How to check whether a port is open at client's network/firewall?

Question

This is solved at last with \"timeout\" attribute of jQuery AJAX (and JSONP). See my own answer !

Please see the updated part, I

Answer 1

I am occasional frontend/javascript/jQuery guy, so this may not be 100% professional, but it is good enough and it solved my similar problem:

ping_desktop_app = $.get({
  url: "http://127.0.0.1:#{desktop_app_port}",
  dataType: 'jsonp',
})

$(@).parent().find(".click-me-to-use-desktop-app").click ->
  if ping_desktop_app.status == 200
    $.get({
      url: "http://127.0.0.1:#{desktop_app_port}/some_command/123123",
      dataType: 'jsonp',
    })
  else
    alert("Please run your desktop app and refresh browser")

I could not check whether port is open (desktop app is running) on server side because views are cached, so I needed to check the localhost/port right before user click in browser

Edits translating to JS are welcome