app listen with all error handling

//For those who will find this useful, here a function to implement busy port handling (if the port is busy, it will try with the next port, until it find a no busy port)


app.portNumber = 4000;
function listen(port) {
    app.portNumber = port;
    app.listen(port, () => {
        console.log("server is running on port :" + app.portNumber);
    }).on('error', function (err) {
        if(err.errno === 'EADDRINUSE') {
            console.log(`----- Port ${port} is busy, trying with port ${port + 1} -----`);
            listen(port + 1)
        } else {
            console.log(err);
        }
    });
}

listen(app.portNumber);