const portfinder = require('portfinder');

portfinder.getPort(function (err, port) {
  //
  // `port` is guaranteed to be a free port
  // in this scope.
  //
});

const portfinder = require('portfinder');

portfinder.getPortPromise()
  .then((port) => {
    //
    // `port` is guaranteed to be a free port
    // in this scope.
    //
  })
  .catch((err) => {
    //
    // Could not get a free port, `err` contains the reason.
    //
  });

portfinder.setBasePort(3000);    // default: 8000
portfinder.setHighestPort(3333); // default: 65535

portfinder.getPort({
  port: 3000,    // minimum port
  stopPort: 3333 // maximum port
}, callback);