MongoDB / Node.js 'MongoServerSelectionError: connect ECONNREFUSED 127.0.0.1:27017' or '::1:27017': how do I fix it?

Cause: The driver could not open a TCP connection to any server, so server selection failed. Typical reasons: mongod is not running or is on a different port, the host in the URI is wrong (in Docker, localhost is the app container itself), a firewall or Atlas IP access list blocks you, or localhost resolved to IPv6 (::1) while mongod listens only on IPv4. Since Node.js 17 the resolver can prefer IPv6 when both sides support it, which turns a previously working localhost URI into ECONNREFUSED. Fix (best first): 1. Check mongod is running and listening: service/container status and logs, and ss -ltnp | grep 27017. The default port is 27017 for a standalone mongod, and net.bindIp defaults to localhost. 2. Use 127.0.0.1 instead of localhost in the URI, or force IPv4 in the driver: new MongoClient(uri, { family: 4 }). Alternatively configure mongod to listen on IPv6 (net.ipv6 must be enabled, and it is not turned on just by setting bindIp to an IPv6 address). 3. Docker/Compose: point the app at the database service name on the same user-defined network, not localhost, and make sure mongod binds to an address reachable from other containers (net.bindIp). 4. Remote servers: add the needed interface to net.bindIp and open the port only to trusted sources. Do not bind to all interfaces (bindIpAll or 0.0.0.0) without enabling authentication and firewalling; MongoDB's manual warns to secure the deployment first. 5. Atlas: confirm your current IP is in Network Access, the IP access list. 6. For slow failures, serverSelectionTimeoutMS controls how long the driver waits before throwing. Verify: mongosh "<your URI>" connects, and the app's first query succeeds without the error. Sources: https://www.mongodb.com/docs/drivers/node/current/connect/connection-troubleshooting/ https://www.mongodb.com/docs/manual/reference/configuration-options/