Find an unused port in Node.

find-unused-port.js

const net = require('net');

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

    // Bind to port 0 to ask the kernel to find a
    // local port from a range of open addresses.
    // Close the connection once a port is found.
    server.listen(0, () => {
      const { port } = server.address();

      server.once('close', () => resolve(port));

      if (typeof port === 'number') {
        server.close();
      } else {
        reject('Failed to find an open dynamic port');
      }
    });
  });
}

(async () => {
  const port = await findUnusedPort();
  console.log('port', port);
})();