How To Fix Cross-Origin Request Blocked Errors

Cross-Origin Resource Sharing (CORS) errors occur when a web page tries to request resources from a different domain that does not allow access. These errors are common in modern web development when working with APIs, external services, or different subdomains.

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1. Understand the Problem

• Browsers enforce the same-origin policy for security.
• A cross-origin request is blocked if the server does not send proper CORS headers.

Example error in the console:

Access to fetch at 'https://api.example.com/data' from origin 'https://mysite.com' has been blocked by CORS policy

• Causes:
  • Server missing Access-Control-Allow-Origin header
  • Using incorrect HTTP methods (GET, POST, etc.)
  • Attempting to send credentials without proper headers

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2. Configure Server-Side CORS

• On the server, set headers to allow requests:

Access-Control-Allow-Origin: *
Access-Control-Allow-Methods: GET, POST, PUT, DELETE
Access-Control-Allow-Headers: Content-Type

• For Node.js with Express:

const express = require('express');
const cors = require('cors');
const app = express();

app.use(cors()); // allows all origins

• Ensure the server configuration matches the type of request.

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3. Use Correct HTTP Methods

• Some APIs restrict certain methods:

fetch('https://api.example.com/data', { method: 'POST' });

• Make sure the server supports the requested method. Otherwise, you may need to adjust the server or use the allowed method.

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4. Handle Credentials Properly

• Sending cookies or authentication headers requires additional configuration:

fetch('https://api.example.com/data', {
  credentials: 'include'
});

• The server must respond with:

Access-Control-Allow-Credentials: true

• Without proper headers, requests with credentials are blocked.

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5. Use a Proxy as a Workaround

• During development, you can use a proxy to bypass CORS restrictions:

fetch('https://my-proxy-server.com/https://api.example.com/data')

• Only use this for development; production servers should be configured correctly.

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6. Debugging Tips

• Check browser console for the exact CORS error.
• Verify server headers using browser DevTools → Network tab.
• Test API requests with Postman to ensure server allows cross-origin access.
• Avoid modifying browser security settings in production.

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7. Best Practices Summary

1. Configure server with proper CORS headers.
2. Ensure HTTP methods match server configuration.
3. Handle credentials with care and correct headers.
4. Use a proxy for development if necessary.
5. Debug using browser DevTools and Postman.

By applying these best practices, cross-origin request errors can be prevented, ensuring smooth communication between your frontend and external APIs.

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Citations

Internal: https://savanka.com/category/savanka-helps/
External: https://developer.mozilla.org/en-US/docs/Web/JavaScript