Hello all,

During work I needed to take in consideration multiple instances of gulp w/ livereload.
Normally Gulp starts livereload on port 35729 and doesn’t auto-select the first available automatically.
This case can also be applied to any Node.js script that requires finding the first available port.

First produce a function that takes 3 arguments: the starting port, an optional ending port and a callback.

var net = require('net');

function getAvailablePort(start, end, cb) {
    if (!cb) {
        if (!end) {
            cb = start;
            start = 8000;
            end = 60000;
        } else {
            cb = end;
            end = 60000;
        }
    }
    if (start >= end) {
        return cb(new Error('No available ports to launch livereload.'));
    }

    var c = net.connect(start, function() {
        c.destroy();
        getAvailablePort(start + 1, end, cb);
    });

    c.on('error', function() {
        cb(null, start);
    });
}

The script will keep calling itself until it finds a free port between the starting port and the ending port. To check the availability of a port it simply create a connection instance and kill it afterwards.

Applied to gulp the result can be the following:

gulpfile.js

gulp.task('connect', function() {
    getAvailablePort(35729, function(err, availableLiveReloadPort) {
        connect.server({
            livereload: {
                port: availableLiveReloadPort
            }
        });
    });
});

This way, before assigning the default livereload port, we will search the first free port starting from 35729.

Neat and minimal!