extension js help: var? Is not a constructer

[Craig Rob l300lvl <atz3 yahoo ca>]

I'm driving myself crazy trying to figure out why I get an error about
popMenu not being a constructer. Looking glass says:
Failed to evaluate init function:TypeError: PopupMenu.PopupSubMenuItem
is not a constructor.

Basically I'm trying to create a PopupSubMenuItem that "menuItem"(which
creates a Switch item) is then added to. Why would it see popMenu as a
var?

      let popMenu = new PopupMenu.PopupSubMenuItem(_("List"));
      this.menu.addMenuItem(popMenu);
      menuItem = new
PopupMenu.PopupSwitchMenuItem(_(thisName),init_val);
      popMenu.menu.addMenuItem(menuItem);

Any knowledge here would be very appreciated.