This is a dynamic way to initialize menu. A typical use case is when you want to create an arbitrary menu, using the data from ctx.session:

const menu = new Menu('root')
menu.dynamic(ctx => ctx.session.data.reduce((range, entry) => range.text(entry)), new Menu.Builder())
bot.command("start", async (ctx) => {
  await ctx.reply("Menu", {
     reply_markup: menu,
  });
});