How to use the @lumino/widgets.MenuBar function in @lumino/widgets

command: 'example:open-task-manager'
  });

  let menu1 = createMenu();
  menu1.title.label = 'File';
  menu1.title.mnemonic = 0;

  let menu2 = createMenu();
  menu2.title.label = 'Edit';
  menu2.title.mnemonic = 0;

  let menu3 = createMenu();
  menu3.title.label = 'View';
  menu3.title.mnemonic = 0;

  let bar = new MenuBar();
  bar.addMenu(menu1);
  bar.addMenu(menu2);
  bar.addMenu(menu3);
  bar.id = 'menuBar';

  let palette = new CommandPalette({ commands });
  palette.addItem({ command: 'example:cut', category: 'Edit' });
  palette.addItem({ command: 'example:copy', category: 'Edit' });
  palette.addItem({ command: 'example:paste', category: 'Edit' });
  palette.addItem({ command: 'example:one', category: 'Number' });
  palette.addItem({ command: 'example:two', category: 'Number' });
  palette.addItem({ command: 'example:three', category: 'Number' });
  palette.addItem({ command: 'example:four', category: 'Number' });
  palette.addItem({ command: 'example:black', category: 'Number' });
  palette.addItem({ command: 'example:new-tab', category: 'File' });
  palette.addItem({ command: 'example:close-tab', category: 'File' });