MongoServerSelectionError: connection timed out means your app can’t reach the MongoDB server within the timeout period.

What causes this error

1. MongoDB Atlas IP whitelist — your IP isn’t allowed
2. Wrong connection string — typo in host, port, or credentials
3. Network/firewall — port 27017 is blocked
4. MongoDB not running — the server is down

Fix 1: Check Atlas IP whitelist

In MongoDB Atlas → Network Access → Add your current IP or 0.0.0.0/0 for development (not production).

Fix 2: Verify connection string

# Test the connection
mongosh "mongodb+srv://user:password@cluster.mongodb.net/mydb"

# Common mistakes:
# - Special characters in password (URL-encode them)
# - Wrong cluster name
# - Missing /database at the end

Fix 3: Check network connectivity

# Can you reach the server?
ping cluster.mongodb.net
telnet cluster.mongodb.net 27017

# Check DNS resolution
nslookup cluster.mongodb.net

Fix 4: Increase connection timeout

mongoose.connect(uri, {
  serverSelectionTimeoutMS: 10000, // 10 seconds (default 30s)
  connectTimeoutMS: 10000,
});

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