Error: listen EADDRINUSE: address already in use :::3000

npm init -y >/dev/null
npm install --silent --no-audit --no-fund express@5 >/dev/null 2>&1
node -e '
const express = require("express");
const first = express().listen(3456, () => {
  const second = express().listen(3456);
  second.on("error", (err) => { console.log(err.code, "-", err.message); first.close(); });
});'

# Linux / macOS
lsof -i :3000
ss -ltnp 'sport = :3000'       # Linux

# Windows (PowerShell)
Get-NetTCPConnection -LocalPort 3000 | Select-Object OwningProcess

const port = Number(process.env.PORT ?? 3000);
const server = app.listen(port, () => console.log(`listening on ${port}`));
server.on('error', (err) => {
  if (err.code === 'EADDRINUSE') console.error(`port ${port} is busy, set PORT to another value`);
  else throw err;
});