main.js

$(function(
  function Menu(cutLabel, copyLabel, pasteLabel) {
    var gui = require('nw.gui')
      , menu = new gui.Menu()

      , cut = new gui.MenuItem({
        label: cutLabel || "Cut"
        , click: function() {
          document.execCommand("cut");
          console.log('Menu:', 'cutted to clipboard');
        }
      })

      , copy = new gui.MenuItem({
        label: copyLabel || "Copy"
        , click: function() {
          document.execCommand("copy");
          console.log('Menu:', 'copied to clipboard');
        }
      })

      , paste = new gui.MenuItem({
        label: pasteLabel || "Paste"
        , click: function() {
          document.execCommand("paste");
          console.log('Menu:', 'pasted to textarea');
        }
      })
    ;

    menu.append(cut);
    menu.append(copy);
    menu.append(paste);

    return menu;
  }

  var menu = new Menu(/* pass cut, copy, paste labels if you need i18n*/);
  $(document).on("contextmenu", function(e) {
    e.preventDefault();
    menu.popup(e.originalEvent.x, e.originalEvent.y);
  });
));