node/test/parallel/test-cluster-listening-port.js

'use strict';
var common = require('../common');
var assert = require('assert');
var cluster = require('cluster');
var net = require('net');

if (cluster.isMaster) {
  cluster.fork();
  cluster.on('listening', common.mustCall(function(worker, address) {
    const port = address.port;
    // ensure that the port is not 0 or null
    assert(port);
    // ensure that the port is numerical
    assert.strictEqual(typeof port, 'number');
    worker.kill();
  }));
} else {
  net.createServer(common.fail).listen(0);
}