Find unused port in node.js

find-port.js

#!/usr/bin/env node
'use strict';
const net = require('net');
const server = net.createServer();
let port;

server.listen(0, () => {
    port = server.address().port;
    server.once('close', () => {
        console.log(`Found unused port: ${port}`);
    });
    server.close();
});