How to Resolve the CORS Error: "Response to preflight request doesn't pass access control check" in JavaScript

The error Response to preflight request doesn't pass access control check is a common and often frustrating issue in web development. It is a CORS (Cross-Origin Resource Sharing) error, and it is a security measure enforced by web browsers. It means that your backend server did not give your frontend web application permission to make the request it was trying to make.

This guide will explain what a "preflight request" is, why this error happens, and show you how to fix it by configuring the correct CORS headers on your server.

The Core Problem: The Same-Origin Policy and Preflight Requests

For security reasons, browsers enforce the Same-Origin Policy, which restricts a web page from making requests to a different domain (or "origin") than the one that served the page.

However, modern web apps often need to make cross-origin requests (e.g., a frontend at https://example.com calling an API at https://api.example.com). To do this safely, the browser performs a "preflight request."

What is a Preflight Request?

Before sending a "complex" cross-origin request (like a POST with a Content-Type of application/json, or any request with custom headers), the browser first sends a separate, preliminary request using the OPTIONS HTTP method. This OPTIONS request is the preflight request.

It essentially asks the server: "Hey, I'm a web app from https://example.com. Am I allowed to make a POST request to /data with a Content-Type header?"

The error occurs when the server's response to this OPTIONS request does not include the correct CORS headers that grant this permission. The browser sees the lack of permission and blocks the actual POST request from ever being sent.

The Solution: Configuring CORS Headers on the Server

To fix this error, you must configure your backend server to respond to the OPTIONS preflight request with the appropriate Access-Control-* headers.

The Most Important Headers:

• Access-Control-Allow-Origin: This is the most critical header. It specifies which origins are allowed to access the resource.

  Access-Control-Allow-Origin: https://example.com

  For development, you can use a wildcard (*), but this is insecure for production:

  Access-Control-Allow-Origin: * 

• Access-Control-Allow-Methods: This specifies which HTTP methods (e.g., GET, POST, DELETE) are allowed for the cross-origin request.

  Access-Control-Allow-Methods: GET, POST, PUT, DELETE, OPTIONS

• Access-Control-Allow-Headers: This specifies which HTTP headers can be used in the actual request.

  Access-Control-Allow-Headers: Content-Type, Authorization, X-Requested-With

A Practical Example (Node.js/Express)

Most modern backend frameworks have libraries or middleware that make it easy to configure CORS. For a Node.js server using the Express framework, the cors middleware is the standard solution.

Problem: your Express API is not configured to handle CORS requests from your React frontend running on http://localhost:3000.

Solution:

1. Install the cors package:

   npm install cors

2. Use the middleware in your server:

   server.js:

   const express = require('express');
   const cors = require('cors');       // Import the cors middleware
   const app = express();
   
   // Configure CORS
   const corsOptions = {
     origin: 'http://localhost:3000',  // Allow only your frontend origin
     methods: ['GET', 'POST', 'PUT', 'DELETE'],
     allowedHeaders: ['Content-Type', 'Authorization'],
   };
   
   // Enable CORS for all routes
   app.use(cors(corsOptions));
   
   // Your API routes
   app.get('/api/data', (req, res) => {
     res.json({ message: 'Hello from the API!' });
   });
   
   app.listen(8000, () => {
     console.log('Server is running on port 8000');
   });

note

This server will now correctly respond to OPTIONS preflight requests from your frontend, allowing the actual API calls to proceed.

How to Debug the Error

The Network tab in your browser's developer tools is your best friend for debugging CORS issues.

1. Open the Network tab (F12).
2. Trigger the API request in your app.
3. Look for the OPTIONS request. It will likely have a red status (e.g., 404, 405, or a CORS error).
4. Click on this request and inspect the Response Headers. You will see that the required Access-Control-Allow-Origin header is missing or incorrect.

What if I Don't Control the Server?

If you are trying to make a request to a third-party API that you do not control and it is not configured for public CORS access, you cannot fix this error in your frontend code. The browser will always block the request.

The solution in this case is to set up a proxy server. Your frontend makes a same-origin request to your own backend, which then makes the request to the third-party API. Since server-to-server requests do not have CORS restrictions, this works perfectly.

Conclusion

The "Response to preflight request doesn't pass access control check" error is a server-side configuration issue, not a client-side code bug.

• The error is caused by the browser's Same-Origin Policy and the CORS preflight request mechanism.
• To solve it, you must configure your backend server to send the correct Access-Control-Allow-Origin, Access-Control-Allow-Methods, and Access-Control-Allow-Headers in response to OPTIONS requests.
• Most backend frameworks have standard middleware (like the cors package for Express) to handle this easily.