How can I check if port is busy in NodeJS?

Check out the amazing tcp-port-used node module!

//Check if a port is open
tcpPortUsed.check(port [, host]) 

 //Wait until a port is no longer being used
tcpPortUsed.waitUntilFree(port [, retryTimeMs] [, timeOutMs])

//Wait until a port is accepting connections
tcpPortUsed.waitUntilUsed(port [, retryTimeMs] [, timeOutMs])

//and a few others!

I've used these to great effect with my gulp watch tasks for detecting when my Express server has been safely terminated and when it has spun up again.

This will accurately report whether a port is bound or not (regardless of SO_REUSEADDR and SO_REUSEPORT, as mentioned by @StevenVachon).

The portscanner NPM module will find free and used ports for you within ranges and is more useful if you're trying to find an open port to bind.

Thank to Steven Vachon link, I made a simple example:

const net = require("net");
const Socket = net.Socket;

const getNextPort = async (port) =>
{
    return new Promise((resolve, reject) =>
    {
        const socket = new Socket();

        const timeout = () =>
        {
            resolve(port);
            socket.destroy();
        };

        const next = () =>
        {
            socket.destroy();
            resolve(getNextPort(++port));
        };

        setTimeout(timeout, 10);
        socket.on("timeout", timeout);

        socket.on("connect", () => next());

        socket.on("error", error =>
        {
            if (error.code !== "ECONNREFUSED")
                reject(error);
            else
                resolve(port);
        });

        socket.connect(port, "0.0.0.0");
    });
};

getNextPort(8080).then(port => {
    console.log("port", port);
});

You could attempt to start a server, either TCP or HTTP, it doesn't matter. Then you could try to start listening on a port, and if it fails, check if the error code is EADDRINUSE.

var net = require('net');
var server = net.createServer();

server.once('error', function(err) {
  if (err.code === 'EADDRINUSE') {
    // port is currently in use
  }
});

server.once('listening', function() {
  // close the server if listening doesn't fail
  server.close();
});

server.listen(/* put the port to check here */);

With the single-use event handlers, you could wrap this into an asynchronous check function.