What is CORS and how do you handle it?

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CORS (Cross-Origin Resource Sharing) is a security mechanism in web browsers that controls how resources are requested from a different domain, protocol, or port than the one the page was loaded from. By default, browsers block cross-origin requests to prevent malicious attacks like CSRF.

When a frontend (e.g., http://example.com) requests data from a backend on another domain (e.g., http://api.example.org), the browser sends a preflight request (OPTIONS) to check if the server allows it.

Handling CORS:

• On the server – Configure allowed origins, HTTP methods, and headers.

  • In Spring Boot: use @CrossOrigin on controllers or global CORS config.

  • In Express.js: use the cors middleware (npm install cors).

• Set HTTP headers – e.g., Access-Control-Allow-Origin: * (or a specific domain), Access-Control-Allow-Methods, and Access-Control-Allow-Headers.

• Avoid using * for sensitive APIs – Always specify trusted domains.

In short, CORS ensures safe cross-domain communication, and developers handle it by configuring server-side rules to allow legitimate requests.

Read More :

How do you manage state between frontend and backend?

What is Axios and how do you use it?

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