node-portfinder 
Installation
$ [sudo] npm install portfinder
Usage
The portfinder module has a simple interface:
var portfinder = require('portfinder');
portfinder.getPort(function (err, port) {
//
// `port` is guaranteed to be a free port
// in this scope.
//
});
Or with promise (if Promise are supported) :
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.
//
});
If portfinder.getPortPromise() is called on a Node version without Promise (<4), it will throw an Error unless Bluebird or any Promise pollyfill is used.
By default portfinder will start searching from 8000. To change this simply set portfinder.basePort.
Run Tests
$ npm test