What is CORS and how do you handle it?

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CORS stands for Cross-Origin Resource Sharing. It’s a security feature implemented by web browsers to control how web pages can request resources (like APIs) from a different domain (origin) than the one that served the web page.

Why CORS Exists:

Browsers enforce the Same-Origin Policy, which restricts scripts on one origin (domain, protocol, port) from accessing data from another origin. This prevents malicious sites from reading sensitive data from other sites.

However, many legitimate use cases require cross-origin requests, like a frontend app hosted on frontend.com calling an API at api.backend.com. That’s where CORS comes in.

How CORS Works:

When a browser makes a cross-origin request, it sends an Origin header to the server indicating the requesting origin. The server responds with specific headers like:

• Access-Control-Allow-Origin: Specifies which origins are allowed to access the resource.

• Access-Control-Allow-Methods: Allowed HTTP methods (GET, POST, etc.)

• Access-Control-Allow-Headers: Allowed custom headers.

If these headers are absent or restrictive, the browser blocks the frontend from accessing the response.

Preflight Requests:

For certain requests (e.g., those with custom headers or methods like PUT), the browser sends an OPTIONS request first (preflight) to check if the actual request is safe.

How to Handle CORS:

1. Configure backend server to send appropriate Access-Control-Allow-Origin headers. For example, allow specific domains or * for any origin (less secure).

2. Use middleware in backend frameworks (like cors in Express.js) to simplify configuration.

3. Proxy requests during development to avoid CORS issues.

4. Use JSONP or server-side requests if CORS can’t be configured (rare today).

In summary, CORS controls cross-origin requests, and handling it involves configuring the backend to explicitly allow trusted origins.

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