Why Does My Fetch Request Fail with a 'Cross-Origin Request Blocked' Error, and How Can I Fix It Using CORS?

Cross-Origin Request Blocked: Understanding CORS and Fetch Syntax

In the realm of cross-origin requests, where browsers prevent scripts from accessing resources from different origins for security reasons, developers often encounter the dreaded "No 'Access-Control-Allow-Origin' header is present on the requested resource" error. To resolve this issue, it's essential to comprehend the concept of CORS (Cross-Origin Resource Sharing) and its implications for our Fetch syntax.

The CORS Conundrum

CORS is a browser-enforced mechanism that safeguards users against malicious code running on other websites from accessing sensitive information stored locally. By default, browsers prevent cross-origin requests from JavaScript code, but they provide a way to relax this restriction by adding an Access-Control-Allow-Origin header to the response from the server. This header specifies which origins are allowed to access the resource.

Decoding the Syntax Error

In the specified code snippet, the developer attempts to use the mode: 'no-cors' attribute in the Fetch object to disable CORS. However, this approach is fundamentally flawed because mode: 'no-cors' effectively instructs the browser to block any access to the response headers and body. Consequently, even if the server were to send a response with an appropriate Access-Control-Allow-Origin header, it would be disregarded by the browser, leading to the syntax error in the fetch call.

The Pitfalls of mode: 'no-cors'

Using mode: 'no-cors' is generally not recommended, as it can create unexpected limitations in the browser's handling of the response. Specifically, this mode blocks the browser from revealing the response's contents and headers, which is often necessary for proper handling of the data in JavaScript code.

The Proxy Solution

To circumvent CORS restrictions without compromising browser security, we can employ a CORS proxy. A proxy acts as an intermediary, making the request cross-origin on behalf of the client and adding the necessary CORS headers to the response before passing it back to the original requester.

Postman Versus Browsers

It's crucial to note that while Postman, a popular HTTP request testing tool, does not enforce CORS restrictions by default, browsers do. This difference stems from the fact that Postman is a debugging tool intended for testing API endpoints, while browsers prioritize user security.

Summary

In conclusion, mode: 'no-cors' should be employed only in limited circumstances where opaque responses are desired. CORS proxies offer a valuable solution for cross-origin requests while preserving browser security. Understanding the intricacies of CORS and applying appropriate techniques is essential for enabling secure and seamless communication between websites and their resources.

The above is the detailed content of Why Does My Fetch Request Fail with a 'Cross-Origin Request Blocked' Error, and How Can I Fix It Using CORS?. For more information, please follow other related articles on the PHP Chinese website!