Why never leak stack traces in production?
Stack traces expose internal file paths, library versions, and logic. Attackers can use them to find weaknesses. In production, send a safe message; log the full error internally.
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More FAQs in How to Handle Errors in an Express Server
Add an error-handling middleware (four parameters) at the end of app.js. Throw or call next(err) in handlers. Use a custom ApiError class with status and message. Use an async wrapper to catch promise rejections. Log externally in production.
Express 4 was designed before async/await and does not catch promise rejections from async handlers. Use an asyncHandler wrapper that catches and passes errors to next. Express 5 catches async errors automatically.
400 bad request, 401 not authenticated, 403 not authorized, 404 not found, 409 conflict (duplicate), 422 unprocessable entity, 500 server error, 503 maintenance or downstream failure.
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