Nodejs random free tcp ports

My project need to setup a new port every time a new instance of my class is instantiated.

In Node.js how I can find a free TCP port to set in my new socket server? Or check if my specified port is already used or not.

You can bind to a random, free port assigned by the OS by specifying 0 for the port. This way you are not subject to race conditions (e.g. checking for an open port and some process binding to it before you get a chance to bind to it).

Then you can get the assigned port by calling server.address().port.

Example:

var net = require('net');

var srv = net.createServer(function(sock) {
  sock.end('Hello world\n');
});
srv.listen(0, function() {
  console.log('Listening on port ' + srv.address().port);
});

For Express app:

const app = require('express')();

const server = app.listen(0, () => {
  console.log('Listening on port:', server.address().port);
});

To find an opened TCP's port you can use the module portastic

You can find a port like this:

port = require('portastic');

options = {
    min : 8000,
    max : 8005
}

port.find(options, function(err, data){
    if(err)
        throw err;
    console.log(data);
});

Port Finder Library:

https://github.com/http-party/node-portfinder

I suggest you use portfinder library, it has over 10 million downloads in a week.

By default portfinder library will start searching from 8000 and scan until the maximum port number (65535) is reached.

const portfinder = require('portfinder');

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

I use this compact form:

import net from "net"

async function getPortFree() {
	return new Promise( res => {
		const srv = net.createServer();
		srv.listen(0, () => {
			const port = srv.address().port
			srv.close((err) => res(port))
		});
	})
}

use:

const PORT = await getPortFree()

Await'ing the port

For those of you trying to do this "synchronously", with downstream code dependent on the port that is chosen by the OS (e.g. while creating a test server and passing the port to tests), the following recipe has served me well:

export const sleepForPort = async (httpServer: Server, ms: number): Promise<number> => {
  return new Promise<number>((resolve, reject) => {
    httpServer.listen(0, async () => {
      try{
        let addr = (httpServer.address() as AddressInfo | null)
        while(! (addr && addr.port) ) {
          await sleep(ms);
          addr = httpServer.address() as AddressInfo | null
        }
        resolve(addr.port);
      }catch(e){
        reject(e);
      }
    });
  });
}

const sleep = (ms: number) => {
  return new Promise(resolve => setTimeout(resolve, ms));
}

If running integration tests against a live server, this allows us to await the port number becoming available, and return this so our testClient can hit localhost on this port!

export const setupTests = async () => {
  const app = createExpressApp(...);
  const httpServer = createServer(app);
  server.installSubscriptionHandlers(httpServer); // Testing graphql subscriptions
  const port = await sleepForPort(httpServer, 100);
  return {port}
}

describe("Http server test", () => {
  let port: number;
  beforeAll(async () => {
    {port} = await setupTests()
  })
  it("Hits the right port", () => {
    const res = await fetch(`http://localhost:${port}/testing`)
    expect(res).toBeDefined()
    expect(res.status).toEqual(200);
  }
})

I've used the accepted answer on numerous occasions when building apps that run locally - thanks @mscdex. However, I had a use case recently when it would be preferable to stick to a given port if possible, but still to fallback to another port if that one is taken.

The reason for this is I wanted to save app preferences using localStorage, which is restricted to the same site, with the same port for security. Therefore, I wanted to always use port 3000 when possible, but if needed fallback to 3001, or 3002, etc. until one is available. If the port being used is not 3000, then I fallback to a copy of the app preferences stored on the user's hard disk.

I do this with this pattern:

const http = require('http');
const server = http.createServer(function(req,res){ ... })

const config = {
  port: 3000,
  launched: false,
};

serverListen(server, config.port);

function serverListen(server, port){
  server.on('error', e => {
    console.log(`port ${config.port} is taken`)
    config.port +=1;
    server.close();
    serverListen(server, config.port);
  }).listen(port, function() {
    if (config.launched){
      return;
    }
    console.log('Listening on port ' + server.address().port);
    launchBrowser();
    config.launched = true;
  });
}

launchBrowser() is just a function that launches the browser to 'http://127.0.0.1:' + server.address().port

The app is a simple local server + browser app. With the browser being the GUI and the server doing the modification of files etc. on the user's harddrive.