CORS error from NextJS endpoint error even with "Access-Control-Allow-Origin" header

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05-10-2024

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CORS Error: The "Access-Control-Allow-Origin" Header Dilemma in Next.js

The Problem: You're building a Next.js application and trying to fetch data from an API endpoint hosted on the same domain. Despite setting the Access-Control-Allow-Origin header, you're still encountering a CORS error. This is a frustrating experience, especially when you expect your application to communicate smoothly within the same domain.

The Scenario: Imagine you have a Next.js application running on http://localhost:3000 and an API endpoint hosted on the same server at http://localhost:3000/api/data. You're trying to fetch data from this endpoint using fetch or axios within your Next.js application. You've meticulously set the Access-Control-Allow-Origin header to http://localhost:3000 in your API endpoint, but the browser still throws a CORS error.

The Code: Here's a typical example of setting the Access-Control-Allow-Origin header in a Next.js API route:

// pages/api/data.js
import { NextApiRequest, NextApiResponse } from 'next';

export default async function handler(
  req: NextApiRequest,
  res: NextApiResponse
) {
  // ... your API logic
  res.setHeader('Access-Control-Allow-Origin', 'http://localhost:3000');
  res.status(200).json({ data: 'Your Data' });
}

The Explanation:

The reason this occurs lies in the browser's security mechanism. When you make a request from a different origin (even if it's the same domain), the browser performs a preflight request (OPTIONS request) to check if the server allows the actual request. This preflight request checks the allowed origins, methods, and headers.

Here's the catch: if the preflight request doesn't find the required headers in the response, the actual request will be blocked, resulting in a CORS error.

The Solution:

The issue stems from the fact that your API route is using a different origin than the one specified in the Access-Control-Allow-Origin header. Next.js API routes run on a different port (by default, 3001), leading to an origin mismatch even though they are on the same domain.

To resolve this, you need to:

1. Configure your API route to use the same origin as your Next.js application.
2. Or explicitly set the Access-Control-Allow-Origin header to the correct origin, including the port.

Here's how you can do it:

• Using the next.config.js file:

  module.exports = {
    reactStrictMode: true,
    serverRuntimeConfig: {
      apiBaseUrl: process.env.NODE_ENV === 'production' ? '/api' : 'http://localhost:3000/api' 
    }
  };
  

  This configuration sets the apiBaseUrl to http://localhost:3000/api in development and /api in production. You can access this variable in your API routes using process.env.serverRuntimeConfig.apiBaseUrl.

• Adjusting the Access-Control-Allow-Origin header:

  // pages/api/data.js
  import { NextApiRequest, NextApiResponse } from 'next';
  
  export default async function handler(
    req: NextApiRequest,
    res: NextApiResponse
  ) {
    // ... your API logic
    res.setHeader('Access-Control-Allow-Origin', 'http://localhost:3000');
    res.status(200).json({ data: 'Your Data' });
  }
  

Additional Tips:

• Be mindful of the wildcard (*). While using * for Access-Control-Allow-Origin might seem convenient, it grants access to all origins, which is a security risk. It's best to be specific with the allowed origins.
• Use a CORS library: For larger projects, consider using a dedicated CORS library like cors to handle these configurations more effectively.

References:

• MDN Web Docs: CORS
• Next.js API Routes Documentation

By understanding the origin mismatch and properly configuring your API routes and headers, you can successfully overcome CORS issues and enable seamless communication within your Next.js application.