Answer
CORS is a browser security rule restricting cross-domain requests. Node must configure CORS to allow approved domains to access APIs.
Quick Answer
CORS (Cross-Origin Resource Sharing) is a browser security mechanism that blocks frontend code from making requests to a different domain. Your React app on localhost:3000 can't call your API on localhost:5000 without CORS headers. The server must send Access-Control-Allow-Origin headers to grant permission. The cors npm package makes this easy in Express.
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Verified Answer
This answer has been peer-reviewed by industry experts holding senior engineering roles to ensure technical accuracy and relevance for modern interview standards.
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Source: SugharaIQ