Node.js: `EADDRINUSE: address already in use :::3000`

Ctrl+Z suspends the process, it doesn't stop it. It's still holding port 3000. Use Ctrl+C to stop a server.

To find and kill whatever is on the port:

macOS / Linux

lsof -i :3000
kill <PID>

Windows (PowerShell)

Get-NetTCPConnection -LocalPort 3000 | Select OwningProcess
Stop-Process -Id <PID>

Or use npx kill-port 3000 on any OS.

In dev, it's also handy to read the port from the environment so you can just pick another one: const port = process.env.PORT ?? 3000; and then PORT=3001 npm run dev.