Node.js: 'Error: listen EADDRINUSE: address already in use :::3000' (or 0.0.0.0:8080) when starting the dev server

Cause: another process is already listening on that port, so your server can't bind to it. Usually it's a previous run of the same app that didn't exit (a crashed or orphaned dev server, a nodemon/ts-node restart race, a second terminal), another app using the same default port (3000, 5173, 8080), or a container publishing that port. How to check which process holds the port: - macOS / Linux: lsof -i :3000 (or ss -ltnp | grep 3000 on Linux). - Windows: netstat -ano | findstr :3000, then look up the PID in Task Manager or with tasklist /FI "PID eq <pid>". - Docker: docker ps and look for 0.0.0.0:3000-> in the PORTS column. Fix: 1. Stop the process that holds the port: kill <pid> (macOS/Linux; use kill -9 only if it ignores a normal kill) or taskkill /PID <pid> /F (Windows), or docker stop <container>. 2. Or run your app on another port: most servers read PORT (PORT=3001 npm start), and tools like Vite or Next.js accept --port. 3. Prevent repeats: handle SIGINT/SIGTERM and call server.close() so restarts release the port; with nodemon, give it time to stop the old process before starting the new one; don't start the server twice from both a dev script and a test setup. 4. In your own code, handle the 'error' event on the server and print a clear message (or try the next port) instead of crashing with a stack trace. After: start the app again and confirm it listens (curl localhost:3000). If the port is held by something you can't identify, check for system services or VPN/proxy software using it. Sources: https://nodejs.org/api/errors.html#common-system-errors https://nodejs.org/api/net.html#serverlisten https://docs.docker.com/engine/network/#published-ports