How does CORS work?

CORS, short for Cross-Origin Resource Sharing, is a mechanism that allows web browsers to make cross-origin requests and access resources from different domains. It is a security feature implemented in web browsers to enforce restrictions on cross-domain requests made by client-side JavaScript code.

By default, web browsers adhere to the Same-Origin Policy, which restricts JavaScript code from making requests to different domains. This policy is in place to prevent malicious websites from accessing sensitive data or performing unauthorized actions on behalf of the user.

CORS enables controlled and secure access to resources from other domains by enforcing server-side permissions and restrictions. Here's how it works:

1. Origin Header: When a web browser makes a cross-origin request, it includes an Origin header in the HTTP request. The Origin header contains the domain (origin) from which the request originated.

2. Preflight Request: If the cross-origin request is deemed to be a "complex" request, the browser first sends a preflight request using the OPTIONS method to the target domain. The preflight request includes additional headers such as Access-Control-Request-Method and Access-Control-Request-Headers, specifying the actual HTTP method and headers the client-side code intends to use.

3. Server Response: The server receiving the preflight request responds with appropriate headers, indicating whether the cross-origin request is allowed. These headers include Access-Control-Allow-Origin, Access-Control-Allow-Methods, Access-Control-Allow-Headers, and possibly others. The Access-Control-Allow-Origin header specifies the domain(s) that are allowed to access the resource.

4. Client Access: If the server response allows the cross-origin request, the client-side JavaScript code can proceed with making the actual request. The browser attaches the appropriate credentials (cookies, authentication headers) if allowed by the server's response.

CORS operates based on the interaction between the client-side code running in the browser and the server responding to the requests. The server needs to explicitly configure the appropriate CORS headers to control access to its resources.

CORS policies can be configured to allow requests from specific domains (using Access-Control-Allow-Origin header), control allowed methods and headers (using Access-Control-Allow-Methods and Access-Control-Allow-Headers headers), set response caching, and handle authentication (using Access-Control-Allow-Credentials header).

By obtaining Web Developer Training, you can advance your career as Web Developer. With this course, you can demonstrate your expertise in HTML5, CSS3, Twitter Bootstrap 3, jQuery, and Google APIs and deploy it to Amazon Simple Storage Service (S3), many more fundamental concepts, and many more critical concepts among others.

CORS plays a crucial role in enforcing security and protecting users' data while allowing controlled access to resources across different domains. It ensures that cross-origin requests are only permitted when explicitly allowed by the server's CORS configuration, mitigating potential security risks associated with cross-domain data access.