Avoiding CORS Issues in React/Next.js

Cross-Origin Resource Sharing (CORS) is a browser and server security mechanism that controls whether a web page can make requests to a different origin, protecting users from malicious sites exploiting authenticated cookies or other browser-held state to access sensitive data. Servers configure CORS rules through response headers to allow or deny particular origins, credentials, HTTP methods, and request types, while browsers enforce those rules. In a local React and Express example, applications running on different ports are treated as different origins, so a frontend on port 3000 requesting an API on port 4000 can trigger a CORS error. The preferred solution is to configure the backend, such as with Express’s CORS middleware, to explicitly allow the frontend origin. When backend configuration is unavailable or when frontend and backend should appear under one origin, React development proxies or Next.js rewrites can forward browser requests to the backend, preventing the browser from treating them as cross-origin.

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