Node.js function to find a free port.

findUnusedPort.js

const net = require('net');

function findUnusedPort() {
  return new Promise(resolve => {
    const server = net.createServer();

    // When the port is omitted, the OS provides an unused one.
    // https://nodejs.org/api/net.html#net_server_listen_port_host_backlog_callback
    server.listen(() => {
      const { port } = server.address();
      server.close();
      resolve(port);
    });
  });
}