How do I fix Socket.io connection failures?
Configure CORS on the backend Socket.io server with the frontend origin and credentials: true. On the frontend, use withCredentials: true and pass the token in auth. In production, configure Nginx with Upgrade and Connection headers for WebSocket support.
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Check if MongoDB is running (mongosh for local), verify the MONGODB_URI in .env, ensure Atlas IP whitelist includes your IP, and check that the database name is in the connection string.
Ensure the frontend uses withCredentials: true in Axios, the backend sets credentials: true in CORS, and cookie options include httpOnly: true, sameSite: 'lax', and secure: true only in production (HTTPS). sameSite: 'none' requires secure: true.
In the global error handler, check if err.code === 11000. Extract the field name from err.keyPattern and return a 409 Conflict status with a descriptive message like 'Duplicate value for email'.
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