CORS Headers: Fix Cross-Origin Errors
Understand and fix CORS — Access-Control-Allow-Origin, preflight requests, and credentialed requests.
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Tags: CORS headers guide, fix CORS error, Access-Control-Allow-Origin
CORS Headers: Fix Cross-Origin Errors CORS errors are among the most common HTTP errors frontend developers encounter. The error message is clear — "No 'Access-Control-Allow-Origin' header" — but the fix requires understanding why the browser blocks the request and what exactly the server needs to return. This guide explains the full CORS mechanism and how to configure it correctly for every scenario. --- All the tools discussed here are available for free at theproductguy.in — client-side, no sign-up required. Part of the HTTP Debugging Tools Guide — a complete toolkit for diagnosing web requests. --- What is CORS Actually? The browser's Same-Origin Policy (SOP) prevents JavaScript on from reading HTTP responses from . Without this restriction, malicious JavaScript on any site could…
Frequently Asked Questions
What is CORS?
CORS (Cross-Origin Resource Sharing) is a browser security mechanism that restricts JavaScript on one origin from reading responses from a different origin. An origin is the combination of protocol, hostname, and port. CORS is enforced by the browser — the server must explicitly grant permission via response headers for cross-origin reads to succeed.
How do I fix a CORS error?
CORS errors are fixed on the server, not in the browser. Add the correct Access-Control-Allow-Origin header to your server's responses. For public APIs, use Access-Control-Allow-Origin: *. For credentialed requests, use the exact requesting origin and add Access-Control-Allow-Credentials: true. For complex requests, also handle the OPTIONS preflight.
What is a preflight request?
A preflight is an automatic HTTP OPTIONS request the browser sends before certain cross-origin requests — specifically those using non-simple methods (PUT, DELETE, PATCH), non-simple headers (Authorization, Content-Type: application/json), or requests with credentials. The server must respond to the OPTIONS request with the appropriate Access-Control headers before the browser sends the actual request.
How do I enable CORS in Express.js?
Install the cors package: npm install cors. Then add app.use(cors()) for all origins, or app.use(cors({ origin: 'https://yourfrontend.com', credentials: true })) for a specific origin with credentials. For route-specific CORS, pass the cors() middleware to individual routes instead of app.use().
What is the difference between simple and complex CORS requests?
Simple requests use GET, HEAD, or POST with only basic headers (Accept, Content-Type: application/x-www-form-urlencoded or text/plain, Cookie). They don't trigger a preflight. Complex (preflighted) requests use other methods or headers like Authorization or Content-Type: application/json, and trigger an OPTIONS preflight that the server must handle.
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