are supported” error in Next.js, it’s essential to understand its causes and implement best practices.

Always use absolute URLs with protocols (e.g., https://example.com/api/data) instead of relative URLs like /api/data.
Store your base URL in environment variables to manage different environments easily and maintain consistency across your project.
Create utility functions to construct absolute URLs, ensuring that you’re using the correct protocol and domain for each request.
Use Next.js configuration files (next.config.js) to define your base URL and other configurations, making it easier to manage and update your application’s settings.
Consider server-side rendering (SSR) and client-side rendering (CSR) contexts when working with URLs in Next.js. Ensure that all routes and redirects use absolute URLs to maintain consistency across both environments.
If using a custom server, ensure that all routes and redirects use absolute URLs.